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ANALYTICAL 
 TABLE OF CONTENTS. 
 
 INTRODUCTION. 
 
 HISTORICAL NOTICE OF THE PROGRESS OP ELECTRICAL 
 DISCOVERY. 
 
 I. ELECTRO-STATICS. 
 
 Page 
 
 (1.) The Ancients ignorant of electrical Science - - 1 
 
 (2.) Electricity of Amber known to THALES - - 2 
 (itf Electrical Light seen on the Lances of Roman Soldiers, and on 
 
 the Masts of Vessels - - - 3 
 
 Sparks emitted from the Body of Walimer, a Gothic Chief - 4 
 
 (4.) GILBERT'S Treatise on the Magnet - 5 
 
 First electrical Machine constructed by Otto Guericke - 5 
 
 (5.) Otto Guericke discovers Attractions and Repulsions of excited 
 
 Bodies - -5 
 
 (6.) NEWTON'S Neglect of Electricity - - 6 
 (7.) HAWKSBEE'S Experiments ; uses a Glass Globe for exciting 
 
 Electricity - - - - - -6 
 
 (8.) GRAY commences his Researches - - - - 7 
 
 (9.) Classifies Bodies as Electrics and Non-electrics 7 
 
 (10.) Discovers the Transmission of Electricity by Contact - 7 
 
 (11.) Discovers that some Bodies are Conductors and others not - 9 
 
 (12.) Discovers that Liquids conduct - - 10 
 (13.) Observes that a Body is electrified by mere Proximity to an 
 
 electrified Body - - - - - 10 
 
 (14.) Finds that Bodies may become electrical in cooling after Fusion 11 
 
 (15.) Researches of DUFAYE - - - * - 12 
 
 Extends the Class of Electrics to all Bodies except Metals and 
 
 Liquids - - - 12 
 VOL.1. A 
 
ANALYTICAL TABLE OF CONTENTS. 
 
 Shows that by Insulation all Bodies may be electrified by 
 
 Contact - . . _ . .12 
 
 (16.) Takes a Spark from the Human Body . . > 13 
 
 (17.) Discovers the Properties of Bodies electrified by Contact - 14 
 
 (13.) Propounds his Hypothesis of two distinct Electricities - 14 
 
 (19.) Researches of DESAGULIERS - - . . 18 
 
 (20.) BOZE of Wittemburgh invents the Prime Conducts - - 16 
 
 (21.) WINKLER invents the Cushion, or Rubber - -17 
 
 GORDON substitutes a Glass Cylinder for a Globe - - 17 
 
 (22.) Inflammable Substances fired by Electricity - -17 
 
 Electrical Bells invented - _ - 17 
 
 (23.) Researches of MUSCHENBROECK, CUNEUS, and KLEIST - 18 
 
 (24.) Invention of the LEYDEN JAR - . . .18 
 
 (25.) Anecdotes of its Effects - - - _ _ jg 
 
 (26.; BEVIS substitutes an external Coating for the Hand - 21 
 
 Substitutes Shot for the Liquid contained in the Jar - - 21 
 (27.) Performs the Leyden Experiment with a Plate of Glass coated 
 
 with Silver Leaf - . - . - 21 
 
 (28.) Experiments of WILSON and Dr. WATSON - -22 
 
 Wilson discovers the lateral Shock, but does not explain it - 23 
 
 (29.) Researches of NOLLET and LEMONNIER - - - 23 
 
 Phial containing rarefied Air charged - - -24 
 (30.) Experiments by Nollet, on the Distance to which Electricity 
 
 could be transmitted - . . - 24 
 
 Discharge passed through 180 Men - _ - 24 
 (31.) Experiments in England conducted by Dr. Watson and a Com- 
 mittee of the Royal Society - . - 24 
 
 A Discharge transmitted over and under Westminster Bridge - 25 
 
 (32.) Similar Experiments at Stoke Newington - - - 25 
 
 (33.) Experiments at Shooter's Hill ... . QQ 
 
 (34.) FRANKLIN begins his Researches - - . - 26 
 
 His Correspondence with Collinson - - 27 
 
 Importance of his Discoveries ... - 27 
 
 (35.) Great Circulation of his Letters on Electricity - - 28 
 
 (36.) His Facts more valuable than his Theory - - - 29 
 
 (37.) Franklin's Notion of the two Electricities . - - - 29 
 
 (38.) Two insulated Persons electrified, one positively and the other 
 
 negatively ... _ .30 
 
 (39.) Opposite Electricities neutralise each other - - 31 
 
 (40.) His celebrated Hypothesis of a single electric Fluid - - 31 
 
 (41.) His Analysis of the Leyden Experiment - - - 33 
 
 (42.) Experimental Verifications of this - - - 34 
 
 (43.) Invents the Charge of a Series of Jars by Cascade - - 34 
 
 (44.) An electric Battery thus charged - - 35 
 (4S.) A Jar charged by connecting it with the Conductor and 
 
 Rubber - . . _ _ - 35 
 
 (46.) A Jar charged externally . . - 36 
 
 (47.) The Charge lies upon the Glass - - - 36 
 
 Experiments to prove this - . - 36 
 
(48.) 
 
 (49.) 
 (50.) 
 (51.) 
 
 (52.) 
 
 (53.) 
 ,54.) 
 (55.) 
 
 (56.) 
 (57.) 
 (58.) 
 (59.) 
 
 (60.) 
 (61.) 
 
 (62.) 
 
 ANALYTICAL TABLE OF CONTENTS. XI 
 
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 Early conjectures of the Identity of Electricity and Lightning 37 
 Dr. WALL'S Allusion to it - 37 
 
 Remarkable Guess of Mr. GREY - - - 38 
 
 Anticipation of their Identity by NOLLET - - - 38 
 
 Character of Franklin's Mind. His Desire for useful Applica- 
 tions of scientific Principles - - - 40 
 His first Suggestions of the probable Identity of Lightning and 
 Electricity - - - - 42 
 
 Analogies supporting these Suggestions - - .43 
 
 Discovers the Property of Points - - 44 
 
 Ingenious Experiment to show their probable Use for drawing 
 
 off Electricity from the Clouds - 44 
 
 Describes the Methods by which Lightning might possibly be 
 
 drawn from the Clouds - - 46 
 
 Makes his celebrated Experiment with a Kite, and draws 
 
 down Lightning along its Cord - - 48 
 
 The same Result obtained, according to his Directions, by M. 
 
 DALIBARD, near Paris, a Month earlier 
 Franklin's Right to the Discovery of the Identity of Lightning 
 
 and Electricity denied by M. Arago - - - 51 
 
 Vindication of Franklin - - - 52 
 
 Franklin proposes pointed Conductors for the Protection of 
 Buildings from Lightning - - - - 56 
 
 Disputes in England, whether blunt Conductors would not be 
 better - - - - - 56 
 
 Blunt Conductors placed on the royal Palace - 57 
 
 Continues his Experiments on Lightning - - - 58 
 
 Finds that the Electricity of the Clouds is sometimes positive 
 and sometimes negative - - - - 58 
 
 Is elected a Fellow of the Royal Society - - 59 
 
 Death of RICHMANN. - - - - - 59 
 
 Franklin's Experiments repeated in England by CANTON, WIL- 
 SON, and BEVIS - - 60 
 Researches of BECCARIA - - 61 
 His Observations of the Circumstances attending a Thunder- 
 storm - - - - - - 61 
 
 FRANKLIN shows the magnetic Effect of Electricity - 64 
 
 BECCARIA shows that the Polarity of the Needle depends on 
 
 the Direction of the Current through it - - - 65 
 
 Remarkable Anticipation, by BECCARIA, of Ampere's Theory 
 of Terrestrial Magnetism 
 
 f(7fi.) 
 (77.) 
 
 BECCARIA explains the Cause of Thunder - 
 
 Observation of BOUGUER confirming this 
 
 Researches of LEMONNIER on Atmospheric Electricity 
 
 Shows the diurnal Variation of the Electricity of the Air 
 
 BECCARIA determines the Law of these Changes 
 
 CANTON discovers that the same Body may be electrified with 
 
 either Kind of Electricity - 
 
 Discovers the Use of Amalgam on the Cushion 
 
 - 66 
 67 
 68 
 68 
 68 
 68 
 
X ANALYTICAL TABLE OF CONTENTS. 
 
 Page 
 
 (78.) CANTON shows that insulated Conductors are rendered electrical 
 by the Proximity of electrified Bodies ; but fails to account 
 for this - - - . . -70 
 
 (79.) FRANKLIN discovers the Principle of INDUCTION - - 70 
 
 (80.) His Experiments illustrative of it - - - 71 
 
 (81.) WILKE and ^EPINUS apply the Principle of Induction, and 
 
 invent the Condenser - - - - .73 
 
 (82.) JEPINUS reduces the Franklinian Theory to mathematical 
 
 Analysis - - . . . - 76 
 
 (83.) Relation of Heat to Electricity - - - - 77 
 
 (84.) JEpiKvs discovers the Polarity of the electric Fluid on Tour- 
 maline - - . . . - 77 
 (85.) BECCARIA, WATSON, and CANTON observe some chemical Effects 
 
 of Electricity . . - . . .78 
 
 / (861) Electroscope invented by NOLLET . - 79 
 
 (87.) Condenser improved by VOLTA, who invents the Electrophorus 79 
 Condensing Electroscope invented by him - - 79 
 
 (88.) BECCARIA shows that the Distribution of the Fluid on Con- 
 ductors is superficial - ... 80 
 Lemonnier shows that the Form of the Conductor affects the 
 Distribution ------ 
 
 (89.) VOLTA'S Researches as to the Form of Conductors- - 
 
 (90.) SYMMER propounds the Theory of two electric Fluids 
 
 (91.) His Theory not generally assented to 
 
 (92.) Researches of Dr. Priestley 
 
 (93.) Labours of COULOMB ; his Balance of Torsion 
 
 (94.) Discovers the Law of electrical Attraction and Repulsion 
 
 (95.) Invents a Method of observing the Distribution of Electricity 
 
 on Conductors - - - - - 84 
 
 (96.) Observes the Dissipation of Electricity by Contact of the Air 
 
 and imperfect Insulators - 85 
 
 (97.) EBERHART and PAUL FRISI explain the Aurora Borealis - 85 
 (98.) Fruitless Attempts to apply Electricity to Medicine - - 87 
 
 (99.) Straw Electroscope invented by Volta ; places a Lamp at the 
 
 Point of the Conductor of the atmospheric Electroscope - 87 
 (100.) Volta proposes large Fires as a means of averting Storms - 87 
 (101.) Arago suggests Observations on the Effects of the Iron Fur- 
 naces of Staffordshire - - 88 
 (102.) Experiment of Laplace, Lavoisier, and Volta, to determine the 
 
 Effect of Evaporation on the Electricity of the Atmosphere - 88 
 (103.) POISSON reduces the Investigation of the Phenomena of common 
 Electricity to strict mathematical Analysis, on the Principles 
 of the Theory of two Fluids - - 90 
 
 II. ELECTRO- DYNAMICS. 
 
 C104.) Propriety of the Division of the Science into Electro-statics and 
 Electro-dynamics 
 
ANALYTICAL TABLE OF CONTENTS. Xlll 
 
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 Origin of the Discovery of Galvanism - - 96 
 
 (105.) GALVANI assumes the Existence of Animal Electricity - 96 
 
 (106.) Is opposed by VOLTA - - 100 
 
 (107.) Contest between these Philosophers - 101 
 
 (108.) Volta's Theory of Contact - 102 
 
 (109.) Experiments in Support of it - -102 
 
 (J10.) FABRONI rejects the Theory of Contact, and ascribes the Elec- 
 tricity to chemical Action - - - 104 
 (111.) Invention of the VOLTAIC PILE - - - -106 
 (112.) Invention of the " Couronne des Tasses " - - 109 
 a (113.)- NAPOLEON invites VOLTA to Paris ... 110 
 His Discourses at the Institute; is honoured with a gold 
 
 Medal - 110 
 
 (114.) NAPOLEON establishes Prizes for VOLTAIC DISCOVERIES - 110 
 
 * (115.) Relation between the VOLTAIC PILE and the LEYDEN JAR - 111 
 (116.) Their respective physiological Effects - - 112 
 
 (117.) Anecdote of NAPOLEON - - - 113 
 
 (118.) Experiments of NICHOLSON and CARLISLE - - 114 
 
 (119.) Experiments of W. CRUICKSHANK - - -117 
 
 (120.) CRUICKSHANK invents the Voltaic Trough - -117 
 
 (121.) RITTER'S Experiments - - - - 118 
 
 (122.) DAVY commences his Researches - 119 
 
 (123.) His Experiments on the Decomposition of Water in separate 
 
 Vessels .... - 120 
 
 (124.) Shows that the Power of the Pile depends on Oxydation - 121 
 (125.) Examines the chemical Action which takes place in the Pile - 122 
 (126.) Shows that a Voltaic Pile may be made with Charcoal and 
 
 Zinc ... - 124 
 
 (127.) Shows that voltaic Action does not depend on the conducting 
 
 Power of the Metals - -124 
 
 (128.) Constructs a Pile with a single Metal and two Liquids - 125 
 
 (129.) Constructs a Pile with Charcoal and two Liquids, without any 
 
 metallic Element - - - 128 
 
 (130.) Researches of BIOT and F. CUVIER - -129 
 
 (131.) WOLLASTON and GAUTHEROT maintain that chemical Action is 
 
 the Source of Voltaic Electricity - 130 
 
 (132.) Experiment of VOLTA and ERHMAN to prove electric Polarity - 130 
 (133.) RITTER'S secondary Piles - 130 
 
 (134.) DAVY shows that the Wire retains its electric Property after 
 
 Separation from the Pile - 131 
 
 (135.) He investigates the calorific Properties of the Pile - - 132 
 
 (136.) Researches of FOURCROY, VAUQUELIN, and THENARD -133 
 
 (137.) Electric Spark transmitted in Water and nitric and sulphuric 
 
 Acids - - - - - - 134 
 
 Charcoal ignited in various Liquids ... 134 
 
 (138.) Theory of GROTTHUS ----- 13; 
 
 (139.) DAVY'S Bakerian Lecture for 1806 - 137 
 
 (140.) He shows that in the Decomposition of Water by the Pile no 
 
 new material Principle is generated - 139 
 
XIV ANALYTICAL TABLE OF CONTENTS. 
 
 Page 
 
 (HI.) Decomposes various solid Substances by the Pile - - 145 
 
 (142.) Decomposes various Salts - 14(> 
 
 (143.) Transfers from Vessel to Vessel the decomposed Elements of 
 
 Solutions ...... 147 
 
 (144.) Shows that the decomposing Power is in the Current and not 
 
 the Wire .... -149 
 
 (145.) .Transfers the decomposed Elements through intermediate Solu- 
 tions - - - - - - - 149 
 
 (146.) Shows that during the Transfer chemical Action is suspended 150 
 (147.) Except in Cases where insoluble Compounds are formed - 151 
 
 (148.) Transmission of Oxides through Acids - -153 
 
 (149.) Transmission of the Constituents of Salts through Solutions of 
 
 neutral Salts - ... - 153 
 
 (150.) Decomposition of vegetable and animal Substances - -154 
 
 (151.) His Ideas of the Mode of Action in these Phenomena - 155 
 
 (152.) His electro-chemical Hypothesis - - 159 
 
 (153.) Experiments in Support of it ... -160 
 
 (154.) Chemical Properties of Bodies consistent with this Hypothesis 161 
 (155.) Relations of different Bodies according to their electrical 
 
 Energies - - 161 
 
 (156.) Heat and Light evolved in chemical Action, accounted for - 162 
 (157.) How Heat influences chemieal Action - 162 
 
 (158.) When Combination is rapid and when slow - - 163 
 
 (159.) Davy's Explanation of the Mode of Action of the Pile -164 
 
 (160.) Shows that chemical Action is indispensable, but still receives 
 
 Volta's Theory of Contact - 166 
 
 (161.) Shows the propable Applications of the Pile to the Uses of 
 
 Life - ... - 168 
 
 (162.) Anticipates the future decomposing Effects of the Pile . 169 
 
 (163.) Attempts to explain geological Phenomena by voltaic Action - 170 
 (164.) Experimental Illustration of these Views - 170 
 
 (165.) GUYTON-MORVEAU adopts and follows out these Views - 171 
 
 (166.) DAVY'S Bakerian Lecture for 1807 - 172 
 
 (167.) Discovery of the Decomposition of Potash . - 173 
 
 (168.) And of Soda - 174 
 
 (169-) Methods of preserving Potassium and Sodium in a separate 
 
 State - 175 
 
 (170.) Extension of the Enquiry to other alkaline Substances - 175 
 
 (171.) BERZELIUS and PONTIN decompose Baryta and Lime - 176 
 
 (172.) Davy repeats these Experiments, and also decomposes Strontia 
 
 and Magnesia - - - - - 177 
 
 (173.) Decomposes Silica, Alumina, Glucinia, and Zirconia - 177 
 
 (174.) Infers in general, that the Alkalies and Earths are metallic 
 
 Oxides - - 179 
 
 (175.) BERZELIUS submits Ammonia to Analysis - - 179 
 
 (17.6.) DAVY'S Hypothesis to explain Volcanoes and Aerolites - 180 
 
 (177.) Physiological Applications of Voltaic Electricity - - 181 
 
 (178.) Experiment and Conjectures of WOLLASTON -182 
 
 (179.) Experiments of GAY LUSSAC and THENARD - 183 
 
ANALYTICAL TABLE OF CONTENTS. XV 
 
 Page 
 (180.) Davy's electro-chemical Theory compared with those of GROT- 
 
 THUS and OERSTED - 
 
 (181.) BERZELIUS and AMPERE support Davy's Views 
 (182.) Berzelius extends and improves them 
 (183.) Fruitless Attempts to construct DRY PILES 
 
 (184.) Deluc's Pile improved by Zamboni - 189 
 
 (185.) Uses of what have been called dry Piles - - isX) 
 
 (186.) CONCLUSION - - - - - - 1* 
 
 III. MAGNETISM. 
 
 (187.) Magnetic Attraction and Polarity * - 190 
 
 (188.) Magnetic Meridian, Variation - - - - 191 
 (189.) Dip of the Magnetic Needle 
 
 (190.) Magnetic Attraction known to the Ancients - - 192 
 
 (191.) Invention of the Mariner's Compass of uncertain Date - 192 
 
 Said to be of Chinese Origin 
 
 (192.) Alluded to in Writings of the 12th Century. > - 193 
 
 (193.) Discovery of the Variation 194 
 (194.) Tables of Variation constructed ... 194 
 
 (195.) Robert Norman discovers the Dip 194 
 
 (19n.) Invention of the Dipping Needle - - 194 
 
 (197.) The Variation of the Variation discovered -195 
 
 (198.) Influence of Magnets on soft Iron observed - -195 
 
 (199.) Polarity of Magnets observed - - - - 196 
 
 (200.) Construction of artificial Magnets - - - 196 
 (201.) Magnetism imparted to Iron by the Earth 
 
 (02.) Laws of Magnetic Attraction discovered by COULOMB - 197 
 (203.) Methods of making artificial Magnets consequent Points -198 
 
 (204.) Knight's improved Method - - 198 
 
 (205.) DuhamePs Improvement 199 
 
 (206.) Coulomb's Researches on artificial Magnets - - 199 
 
 (207.) Influence of Heat on Magnetism - 200 
 
 (208.) Local and periodical Changes of the Variation - - 200 
 
 Diurnal Variation - - - - 200 
 
 ., (209.) Cassini's Observations at Paris - - - .201 
 
 (210.) Advancement of Magnetic Geography . - -201 
 
 (211.) Magnetic Equator - - - - - 202 
 
 (212.) Magnetic Poles . - - - .202 
 
 IV. ELECTRO-MAGNETISM. 
 
 (213.) Electro-magnetism very recently discovered - - 203 
 
 (214.) OERSTED'S Experiments at Copenhagen - 204 
 
 (215.) AMPERE follows them up by a Series of splendid Memoirs - 206 
 
XVI ANALYTICAL TABLE OF CONTENTS. 
 
 Page 
 
 (216.) Shows the Law according to which the Needle is deflected - 206 
 (217.) Discovers the Law of Attraction and Repulsion of electric 
 
 Currents ... -207 
 
 (218.) Supposes electric Currents circulating round the Globe - 208 
 
 {219.) ARAGO shows that the conducting Wire has magnetic Pro- 
 perties - 208 
 (220.) ARAGO magnetises Needles by the electric Current - - 209 
 (221.) DAVY effects the same Object by another Method - - 209 
 (222.) BIOT and SAVART determine the Variation of the Attraction 
 
 of the Current at different Distances - - 210 
 
 LAPLACE reduces this Result to an analytical Formula - 210 
 
 (223.) AMPERE reduces the whole Body of Electro-magnetic Phe- 
 nomena to analytical Calculation - - 210 
 (224.) FARADAY begins his Researches makes an electric Cur- 
 rent revolve round a Magnet - - -211 
 (225.) DAVY imparts Rotation to Mercury by means of the Magnet 
 
 and electric Current - - 212 
 
 (226.) DAVY shows the magnetic Influence of a Current passing 
 
 through Air - 214 
 
 (227.) SCHWEIGER invents the Multiplier, or Galvanometer - - 215 
 
 Its Construction and Application - - 215 
 
 (228.) AMPERE shows that the Earth affects electric Currents in the 
 
 same Manner as it affects Magnets - 216 
 
 (229.) AMPERE'S Theory of Terrestrial Magnetism - 217 
 
 (230.) Researches of M. DE LA RIVE - - 18 
 
 (231.) SAVARY shows the magnetising Power of the Current at different 
 
 Distances, and the Law of its Variation - - 219 
 
 (232.) Shows the Effect produced by transmitting it through Metals - 219 
 (233.) Shows that these Facts indicate an undulatory Theory of 
 
 Electricity, similar to that of Light - - - 220 
 
 V. THERMO-ELECTRICITY. 
 
 (234.) Thermo-electric Effects observed by Professor SEEBECK - 220 
 
 (235.) His Experiment with Antimony and Copper - - 221 
 
 (236.) Researches of YELIN, MARSH, and CUMMING - 222 
 
 OERSTED and FOURIER construct a Thermo-electric Pile - 222 
 
 BECQUEREL decomposes Water with such an Instrument - 223 
 
 (237.) Thermo-electric Scale of Metals - - - - 23 
 
 (238.) CONCLUSION - - - - 224 
 
ANALYTICAL TABLE OF CONTENTS. XVil 
 
 BOOK THE FIRST. 
 
 ELECTRO-STATICS. 
 
 CHAPTER I. 
 DEFINITIONS AND PRIMARY FACTS. 
 
 Page 
 
 (1.) Properties of Matter - - 225 
 
 Imponderable Agents - 225 
 
 Material Substances have extensive interstitial Spaces - 226 
 
 Proved by Expansion and Contraction by Change of Temper- 
 ature * - - - - - 226 
 Imponderable Ether pervades these Spaces - - 227 
 The probable Cause of Heat and Light - - - 227 
 And of Electricity - - 227 
 Electricity independent of the mechanical Properties of the 
 
 Bodies it invests - - 227 
 
 (2.) Electrical Excitation by Friction - - - - 227 
 
 (3 ) Electricity defined - 228 
 
 Origin of the Name - 228 
 
 (4.) Attraction of excited Body on light Substances - .228 
 
 (5.) Succeeded by Repulsion - - 230 
 
 (6.) Meaning of the Term Electric Fluid ... 230 
 
 (7.) Attractive and repulsive Power of excited Substances may be 
 
 imparted to others by Contact - - 231 
 
 Effect of an excited Substance on the Skin . - 232 
 
 (9.) Luminous Spark produced - - . - 232 
 
 Bluish Light attends the Friction - - 232 
 
 (10.) Substances capable of being excited ; numerous - -232 
 
 (11.) The Metals are apparently incapable of it - - -232 
 
 How shown to be capable of Excitation ... 232 
 (12.) Effects of Suspension by silk Threads, or Support on Glass Pillars 233 
 (14.) Conductors and Non-conductors of Electricity - - -234 
 
 (15.) Impropriety of the Term . . -234 
 
 Electrics and Non-electrics ... -234 
 
 (18.) Effect of suspending Pith Balls by conducting and non-conduct- 
 ing Threads ... . 235 
 (20.) Water in all States is a Conductor . . - 236 
 (21.) Effects of Humidity of the Air . . . -236 
 Injures the insulating Power of Support - - - 237 
 
XV111 ANALYTICAL TABLE OF CONTENTS. 
 
 Page 
 
 No Substance is either a perfect Conductor or Non-conductor 237 
 (22.) The conducting Power exists in various Degrees - - 238 
 
 (23.) Metals the best Conductors - - 238 
 
 (24.) Method of measuring the conducting Power - - 238 
 
 (25.) Table of Substances in the Order in which they possess the con- 
 ducting Power - 239 
 (27.) Conducting Power of rarefied Air - - - 241 
 (28.) Meaning of the Term conducting Power -241 
 (29.) Conductors of Electricity compared with Conductors of Heat 
 
 and Light - - 242 
 
 (31.) Use of the conducting Power of Metals in the Construction 
 
 of Electrical Apparatus - 244 
 
 Insulated Conductor - 245 
 
 jf General Account of the Electrical Machine - 245 
 
 (32.) The Human Body a Conductor - - 248 
 
 (33.) The Earth the common Reservoir of Electricity - - 249 
 
 CHAP. II. 
 
 * 
 OPPOSITE ELECTRICITIES. 
 
 (34.) Mutual Repulsion of electrified Bodies - - - 251 
 
 (35.) Different Substances excited by Friction produce different 
 
 Effects .... - 251 
 
 Attraction of Bodies electrified by Contact with different Sub- 
 stances .... _ 252 
 Electricity of two Kinds . 252 
 Vitreous and resinous - - 253 
 Positive and negative ... . 253 
 The latter Terms preferred - - - 253 
 (36.) Bodies similarly electrified repel each other . . 253 
 Bodies oppositely electrified attract each other . - 253 
 (38.) Electrical Attractions and Repulsions are transmitted through 
 
 material Substances - - - - - 254 
 
 (40.) Usefulness of suspended Pith Balls in electrical Researches - 254 
 Mode of increasing the Sensibility of such an Apparatus - 254 
 
 (41.) Substances excited by mutual Friction are both electrified - 255 
 They have opposite Electricities - ... 255 
 
 (42.) The same Substance may be electrified by Friction with Elec- 
 tricity of either Kind - 256 
 (43.) Friction produces opposite Electricities ... 256 
 (44 ) Table of Substances, arranged according to the Electricity they 
 
 acquire by mutual Friction .... 257 
 (46.) Example of opposite Electricities produced in the Bodies of two 
 
 Persons --.-.. 259 
 
 The Fur of the Cat useful in electrical Experiments - 259 
 
ANALYTICAL TABLE OF CONTENTS. XIX 
 
 Page 
 
 Electricity of the Human Hair - - 2GO 
 
 (47.) The Production of contrary Electricities by Friction in the 
 
 Electrical Machine - - 260 
 
 (48.) Electricity excited by the Friction of a Liquid against a Solid - 260 
 
 Mercury rubbed against Glass - - - -260 
 
 (49.) Electricity of Mercury in the Barometer ... 261 
 
 (50.) Electricity produced by the Friction of Gases against Solids - i'61 
 
 (51.) Friction not the only Method of exciting Electricity - -261 
 
 Excited in the Fusion of Solids - - 261 
 
 By Change of Temperature - 261 
 
 Chemical Changes - - 262 
 
 By Contact of dissimilar Bodies - - 62 
 
 CHAP. III. 
 LAWS OF ELECTRICAL ATTRACTION AND REPULSION. 
 
 (52.) Coulomb's Electrometer of Torsion . .263 
 
 (54.) Its extreme Sensibility - - 266 
 
 (56.) Another Modification of it - .267 
 
 (57.) Method of using it ..... 268 
 
 (58.) Electrical Attraction and Repulsion ... 269 
 
 (59.) Application of the Electrometer to its Investigation - 270 
 
 (60.) Coulomb's Experiments, proving that the Repulsion varies in- 
 versely as the Square of the Distance - . - 271 
 (62 ) Objections to these Experiments answered - 272 
 (65.) Similar Experiments establish the same Law of electrical At- 
 traction ...... 73 
 
 (66.) Method of establishing the same End by Experiments with an 
 
 electrical Pendulum ..... 274 
 
 Coulomb's Experiments for this Purpose - 275 
 
 (68.) The Share which each of two electrified Bodies has in the 
 
 Effects of Attraction and Repulsion examined - - 280 
 
 (69.) Diffusion of Electricity on Conductors is independent of the 
 
 Kind of Matter of which they are formed - - 281 
 
 70.) Hence different Kinds of Substances have no peculiar Affinity 
 
 for Electricity - .... 282 
 
 V71.) Mutual Attraction or Repulsion of electrified Bodies at a given 
 Distance, depends conjointly on the Quantities of Electricity 
 upon them, and on these only .... 282 
 
 Mathematical Expression of the general Law of electrical At- 
 traction and Repulsion .... 282 
 
ANALYTICAL TABLE OF CONTENTS. 
 
 CHAP. IV. 
 
 DISSIPATION OF ELECTRICITY BY THE AIR, AND BY IMPERFECT 
 INSULATORS, 
 
 Page 
 
 (72.) Electricity lost by the Contact of the Air - - 284 
 
 (73.) Escape of Electricity along the insulating Supports - - 284 
 
 (75.) Deposition of Humidity on the Supports - - 285 
 
 (76.) Manner in which Electricity is absorbed by Contact of the Air 286 
 (79.) Coulomb's Experiments to investigate the Law of the Dis- 
 sipation of Electricity by the Contact of the Air - - 288 
 (80.) Other Circumstances being the same, the Loss of Electricity is 
 
 equal in equal Intervals of Time - - 291 
 
 Other Circumstances being the same, the Loss of Electricity 
 is proportional to the Force of Repulsion, or the Tension of 
 the Fluid - - - - - - 291 
 
 (81.) The Loss of Electricity affected by the hygrometric State of 
 
 the Air - - 291 
 
 (83.) Mathematical Formulae for calculating the Loss of Electricity 
 
 by the Contact of the Air - .292 
 
 Practical Application of these Formulae ... 292 
 (84.) Formulae for electrical Attractions and Repulsions, including 
 Correction for the Loss of Electricity by the Contact of 
 the Air - ... . 296 
 
 (85.) The Law of the Loss of Electricity deduced from Coulomb's 
 
 Experiments is general - 298 
 
 Biot's Experiments on the Dissipation of Electricity of dif- 
 ferent Kinds - - - - - - 301 
 
 (86.) General Inferences from Coulomb's Experiments - - 301 
 
 (87.) Coulomb's Investigation of the Loss of Electricity by imperfect 
 
 Insulators ...... 302 
 
 The Loss ceases when the Tension of the Fluid is reduced to a 
 
 certain Limit . . .302 
 
 Mathematical Analysis of these Effects - - 303 
 
 CHAP. V. 
 DISTRIBUTION OF ELECTRICITY ON CONDUCTORS. 
 
 (89.) Experiment to prove the Electricity of a Conductor superficial 308 
 
 (90.) Another Experiment with the same Object - 309 
 
 (91.) Another Experiment illustrating the same Principle - 311 
 
 (92.) Electricity not strictly speaking superficial - - 312 
 
ANALYTICAL TABLE OF CONTENTS. XXI 
 
 Page 
 
 (93.) Distribution of Electricity on a Sphere - - 513 
 
 (94.) Density or Depth of Electricity on a Conductor - - 314 
 
 (95.) Coulomb's Proof -Plane its Use - -315 
 
 (96.) On the same Conductor the Distribution is always the same - 316 
 (97.) The total Change proportional to the Depth of a given Point - 317 
 (98.) Dissipation by the Air verified - - - - 317 
 
 (99.) Method of determining the relative Depths at different Points 
 
 at the same Time - - - - 318 
 
 (100.) Experiment to prove the Efficacy of the Proof Plane - 319 
 
 [101.) Determination of the actual Depth at a given Point by the 
 
 Proof Plane ... -319 
 
 (102.) Method of rendering successive Observations comparable -321 
 (103.) Precautions respecting the Gum-lac Handle of the Proof Plane 322 
 C104.) Practical Example of these Methods of experimenting. Dis- 
 tribution of Electricity on a flat oblong metallic Plate - 323 
 (105.) Depth increases towards the Ends - 324 
 (106.) Is still greater beyond the Extremity - - -325 
 (107.) General Inference from such Experiments - - 325 
 (108.) Geometrical Illustration of these Results - - - 326 
 (109.) Extension of the same Property to oblong Cylinders, Prisms, &c. 326 
 (110.) Distribution of Electricity on a thin circular Plate - -327 
 (111.) Mathematical Expression of this - -327 
 (112.) Depth of Fluid always increases towards Edges - -329 
 (113.) Conductors with rounded Extremities admit of uniform Dis- 
 tribution ....._ 329 
 (114.) Effect of Points ... .329 
 (115.) Escape of Electricity at a Point - - - -329 
 (116.) Experimental Illustration - - - .330 
 (117.) Rotation by Recoil produced .... 330 
 (118.) Another experimental Illustration - - 331 
 (119.) Electrical Orrery - - . . .332 
 
 CHAP. VI. 
 INDUCTION. 
 
 (120.) Action of the electric Fluid at a Distance - - - 333 
 
 (121.) Action of an electrified Sphere on the Electricity diffused on 
 
 a cylindrical Conductor placed near it - 334 
 
 (122.) The electric Fluid on one Conductor repels a similar electric 
 Fluid, and attracts an opposite electric Fluid on any neigh- 
 bouring Conductor ..... 335 
 
 (124.) The Attractions and Repulsions manifested between electrified 
 Bodies, really belong to the electric Fluids with which they 
 are charged -.... 336 
 
 (125.) How then can a Body not electrified be acted on by one that is ? 336 
 
XX11 ANALYTICAL TABLE OF CONTENTS. 
 
 Page 
 (126.) A cylindrical Conductor in its natural State placed under the 
 
 Influence of two Spheres oppositely electrified - - 337 
 
 (127.) Effects produced by measuring and diminishing the Distance 
 
 of the Spheres - - 338 
 
 (128.) Equal Quantities of contrary Electricities are diffused on a 
 
 Conductor in its natural State - - 339 
 
 (129.) Effect of placing the electrified Spheres at unequal Distances 
 
 from the Conductor ... 340 
 (130.) Effect of removing one of the Spheres - 341 
 
 (131.) Further Demonstration of the natural Electricities of a Con. 
 
 ductor - - - - " 342 
 
 (132.) Inductive Action of a Series of Conductors - 343 
 
 (133.) Inductive Action reciprocal - 344 
 
 (134.) Reciprocal Decomposition of the natural Electricities of two 
 
 Conductors - - - - - - 346 
 
 (135.) Experiments illustrative of this - - 346 
 
 (136.) Analysis of these Effects -347 
 
 (137.) Further experimental Illustrations - - 348 
 
 CHAP. VII. 
 THEORY OF ELECTRICITY. 
 
 (141.) Tests of the Validity of Theories - - - -352 
 
 (142.) The two electrical Theories - - 354 
 
 (143.) Hypothesis of two Fluids -355 
 
 (144.) The natural Electricities of Bodies independent of their free 
 
 electrical Changes ... 355 
 
 (145.) Induction and Excitation explained - - 35(i 
 
 (146.) Effect of the natural Electricities proved experimentally - 357 
 
 (147.) Franklinian Theory - - - - - 359 
 
 (148.) Experiment in support of it - - - o62 
 
 (149.) Another - - - 363 
 
 (150.) Another ..... - 363 
 
 (151.) Objections to the Franklinian Theory - - 364 
 
 (152.) Distribution of free Electricity on an insulated Conductor - 367 
 
 (153.) Distribution of free Electricities on a spherical Conductor - 369 
 Distribution on an elliptical Spheroid - - 369 
 
 (154.) On a very elongated Ellipsoid - - 370 
 
 (155.) Property of Points inferred - 371 
 
 (156,) Experimental Verification - - 371 
 
 (157.) Why Conductors should be every where rounded - - 371 
 
 (158.) Distribution of Electricity on Conductors in contact 
 
 (159.) Condition determining it - 372 
 
 (160.) Mutual Influence of two Spheres, one electrified and the other 
 
 in its natural State ..... 373 
 
ANALYTICAL TABLE OF CONTENTS. XX111 
 
 Page 
 
 (161.) Mutual Effects of their electric Charges acting at a Distance - 374 
 (162.) Effects of Contact on the Distribution - - 375 
 
 (164.) Analysis of the Distribution on two Spheres in Contact - 376 
 
 (165.) Experimental Method of estimating the Distribution practised 
 
 by Coulomb - - 377 
 
 (166.) A second Method - - 379 
 
 (167.) Comparison of the Results of Experiment and Theory - 381 
 
 (168.) Analysis of the Distribution after Separation - 383 
 
 (169-170.) Analysis of Depth at extreme Points when in Contact - 384 
 (171-172.) Analysis in particular Cases - 386 
 
 (173.) Comparison of the Results of Theory and Experiment - 388 
 
 (175.) Analysis for equal Spheres - - -389 
 
 (176.) Comparison of the Results of Theory and Experiment - 389 
 
 (177.) Analysis when the Radius of one Sphere is double the other - 391 
 (178.) Comparison of the Results of Theory and Experiment - 391 
 
 (180.) Analysis when the Radii are as 1 to 4 - - 393 
 
 (181.) Correspondence of Theory and Experiment - 394 
 
 (182.) Analysis of two electrified Spheres acting at a Distance - 394 
 
 (183.) When the lesser Sphere is in its natural State - 395 
 
 (184.) State of the lesser at particular Points - 396 
 
 (185.) Locus of the unelectrified Points - - 397 
 
 (187.) Unequal Spheres separated after Contact - - .399 
 
 (188.) Recapitulation - .... 400 
 
 (189.) Comparison of Theory and Experiment - 400 
 
 (190.) Further Comparison - - - 402 
 
 (191.) Results of Poisson's Researches - - - -402 
 
 (192.) Analysis of the Electric Spark - -403 
 
 (193.) Particular Case to verify Theory - - - - 405 
 
 (194.) Method of electrifying two Spheres in any required Proportion 406 ' 
 (195.) Experimental Illustration of the Effect of the Spark - -407 
 
 CHAP. VIII. 
 
 ELECTRICAL ATTRACTIONS AND REPULSIONS EXPLAINED. 
 
 u9ti.) Effect of the Atmosphere on an an elecmned i5ody - - *o9 
 (Iy7.) Attraction or Repulsion of an electrified Non-conductor ex- 
 plained . . 410 
 (198.) Illustration of this Action - 419 
 (199.) Attraction or Repulsion of an electrified Conductor - - 411 
 (200.) Illustrations of this - . . . _ 41i > 
 (201.) Cases apparently exceptional explained - . .416 
 (202.) Experimental Illustration - . . . 41S 
 
XXIV ANALYTICAL TABLE OP CONTENTS. 
 
 CHAP. IX. 
 ELECTRICAL MACHINES. 
 
 Page 
 (203.) Parts of an Electrical Machine - -421 
 
 The common Cylindrical Machine. 
 
 (205.) Cylinder Cushion and Flap. Use of Amalgam - -424 
 
 (206.) The Prime Conductor - - - - - 426 
 
 (207.) Frame of the Cylinder .... . 426 
 
 (208.) Varnish for Insulaters - - - - - 427 
 
 (209.) Necessity of dryness - - - . -427 
 
 (210.) Cement for Sockets - .. - - - 427 
 
 (211.) Conditions for a good Prime Conductor - - 428 
 
 (212) Operation of the Machine -429 
 
 (213.) Practical Limit of its accumulating Power - - 432 
 
 Nairne's Cylindrical Machine. 
 
 (214.) Description of its Form and Operation - - 432 
 
 Capable of evolving and accumulating other Fluid - 433 
 
 The common Plate Machine. 
 
 (215.) Its Form and Operation - - - - 434 
 
 Its accumulating Power greater than the Cylindrical Machines 435 
 
 The Haerlem Plate Machine. 
 
 (216.) Its Form and Operation - - - 435 
 
 (217.) Method of charging the Conductor with either Fluid - 436 
 
 Appendages to Electrical Machines. 
 
 (219.) Insulating Stools - - 
 
 Metallic Points and insulated Rods .. 
 
 (220.) Jointed Dischargers - - . . 
 
ELECTRICITY, MAGNETISM, 
 
 AND 
 
 METEOKOLOGY, 
 
 INTRODUCTION. 
 
 HISTORICAL NOTICE OP ELECTRICAL DISCOVERY. 
 
 I. ELECTRO-STATICS. 
 
 (1.) ALTHOUGH it has been reserved for modern 
 times to bring to perfection the methods of investi- 
 gation pursued in physical researches, these great 
 divisions of human knowledge have nevertheless 
 been always progressive. If the labours of the an- 
 cients were obstructed, their advancement retarded, 
 and their productions disfigured by fantastical theo- 
 ries ; the facts they accumulated, the phenomena 
 they described, and the observations they recorded, 
 have formed a bequest of the highest value to the 
 better disciplined inquirers and observers of later 
 days. Astronomy, the mechanics of solid and fluid 
 bodies, and the physics of the imponderable agents, 
 light and heat, received severally more or less atten 
 TOL. i. B 
 
INTRODUCTION. 
 
 tion at an early epoch of the progress of human 
 knowledge ; and the results of ancient researches in 
 some of these branches of science, astronomy for 
 example, form an important element of the know- 
 ledge we now possess. Electricity, however, is a 
 remarkable exception to this state of progressive 
 movement. To that particular division of physics 
 antiquity has contributed absolutely nothing. The 
 vast discoveries which have accumulated respecting 
 this extraordinary agent, by which its connection 
 with and influence upon the whole material uni- 
 verse, its relations to the phenomena of organized 
 bodies, the part it plays in the functions of animal 
 and vegetable vitality, its subservience to the uses 
 of man as a mechanical pov/er, its intimate con- 
 nection with the chemical constitution of material 
 substances, in fine, its application in almost every 
 division of the sciences, and every department of 
 the arts, have been severally demonstrated, are 
 exclusively and peculiarly due to the spirit of 
 modern research, and in a great degree to the 
 labours of the present age. 
 
 (2.) The beginnings of science have often the ap- 
 pearance of chance. A felicitous accident throws a 
 certaia natural fact under the notice of an inquiring 
 and philosophic mind. Attention is awakened and 
 investigation provoked. Similar phenomena under 
 varied circumstances are eagerly sought for; and if 
 in the natural course of events they do not present 
 themselves, circumstances are designedly arranged 
 so as to bring about their production. The seeds of 
 science are thus sown, and soon begin to germinate. 
 Whether such primary facts are really fortuitous, 
 
ELECTRICITY A MODERN SCIENCE. 3 
 
 or ought not rather to be viewed as the prompting 
 of HIM, whose will is that intellectual progression 
 shall be incessant, it is certain that they not only 
 give the first impetus to science, but their occa- 
 sional and timely occurrence in its progress pro- 
 duces frequently greater effects on the celerity of 
 its advancement than the most exalted powers of 
 the human mind, unsupported by such aid, have ever 
 accomplished. It may then be imagined that if 
 any such hints were offered by ordinary pheno 
 mena, an agent so all-pervading as electricity could 
 scarcely have eluded notice, or failed to command 
 attention, during a succession of ages which wit- 
 nessed the growth and extension of so many other 
 parts of natural knowledge. On the contrary, the 
 class of effects in which electricity originated was 
 observed by and well known to the early philo- 
 sophers of Greece. THALES, six centuries before 
 the Christian era, was acquainted with the property 
 of amber, from which electricity derives its name * ; 
 and Theophrastus and Pliny, as well as other writers, 
 Greek and Roman, mention the property of this and 
 certain other substances, in virtue of which, when 
 submitted to friction, they acquire the power to 
 attract straws and other light bodies, as a magnet 
 attracts iron. In the spirit which characterized the 
 times, such effects were regarded with feelings of 
 superstition. A soul was ascribed to amber, and 
 it was held sacred. 
 
 (3.) Nor were these the only phenomena which 
 presented themselves to the ancients, and afforded 
 them a clue to the foundation of this part of physics. 
 
 * "HAe/rrpoi/, amber. 
 B 2 
 
4 INTRODUCTION. 
 
 Various other scattered facts are recorded, which 
 p^pve that Nature did not conceal her secrets with 
 more than usual coyness in this case. The luminous 
 appearance attending the friction of those substances 
 which exhibited electrical effects, was observed. 
 The Roman hist6rians record the frequent appear- 
 ance of a flame at the points of the soldiers' javelins, 
 at the summits of the masts of ships, and sometimes 
 even on the heads of the seamen. * The effects of 
 the torpedo and electrical fishes are referred to by 
 Aristotle, Galen, and Oppian ; and at a period less 
 remote, Eustathius, in his Commentary on the Iliad 
 of Homer, mentions the case of Walimer, a Gothic 
 chief, the father of Theodoric, who used to eject 
 sparks from his body ; and further refers to a certain 
 ancient philosopher, who relates of himself that on 
 one occasion, when changing his dress, sudden 
 sparks were emitted from his person on drawing off 
 his clothes, and that flames occasionally issued from 
 him, accompanied by a crackling noise.t 
 
 Such phenomena attracted little attention, and 
 provoked no scientific research. Vacant wonder 
 was the most exalted sentiment they raised ; and 
 they accordingly remained, while twenty centuries 
 rolled away, barren and isolated facts upon the sur- 
 face of human knowledge. The vein whence these 
 precious fragments were detached, and which, as we 
 have shown, cropped out sufficiently often to chal- 
 lenge the notice of the miner, continued unexplored 
 and undiscovered ; and its splendid treasures were 
 
 * Caesar, de Bell. Afr. cap. vi. Liv. cap. xxxii. Plut. Vita 
 Lys. Plin. sec. Hist. Mun. hb. ii. 
 f Eustath. in Iliad, E. 
 
GILBERT. OTTO GUERICKE. 6 
 
 reserved to reward the toil and crown the enter- 
 prise of our generation. 
 
 (4.) The work of classification and generalisation: 
 was first commenced upon the phenomena of elec- 
 tricity by GILBERT, an English physician, in a work 
 entitled De Magnete, published in the beginning of 
 the seventeenth century. In this treatise, the sub- 
 stances then known to be susceptible of electrical 
 excitement were enumerated, and several of the 
 circumstances which affect the production of elec- 
 trical phenomena, such as the hygrometric state 
 of the atmosphere, were explained. Between that 
 period and the earlier part of the last century the 
 science was not advanced by any capital discoveries. 
 In that interval, however, Otto Guericke, celebrated 
 as the inventor of the air-pump, contrived the first 
 electrical machine. This apparatus consisted of a 
 globe of sulphur, mounted upon a horizontal axis, 
 from which it received a motion of rotation, by 
 means of a common handle or winch. The operator 
 turned this handle with one hand, while with the 
 other he applied a cloth to the globe, the friction of 
 which produced the electrical state. 
 
 (5.) Aided by such apparatus, this philosopher 
 discovered, that after a light substance has been 
 attracted by and brought into contact with an 
 electrified body, it will not be again attracted, but, 
 on the contrary, will be repelled by the same body ; 
 but that after it has been touched by the hand, its 
 primitive condition is restored, and it is again at- 
 tracted. He also showed that a body becomes 
 electric by being brought near to an electrified 
 body without touching it ; but offered no explanation 
 B 3 
 
O INTRODUCTION. 
 
 of this fact, which, as will be seen hereafter, indi- 
 cated one of the most important principles of elec- 
 trical science. 
 
 (6.) Whether it was that his attention was alto- 
 gether engrossed by the researches which he prose- 
 cuted with such splendid results in astronomy, the 
 higher mechanics, and optics, or that facts had not 
 yet accumulated in sufficient number and variety to 
 impress him with a just notion of the importance 
 of electricity as a general physical agent, NEWTON 
 bestowed on it no attention. One experiment only 
 proceeding from him is recorded, in which he shows 
 that when one surface of a plate of glass is elec- 
 trified, the attraction will be transmitted through 
 the glass, and will be manifested by its effect on 
 any light substances placed on the other side 
 of it. 
 
 (7.) In the beginning of the eighteenth century, 
 HAWKESBEE made a series of experiments on elec- 
 trical light produced in rarefied air ; but as no con- 
 sequences were deduced from them affecting the 
 progress of the science, we shall not further notice 
 them. In the construction of the apparatus for 
 producing electricity, he substituted a glass sphere 
 for the globe of sulphur proposed by Otto Guericke. 
 This was a considerable improvement; and yet the 
 experimentalists who followed abandoned it, and 
 used no more convenient apparatus than glass tubes, 
 which were held in one hand, and rubbed with the 
 other. To this circumstance Dr. Priestley ascribes, 
 in a great degree, the slow progress made by the 
 immediate successors of Hawkesbee in electrical 
 discoveries. 
 
STEPHEN GREY. 
 
 (8.) About the year 1730 commenced that splen- 
 did series of discoveries which has proceeded with 
 accelerated speed to the present day, and now forms 
 the body of electrical science. Mr. STEPHEN GREY, 
 a pensioner of the Charter House, impelled by a 
 passionate enthusiasm, engaged in a course of ex- 
 perimental researches, in which were developed some 
 general principles, which produced important effects 
 on subsequent investigations. 
 
 (9.) The most considerable discovery of Mr. Grey 
 was, that all material substances might be reduced, 
 in reference to electrical phenomena, to two classes, 
 electrics and non-electrics; the former, including all 
 bodies then supposed to be capable of electric ex- 
 citation by friction; and the other, those which 
 were incapable of it. He also discovered that non- 
 electrics were capable of acquiring the electric state 
 by contact with excited electrics. As the experi- 
 ments which led to these conclusions were of the 
 highest interest, we shall here state them. 
 
 (10.) Desiring to make some experiments with an 
 excited glass tube, he procured one about three feet 
 and a half long, and an inch and a quarter in dia- 
 meter. To keep the interior free from dust, he 
 stopped it at the ends with corks. When this tube 
 was recited, he happened to present one of the 
 corks to a feather, and was surprised to observe 
 that the feather was first attracted, and then re- 
 pelled by the cork, in the way it was wont to be by 
 the glass tube itself. He concluded from this, that 
 the electric virtue conferred on the tube by friction 
 I spontaneously to the cork. 
 
S INTRODUCTION. 
 
 It then occurred to him to inquire whether this 
 transmission of electricity would be made to other 
 substances besides cork. With this view he obtained 
 a deal rod about four inches in length, to one end of 
 which he attached an ivory ball, and inserted the 
 other in the cork, by which the glass tube was 
 stopped. On exciting the tube, he found that the 
 ivory ball attracted and repelled the feather even 
 more vigorously than the cork. He then tried longer 
 rods of deal ; next rods of brass and iron wire, with 
 like results. He then attached to one end of the 
 tube a piece of common packthread, and suspended 
 from the lower end of this thread the ivory ball and 
 various other bodies, all of which he found capable 
 of acquiring the electric state when the tube was ex- 
 cited. Experiments of this kind were made from the 
 balconies of his house, and other elevated stations. 
 With a true philosophic spirit, he now deter- 
 mined to inquire what circumstances attending the 
 manner of experimenting produced any real effect 
 upon the results ; and, first, whether the position or 
 direction of the rods, wires, or cords, by which the 
 electricity was transmitted from the excited tube, 
 affected the phenomena. For this purpose he ex- 
 tended a piece of packthread in a horizontal direc- 
 tion, supporting it at different points by other pieces 
 of similar cord, which were attached to nails driven 
 into a wooden beam, and which were therefore in a 
 vertical position. To one end of the horizontal 
 cord he attached the ivory ball, and to the other he 
 tied the end of the glass tube. On exciting the 
 tube, he found that no electricity was transmitted 
 
WHEELER AND GREY. 
 
 to the ball, a circumstance which he rightly ascribed 
 to its escape by the vertical cords, the nails sup- 
 porting them and the wooden beam. 
 
 Soon after this, Grey was engaged in repeating 
 his experiments at the house of Mr. Wheeler, who 
 was afterwards associated with him in these inves- 
 tigations, when that gentleman suggested that 
 threads of silk should be used to support the hori- 
 zontal line of cord instead of pieces of packthread. 
 It does not appear that this suggestion of Wheeler , 
 proceeded from any knowledge or suspicion of the 
 electric properties of silk ; and still less does it ap- 
 pear that Grey was acquainted with them ; for, in 
 assenting to the proposition of Wheeler, he ob- 
 served, that " silk might do better than packthread 
 on account of its smallness, as less of the virtue 
 would probably pass off by it than by the thickness 
 of the hempen line which had been previously 
 used." 
 
 (11.) They accordingly extended a packthread 
 through a distance of about eighty feet in an hori- 
 zontal direction, supporting it in that position by 
 threads of silk. To one end of this packthread they 
 attached the ivory ball, and to the other the glass 
 tube. Wlien the latter was excited, the ball imme- 
 diately became electric, as was manifested by its 
 attraction upon metallic leaf held near it. After 
 this, they extended their experiments to lines of 
 packthread still longer, when the silk threads used 
 for its support were found to be too weak, and were 
 broken. Being under the erroneous impression 
 that the escape of the electricity was prevented by 
 the fineness of the silk, they now substituted for it 
 
10 INTRODUCTION. 
 
 thin brass wire, which they expected, being still 
 smaller than the silk, would more effectually inter- 
 cept the electricity; and which, from its nature, 
 would have all the necessary strength. The ex- 
 periment, however, completely failed. No elec- 
 tricity was conveyed to the ivory ball, the whole 
 having escaped by the brass wire, notwithstanding 
 its fineness. They now saw that the silk threads 
 intercepted the electricity, because they were silky 
 and not because they were small. 
 
 Having thus accidentally discovered the insulat- 
 ing property of silk, they proceeded to investigate 
 its generalization, and found that the same property 
 was enjoyed by resin, hair, glass, and some other 
 substances. In fact, it soon became apparent that 
 this property belonged in a greater or less degree 
 to all those substances which were then known to 
 be capable of being rendered electrical by friction, 
 and which were denominated electrics. 
 
 (12.) Grey now extended his inquiry to fluids and 
 animal bodies. Having at that time no other test 
 of the electrical state of a body than its attraction 
 for light substances placed on a stand under it, the 
 application of such a test to liquids presented at 
 first some difficulty. This was soon surmounted 
 by the expedient of blowing a soap-bubble from 
 the bole of a tobacco-pipe. The bubble was held 
 suspended over some leaf metal, and on bringing 
 the excited tube to the small end of the pipe, the 
 bubble immediately became electrical. 
 
 (13.) It was in the prosecution of these experi- 
 ments that he discovered that, when the electrified 
 tube was brought near to any part of a non-electric 
 
GREY'S RESEARCHES. 1 1 
 
 body, without touching it, the part most remote from 
 the tube became electrified. He thus fell upon the 
 fact, which afterwards led to the principle of INDUC- 
 TION. The science, however, was not yet ripe for 
 that great discovery, and Grey accordingly con- 
 tinued to apply the principle of inductive electricity 
 without the most remote suspicion of the rich mine 
 whose treasures lay beneath his feet. 
 
 (14.) In another series of experiments, Grey was 
 also unfortunate in missing a subsequent discovery 
 on which he just touched. He found that certain 
 electric bodies were capable of becoming perma- 
 nently excited without the previous process of attri- 
 tion. He took nineteen different substances, among 
 which were resin, gum-lac, shell-lac, sulphur, and 
 pitch, and the remainder of which were various 
 compounds of these. The sulphur he melted in a 
 glass vessel, the others in a spherical iron ladle. 
 When they became solid, and cooled, and were re- 
 moved from the moulds in which they were, in this 
 manner, cast, he found them to be electrified, and 
 that, on preserving them from exposure to the air, 
 by wrapping them in paper or wool, this electrified 
 state continued for an indefinite time. In the case 
 of sulphur, he found that not only the sulphur was 
 electrical, but also the glass from which it was re- 
 moved. Had he carried these inquiries further, 
 and looked carefully into the circumstances of the 
 attraction exhibited by the sulphur and the glass, 
 he could not have failed in discovering the exist- 
 ence of the two opposite electricities, and would 
 probably have also found the reason why the iron 
 ladle did not exhibit electrical signs as well as the 
 
12 INTRODUCTION. 
 
 glass. This, however, escaped him, and the honour 
 of the discovery was reserved for a contemporary 
 philosopher. 
 
 In his investigations respecting the power of 
 liquids to receive electricity from excited glass, 
 Grey exhibited, in a manner which at that period 
 appeared striking, the attraction of the glass tube 
 for liquids. We shall, however, pass over these 
 and some other experiments of less importance, 
 since they did not conduce to the development of 
 any general principle. 
 
 (15.) Contemporary with Grey was the celebrated 
 DUFAYE, who, though not impelled by the same 
 enthusiasm, nor exhibiting the same unwearied 
 activity in multiplying experiments, was endowed 
 with mental powers of a much higher order, and 
 consequently was not slow to perceive some im- 
 portant consequences flowing from the experiments 
 of Grey, which had eluded the notice of that phi- 
 losopher. Dufaye, in the first place, extended the 
 class of substances called electrics ; showing that all 
 substances whatever, except the metals and bodies 
 in the soft or liquid state, were capable of being 
 electrified by friction with any sort of cloth, and 
 that, to secure the result, it was only necessary to 
 warm the body previously. He also showed that 
 the property of receiving electricity by contact 
 with an excited electric was much more general 
 than was supposed by Grey, and that most sub- 
 stances exhibited that property in a greater or 
 less degree, when supported by glass well warmed 
 and dried. Dufaye also showed that the conduct- 
 ing power of the packthread used in the experi- 
 
DUFAYE. 13 
 
 ments of Grey depended on the moisture contained 
 in it, and that the conducting power was consider- 
 ably increased by wetting it. By this expedient 
 he transmitted electricity along a cord to the dis- 
 tance of about 1300 feet. 
 
 (16.) It had been previously ascertained that 
 when any substance charged with electricity com- 
 municated the electric principle to another body 
 near it, but not in contact with it, the electricity 
 passed from the one body to the other in the form 
 of a spark, accompanied by a snapping or cracking 
 noise, like that of a slight explosion. It had also 
 been discovered by Grey and Wheeler, that the 
 bodies of men and animals would become charged 
 with electricity, if placed in the usual manner in 
 contact with an excited glass tube, provided they 
 were suspended by silk cords, so as to prevent the 
 escape of the electricity. Dufaye, therefore, rea- 
 soned, that a man being so suspended by silk cords, 
 the electricity imparted to his person could not 
 escape ; and being charged by the excited glass 
 tube, sparks of fire ought to issue from his body, 
 if any body capable of receiving the electricity 
 were presented to it. To reduce this to the imme- 
 diate test of experiment, Dufaye suspended his 
 own person by silk lines ; and being electrified, the 
 Abbe Nollet, who assisted him in these experi- 
 ments, presented his hand to his body, when im- 
 mediately a spark of fire issued from the person of 
 the one philosopher, and entered the body of the 
 other. Although such a result had been predicted 
 as a consequence of the arrangement, the astonish- 
 ment was not the less great at its occurrence. 
 
INTRODUCTION. 
 
 Nollet states that he can never forget the surprise 
 of both Dufaye and himself when they witnessed 
 the first explosion from the body of the former. 
 
 (17.) The celebrity of Dufaye rests, however, not 
 on his experiments, but on the sagacity which led 
 him to evolve natural laws of a high degree of 
 generality from his own experiments, and from those 
 of the philosophers who preceded him. He repro- 
 duced in a more definite form the principles of at- 
 traction and subsequent repulsion, which had pre- 
 viously been announced by Otto Guericke. "I 
 discovered," says Dufaye, " a very simple principle, 
 which accounts for a great part of the irregularities, 
 and, if I may use the term, the caprices, that seem 
 to accompany most of the experiments in electri- 
 city." This principle was, first, that excited 
 electrics attract all bodies in their natural state ; 
 second, that after a body is so attracted, and has 
 touched the excited electric, then such body is re- 
 pelled by the excited electric ; third, that if, 
 after being so repelled, such body touches any 
 other, it will be again attracted, and again repelled 
 by the excited electric, and so on. 
 
 (18.) But a discovery of a much higher order was 
 due to Dufaye. " Chance," says he, "threw in my 
 way another principle more universal and remark- 
 able than the preceding one; and which casts a 
 new light upon the subject of electricity. The 
 principle is, that there are two distinct kinds of 
 electricity, very different from one another ; one of 
 which I shall call vitreous, and the other resinous 
 electricity. The first is that of glass, rock-crystal, 
 precious stones, hair of animals, wool, and many 
 
VITREOUS AND RESINOUS ELECTRICITY. 15 
 
 other bodies. The second is that of amber, copal, 
 gum-lac, silk-thread, paper, and a vast number of 
 other substances. The characteristic of these two 
 electricities is, that they repel themselves and attract 
 each other. Thus a body of the vitreous electricity 
 repels all other bodies possessed of the vitreous, 
 and on the contrary attracts all those of the 
 resinous electricity. The resinous also repels the 
 resinous and attracts the vitreous. From this 
 principle one may easily deduce the explanation of 
 a great number of other phenomena, and it is pro- 
 bable that this truth will lead us to the discovery 
 of many other things." 
 
 This was a discovery of the highest order, and 
 in its consequences fully justified the anticipation 
 that " it would lead to the discovery of many other 
 things." It is the basis of the only theory of electri- 
 city which has been found sufficient to explain all the 
 phenomena of the science, and with the subsequent 
 hypothesis of Symmer, and the laws of attraction 
 developed by the researches of Coulomb, it has 
 brought the most subtle and incontrollable of all 
 physical agents under the subjection of the rigor- 
 ous canons of mathematical calculation. 
 
 A new question now arose respecting &ny body 
 which has been rendered electrical, whether by 
 immediate excitation, or by contact with another 
 body already excited. It was not enough to ascer- 
 tain that it was electrified ; but it was necessary to 
 know with which of the two kinds of electricity it 
 was invested. The test of this proposed by Dufaye 
 was the same which has ever since his time been 
 adhered to. He electrified a light substance freely 
 
1 6 INTRODUCTION. 
 
 suspended with a known species of electricity ; say, 
 for example, with resinous electricity. If this sub- 
 stance was repelled on bringing it near another 
 electrified body, then the electricity of that body 
 was known to be resinous ; but if, on the contrary, 
 it was attracted, then the electricity of the other 
 body was known to be vitreous. 
 
 (19.) Dr. DESAGULIERS, whose works in other 
 parts of physical science are well known, devoted 
 some attention to electricity from the close of the la- 
 bours of Grey till the year 174-2, but the researches 
 of this philosopher contributed nothing to the exten- 
 sion of the science. He methodised the elements 
 which had already accumulated, and improved in 
 some important instances the nomenclature. He 
 denominated all substances in which electricity may 
 be excited electrics per se y and defined in a distinct 
 manner their characters. He also first applied the 
 term conductor to bodies which freely transmitted 
 electricity, and showed that animal substances owed 
 this property to the fluids which they contain. He 
 however failed to discover that moisture was the 
 conducting agent in many other bodies which at 
 that time were used to propagate electricity to a 
 distance. 
 
 (20.) The subject of electricity now began to at- 
 tract the attention of the Germans, and the first con- 
 sequence was considerable improvement in the power 
 and efficiency of electrical apparatus. The globe of 
 glass revolving on an horizontal axis, which had 
 originated with Hawkesbee, but had, ever since 
 that time, greatly to the detriment of the science, 
 been abandoned in favour of the glass tube, was 
 
BOZE. WINKLEB. GORDON. 17 
 
 now resumed by Professor BOZE of Wittemburgh, 
 who added, for the first time, the prime conductor 
 to the machine. This conductor consisted of an 
 oblong cylinder, or tube, of iron or tin. It was at 
 first supported by a man, who was insulated by 
 standing on cakes of rosin ; but it was subsequently 
 suspended by silken cords. 
 
 (21.) The method of exciting the globe or tube 
 hitherto generally practised, and, indeed, long after- 
 wards persevered in, was to rub them with the hand, 
 taking care that it was dry and warm. WINKLER, 
 a professor in the university of Leipsic, substituted 
 the more convenient expedient of a cushion fixed 
 in contact with the globe, and gently pressed upon 
 its surface by springs, or any similar means. Gor- 
 don, a Scottish Benedictine monk, who was pro- 
 fessor of philosophy at Erfurt, abandoned the use 
 of the globe, and substituted for it a cylinder of 
 glass, having its geometrical axis horizontal, and 
 supported on pivots so as to revolve on that axis. 
 The cylinders he used were eight inches long, and 
 four inches in diameter. Thus the electrical ma- 
 chine assumed a form very nearly identical with 
 the cylindrical machines of the present day. 
 
 (22.) The effects produced by these improved 
 and powerful apparatus are related to have been ex- 
 traordinary. Various inflammable substances, such 
 as spirits, heated oil, pitch, and wax, were fired. 
 Appearances of electrical light issuing from points, 
 and the experiment since known as the electrical 
 bells, were the productions of this epoch. The 
 spark drawn from the conductor by the finger ia 
 described as being so intense as to burst the skin, 
 
1 8 INTRODUCTION. 
 
 draw blood, and produce a wound. Other effects 
 on the animal system are related, in which there is 
 probably some exaggeration. 
 
 (23.) The year 1746 forms a remarkable epoch in 
 the history of electricity, being signalized by the in- 
 vention of the LEYDEN PHIAL. The merit of this 
 discovery is disputed, being claimed for Professor 
 MUSCHENBROEK, CUNEUS, a native of Leyden, and 
 KLEIST, a monk of that place. Probably all these 
 individuals were engaged in the proceedings in 
 which the discovery originated. Dr. Priestley, a 
 -contemporary writer, gives an account of this inven- 
 tion, apparently obtained by personal inquiry, of 
 which the following is the substance. 
 
 (24.) Professor Muschenbroek and his associates 
 having observed that electrified bodies exposed to 
 the atmosphere speedily lost their electric virtue, 
 which was supposed to be abstracted by the air 
 itself, and by vapour and effluvia suspended in it, 
 imagined that if they could surround them with 
 any insulating substance, so as to exclude the 
 contact of the atmosphere, they could communicate 
 a more intense electrical power, and could preserve 
 that power for a longer time. Water appeared one 
 of the most convenient recipients for the electrical 
 influence, and glass the most effectual and easy in- 
 sulating envelop. It appeared, therefore, very 
 obvious, that water inclosed in a glass bottle must 
 retain the electricity given to it, and that, by such 
 means, a greater charge or accumulation of electric 
 force might be obtained than by any expedient be- 
 fore resorted to. In the first experiments made in 
 conformity with these views, no remarkable results 
 
THE LEYDEN EXPERIMENT. 19 
 
 were obtained. But it happened on one occasion 
 that the operator held the glass bottle in his right 
 hand, while the water contained in it communicated 
 by a wire with the prime conductor of a powerful 
 machine. When he considered that it had received 
 a sufficient charge, he applied his left hand to the 
 wire to disengage it from the conductor. He was 
 instantly struck with the convulsive shock with 
 which electricians are now so familiar, and which 
 has been since, and is at present, so frequently 
 suffered from motives of curiosity or amusement. 
 
 (25.) It is curious to observe how much effects on 
 the organs of sense depend on the previous knowledge 
 of them, which may or may not occupy the minds 
 of those who sustain them. Those who now think 
 so lightly of the shock, produced even by a power- 
 ful Leyden phial, would be surprised at the letter in 
 which Muschenbroek gave Reaumer an account of 
 the effect produced upon him by the first experi- 
 ment. He states, that " he felt himself struck in his 
 arms, shoulders, and breast, so that he lost his 
 breath, and was two days before he recovered from 
 the effects of the blow and the terror." He declared, 
 " that he would not take a second shock for the 
 whole kingdom of France." 
 
 Nor was Muschenbroek singular in this extra- 
 ordinary estimate of the effects of the shock. M. 
 Allamand, who made the experiment with a com- 
 mon beer glass, stated that he lost the use of his 
 breath for some moments, and then felt so intense 
 a pain along his right arm that he feared permanent 
 injury from it. Professor Winkler, of Leipsic, 
 c 2 
 
20 INTRODUCTION. 
 
 stated, that the first time he underwent the experi- 
 ment he suffered great convulsions through his body ; 
 that it put his blood into agitation ; that he feared 
 an ardent fever, and was obliged to have recourse 
 to cooling medicines ! That he also felt a heavi- 
 ness in his head, as if a stone were laid upon it. 
 Twice it gave him bleeding at the nose, to which he 
 was not subject. The lady of this professor, who 
 appears to have been as little wanting in the cu- 
 riosity which is ascribed to her own as in the 
 courage assumed for the other sex, took the shock 
 twice, and was rendered so weak by it that she 
 could hardly walk. In a week, nevertheless, her 
 curiosity again got the better of her discretion, and 
 she took a third shock, which immediately produced 
 bleeding at the nose. 
 
 No sooner were these experiments made known, 
 than the amazement of all classes of people of every 
 age, sex, and rank, was excited at what was re- 
 garded as ie a prodigy of nature and philosophy." 
 Philosophers everywhere repeated the experiment, 
 but none succeeded in explaining its effects. After 
 the first emotions of astonishment had abated, the 
 circumstances which influenced the force of the 
 shock were examined. Muschenbroek observed, 
 that if the glass were wet on the outer surface the 
 success of the experiment was impaired ; and Dr. 
 WATSON proved that the force of the shock was 
 increased by the thinness of the glass of which the 
 bottle containing the water was made. He also ob- 
 served, that the force of the charge did not depend 
 on the power of the electrical machine by which the 
 phial was charged. Dr. Watson also showed that 
 
THE LEYDEN JAB. 
 
 the shock could be transmitted undiminished through 
 the bodies of several men touching each other. 
 
 (26.) By further repeating and varying the ex- 
 periment, Watson found that the force of the charge 
 depended on the extent of the external surface of the 
 glass in contact with the hand of the operator ; and 
 it occurred to Dr. BE vis that the hand might be 
 efficient merely as a conductor of electricity, and 
 in that case the object might be more effectually 
 and conveniently attained by coating the exterior 
 of the phial with sheet-lead or tin-foil. This expe- 
 dient was completely successful ; and the phial, so 
 far as related to its external surface, assumed its 
 present form. 
 
 Another important step in the improvement of 
 the Leyden jar was also due to the suggestion of 
 Dr. Be vis. It appeared that the force of the charge 
 increased with the magnitude of the jar, but not in 
 proportion to the quantity of water it contained. 
 It was conjectured that it might depend on the ex- 
 tent of the surface of glass in contact with water ; 
 and that as water was considered to play the part 
 merely of a conductor in the experiment, metal, 
 which was a better conductor, would be at least 
 equally effectual. Three phials were therefore pro- 
 cured and filled to the usual height with shot instead 
 of water. A metallic communication was made 
 between the shot contained in them respectively. 
 The result was a charge of greatly augmented force. 
 This was, in fact, the first electric battery. 
 
 (27-) Dr. Bevis now saw that the seat of the elec- 
 tric influence was the surface of contact of the metal 
 and the glass, and rightly inferred that the form of 
 c 3 
 
22 INTRODUCTION. 
 
 a bottle or jar was not in any way connected with the 
 principle of the experiment. He therefore took a 
 common pane of glass,, and having coated the op* 
 posite faces with tin-foil, extending on each surface 
 within about an inch of the edge, he was able to ob- 
 tain as strong a charge as from a phial having the 
 same extent of coated surface. Dr. Watson being 
 informed of this, coated large jars made of thin glass 
 both on the inside and outside surface with silver 
 leaf, extending nearly to the top of the jars, the 
 effects of which fully corroborated the anticipations 
 of Dr. Bevis, and established the principle that the 
 force of the charge was in proportion to the quan- 
 tity of coated surface. 
 
 The results of all these experiments led to the 
 inference that, in the discharge of the phial, the 
 electricity passed through the circle of conducting 
 matter which was extended between the inside and 
 the outside coating of the jar. If that circle were any 
 where interrupted by the presence of non-conduct- 
 ing matter, or electrics per se, as they were then 
 called, no discharge took place. Also, if any portion 
 of the circle were formed of living animals, each 
 animal sustained the shock. To carry the de- 
 monstration of this further, Dr. Watson placed, at 
 several points in the circuit, spoons filled with spi- 
 rits between the extremities of iron bars, but not in 
 contact with them. In such cases the spirits in all 
 the spoons were inflamed apparently at the instant 
 of the discharge. 
 
 (28.) Many of these properties were simulta- 
 neously discovered by Mr. WILSON, who experiment- 
 ed in Dublin. He coated the external surface of the 
 
WILSON. CANTOtf. 23 
 
 ar in the first experiments by plunging it in water. 
 He also made several experiments with a view to 
 affect by a shock one part of the human body with- 
 out affecting the other parts. But the most remark- 
 able discovery of this electrician was the lateral 
 shock. He observed, that a person standing near 
 the circuit through which the shock is transmitted, 
 would sustain a shock if he were only in contact 
 with any part of the circuit, or even placed very 
 near it. 
 
 Those who are conversant with the science, and 
 aware of the important principle of induction, will 
 see, with much interest, how nearly many of the 
 philosophers engaged in these researches touched, 
 from time to time, on that property, and yet missed 
 the honour of its discovery. Without it, the ex- 
 plication of the phenomenon of the charge and 
 discharge of the Leyden phial was impossible. The 
 lateral shock just adverted to, and observed by 
 Wilson, was almost a glaring instance of it ; but a 
 still more striking manifestation of the theory of 
 the Leyden phial was afforded by an experiment of 
 Mr. CANTON, who showed that if a charged phial 
 be insulated, the internal and external coatings 
 would give alternate sparks, and then, by continuing 
 the process, the phial might be gradually discharged. 
 Canton just touched on the discovery of dissimulated 
 electricity. 
 
 (29.) While these investigations were proceeding 
 in England, the philosophers of France were not want- 
 ing in that zeal and activity which they have always 
 manifested for the advancement of physical science. 
 The Abbe Nollet, M. de Monnier, and others, pro- 
 c 4 
 
24 INTRODUCTION. 
 
 secuted similar experimental researches, and arrived 
 at the knowledge of several of the important circum- 
 stances developed in England. Nollet showed that 
 a phial containing rarified air admitted of being 
 charged as readily as one which contained water, 
 and stated that the water in the Leyden experiment 
 served no purpose, except to conduct the electricity 
 to the glass. 
 
 (30.) From this time to the period at which Dr. 
 Franklin commenced his researches, no important 
 progress was made in the science, although at no 
 former period were experiments on so grand a scale 
 projected and executed ; nor was public attention 
 ever before so powerfully attracted to any scientific 
 subject. Numerous and extensive experiments were 
 made, both in England and France, to determine the 
 distance through which the electric shock could be 
 transmitted, the nature of the substances through 
 which it could be propagated, and the rate at which 
 it moved. At Paris, M. Nollet transmitted it through 
 a chain of 180 soldiers; and at the convent of the 
 Carthusians he formed a chain measuring 54-00 feet, 
 by means of iron wires extending between every two 
 persons, and the whole company gave a sudden 
 spring, and sustained the shock at the same instant. 
 
 (31.) But it was in England that the experiments 
 on this subject were made on the most magnificent 
 scale. Mr. Martin Folkes, then president of the 
 Royal Society, Lord Charles Cavendish, Dr. Bevis, 
 qjid several other fellows of the Society, formed a 
 committee to witness these experiments, the chief 
 direction and management of them being undertaken 
 by Dr. Watson. A circuit was first formed by a 
 
EXPERIMENTS NEAR LONDON. 25 
 
 wire carried from one side of the Thames to the 
 other over Westminster Bridge. One extremity of 
 this wire communicated with the interior of a charged 
 jar ; the other was held by a person on the opposite 
 bank of the river. This person held in his other 
 hand an iron rod, which he dipped into the river. On 
 the other side, near the jar, stood another person, 
 holding in one hand a wire communicating with the 
 exterior coating of the jar, and in the other hand an 
 iron rod. This rod he dipped into the river, when 
 instantly the shock was received by both persons, 
 the electric fluid having passed over the bridge, 
 through the body of the person on the other side, 
 through the water across the river, through the rod 
 held by the other person, and through his body to 
 the exterior coating of the jar. Familiar as such a 
 fact may now appear, it is impossible to convey an 
 adequate idea of the amazement, bordering on in- 
 credulity, with which it was at that time witnessed. 
 (32.) The next experiment was made at Stoke 
 Newington, near London, where a circuit of about 
 two miles in length was formed, consisting, as in the 
 former case, partly of water and partly of wire. In 
 one case there were about 2800 feet of wire, and 8000 
 feet of water. The result was the same as in the case 
 of the experiment at Westminster Bridge. In this 
 case, on repeating the experiment, the rods, instead 
 of being dipped in the water, were merely fixed in the 
 soil at about twenty feet from the water's edge, when 
 it was found that the shock was equally transmitted. 
 This created a doubt whether, in the former case, 
 the shock might not have been conveyed through 
 the ground between the two rods, instead of passing 
 
20 INTRODUCTION. 
 
 through the water, and subsequent experiments 
 proved that such was the case. 
 
 (33.) The same experiments were repeated at 
 Highbury, and finally at Shooter's Hill, in August, 
 1747. At the latter place the wire from the inside 
 of the jar was 6732 feet, and that which touched the 
 outside coating was 3868 feet long. The observers 
 placed at the extremity of these wires were two miles 
 distant from each other. The circuit, therefore, con- 
 sisted of two miles of wire, and two miles of soil or 
 ground, the latter being the space between the two 
 observers. The result of the experiment proved 
 that no observable interval elapsed between the mo- 
 ments at which each observer sustained the shock. 
 In this experiment the wires were insulated by being 
 supported on rods of baked wood. 
 
 We shall here pass over a variety of experiments 
 made in England, France, and Germany on the 
 effects of electricity on organized bodies, and on 
 some proposed medical applications of that agent, 
 such researches not having led to any general prin- 
 ciples affecting the real advancement of the science. 
 
 (34.) Passing from the analysis of the confused 
 experimental labours of his immediate predecessors, 
 labours which contributed so little to the develop- 
 ment of the nature and laws of electrical phenomena, 
 to the researches of FRANKLIN, is like the transition 
 from the mists and obscurity of morning twilight to 
 the unclouded splendour of the noontide sun. It was 
 in the summer of the year 1747, that a fortuitous cir- 
 cumstance, happily for the progress of knowledge, 
 first drew the attention of this truly great and good 
 man, and (as he afterwards proved) acute philo- 
 
BENJAMIN FRANKLIN. 27 
 
 sopher, to the subject of electricity. Mr. Peter Col- 
 linson, a fellow of the Royal Society of London, and 
 a gentleman who took much interest in scientific 
 affairs, made a communication to the Literary So- 
 ciety of Philadelphia, explaining what had been re- 
 cently done in England in electrical experiments, 
 and with his letter he sent a present of one of the 
 glass tubes then commonly used to excite electricity, 
 with directions for its use. Previous to this time, 
 Franklin does not appear to have ever given his 
 attention to physical science. Nevertheless he now 
 commenced repeating the European experiments 
 with all the ardour of an enthusiast, and extending, 
 varying, and adapting them to the development of 
 great general laws, with all the skill and sagacity of 
 a practised experimental philosopher. Within the 
 brief period of four months after the arrival of the 
 tube, he commenced a series of letters to Mr. Col- 
 linson, in which are related a body of discoveries, 
 which for the high generality of the laws they un- 
 folded, the surpassing beauty and clearness of the 
 experimental demonstrations on which they were 
 based, and their intimate connection with the uses 
 of life, are well worthy to be put in juxtaposition 
 with the discoveries of Newton respecting the analy- 
 sis and properties of light. How different, however, 
 was the position of these two great discoverers and 
 benefactors of the human race ! One brought to 
 bear on the subject of his inquiry a mind early dis- 
 ciplined in scientific investigation, a memory stored 
 with profound mathematical erudition, faculties ren- 
 dered more acute and strong by the severe studies 
 exacted from all aspirants to academical honour and 
 
28 INTRODUCTION. 
 
 office in the universities of the old countries, zeal 
 awakened, emulation stimulated, and enthusiasm 
 kindled by associates, among whom were included 
 all that was most distinguished in the physical 
 sciences ; the other, first a tallow-chandler's appren- 
 tice, and next a poor printer's boy, unschooled, undis- 
 ciplined, self-informed, having nothing to aid him 
 but the inborn energy of his mind, separated by an 
 ocean three thousand miles wide from the countries 
 which alone were the seats of the sciences, and 
 where alone those aids and encouragements deriv- 
 able from the society of others engaged in like in- 
 quiries could be obtained. Such was the individual 
 whose researches we must now briefly notice. The 
 series of letters in which he embodied the details of 
 his experiments, and developed the laws which re- 
 sulted from them, were continued from 1747 to 1754, 
 and were subsequently collected and published. 
 
 (35.) " Nothing," says Priestley, "was ever written 
 upon the subject of electricity which was more gene- 
 rally read and admired in all parts of Europe than 
 these letters. There is hardly any European language 
 into which they have not been translated; and, as 
 if this were not sufficient to make them properly 
 known, a translation of them has lately been made 
 into Latin. It is not easy to say whether we are 
 most pleased with the simplicity and perspicuity 
 with which these letters are written, the modesty 
 with which the author proposes every hypothesis of 
 his own, or the noble frankness with which he re- 
 lates his mistakes when they were corrected by 
 subsequent experiments." * 
 
 * History of Electricity, per. ix. sect. i. 
 
FRANKLIN'S EXPERIMENTS. 29 
 
 (36.) In the analysis of Franklin's discoveries, it 
 is necessary to distinguish carefully fact from hypo- 
 thesis^ and to separate the great natural laws which 
 he brought to light, the truth and reality of which can 
 never be shaken, based, as they are, on innumerable 
 observed phenomena, from the theory by which 
 these phenomena and their laws are attempted to be 
 explained by him ; which latter, though marked by 
 great sagacity and ingenuity, and adequate to the 
 explication of most of the ordinary effects of elec- 
 tricity, has been found insufficient to represent the 
 results of subsequent researches, and has been ge- 
 nerally superseded by another theory, which will be 
 noticed hereafter. 
 
 (37.) The first step made by this philosopher in the 
 brilliant series of discoveries by which he rendered 
 his name so memorable, was one which produced a 
 material influence on his subsequent proceedings, 
 since it formed the foundation of his celebrated hy- 
 pothesis of positive and negative electricity, which 
 served him as the link by which many scattered 
 facts might be grouped and connected, and as a clue 
 to the development of new and unobserved pheno- 
 mena. To reduce to the most brief, simple, and 
 general terms the expression of the first-fruit of his 
 observations, it may be said to consist in the esta- 
 blishment of the general principle, that when electri- 
 city is excited by the mutual friction or attrition of 
 any two bodies, both these bodies become electrified ; 
 and if both are insulated they will continue to be so 
 electrified. They will, however, be in different 
 electrical states, since, if moveable, they would at- 
 tract and not repel each other; but, nevertheless, 
 
SO INTRODUCTION. 
 
 each of them will exhibit in relation to other bodies 
 not electrified the same properties. Thus sparks 
 may be drawn indifferently from either ; and each of 
 them may be de-electrised, or discharged of their 
 electricity, by being put in metallic communication 
 with the ground. These general facts he proved by 
 direct experiment. 
 
 (38.) He placed two persons, A. and B., on insu- 
 lating supports. In the hand of A. he put a glass 
 tube, which being rubbed by A. became electrified. 
 This tube was then touched at every part of the 
 rubbed surface by B. ; after which the same process 
 was several times repeated, the tube being deprived 
 of its electricity as often as it was touched by B. A 
 third person, C., not insulated, now presented his 
 finger or a metallic sphere to B., from whom a spark 
 was drawn ; and by repeating this, or by touching 
 the person of B., the latter was deprived of the elec- 
 tricity he had received from the tube. This was no 
 more than was expected. But on subjecting A. to 
 the same process, the very same effects were pro- 
 duced. It appeared, therefore, that both A. and B. 
 were electrified. 
 
 Being again electrified, as before, by the friction 
 of the tube, instead of A. and B. being successively 
 touched by C., they were made to touch each other, 
 both remaining insulated. After this both were 
 found to be as completely de-electrised and restored 
 to their ordinary state as when they had been 
 touched by C. 
 
 A cork ball, suspended by a silk thread, being 
 electrified by contact with the excited glass tube, 
 was repelled when brought near the person of B,, 
 
FRANKLIN'S EXPERIMENTS. 31 
 
 but it was attracted when brought near the person 
 of A. 
 
 From these experiments it appeared the electrical 
 states of A. and B. were different. Franklin called 
 the state of B., and consequently that of the glass 
 tube from which he drew the electricity, positive, and 
 that of A. negative. The one was said to be posi- 
 tively, the other negatively electrified. The cloth 
 with which A. rubbed the glass tube was, like A., 
 negatively electrified it attracted the cork ball ; and 
 the glass tube, like B., was positively electrified it 
 repelled the cork ball. 
 
 (39.) The generality of this result was established 
 by a great variety of experiments. In all cases it ap- 
 peared that the opposite electrical charges of the 
 two bodies submitted to friction, or of any insulated 
 bodies in communication with them, had the same 
 reciprocally neutralising power ; in virtue of which, 
 when brought into contact, or when a metallic com- 
 munication was established between them, all signs 
 of electricity would disappear. 
 
 (40.) Such is a strict statement of the facts as 
 evolved in the experiments. The hypothesis pro- 
 posed by Franklin for their explication was as fol- 
 lows : All bodies in their natural state are charged 
 with a certain quantity of electricity, in each body 
 this quantity being of definite amount. This quantity 
 of electricity is maintained in equilibrium upon the 
 body by an attraction which the particles of the 
 body have for it, and does not therefore exert any 
 attraction for other bodies. But a body may be 
 invested with more or less electricity than satisfies 
 its attraction. If it possess more, it is ready to give 
 
32 INTRODUCTION. 
 
 up the surplus to any body which has less, or to 
 share it with any body in its natural state ; if it have 
 less, it is ready to take from any body in its natural 
 state a part of its electricity, so that each will have 
 less than their natural amount. A body having 
 more than its natural quantity is electrified posi- 
 tively or plus, and one which has less is electrified 
 negatively or minus. 
 
 When two bodies are submitted to mutual attri- 
 tion and become electrified, one parts with a portion 
 of its proper electricity, which is received by the 
 other. The latter then has more than its natural 
 amount, and is positively electrified ; the former has 
 less, and is negatively electrified. 
 
 In the instance above stated, when A. rubs the 
 glass tube, he loses a portion of his natural elec- 
 tricity, and is negatively electrified ; while the tube 
 receives what he loses, and becomes positively elec- 
 trified. When B. touches the tube, he takes from 
 it nearly all the electricity with which it is charged 
 over and above its natural amount ; for his body 
 being of so much greater magnitude than the tube, 
 the proportion which will remain on the tube will 
 be insignificant. 
 
 If when A. rubs the tube he were not insulated, 
 he would not be electrified, because, as fast as his 
 body would lose its proper amount of electricity, 
 the deficiency would be made up from the earth 
 with which he is in free electrical communication ; 
 whereas in the former case being insulated, this 
 supply could not be obtained. Hence, in this theory, 
 the earth is regarded as the common reservoir of 
 electricity, from which bodies negatively electrified 
 
FRANKLIN'S THEORY. 33 
 
 receive what they want, and to which bodies posi- 
 tively electrified give up their surplus, except in the 
 case in which the one or the other are insulated. 
 
 Such, in general, was the Franklinian theory ; 
 whichj however, will be more fully developed here- 
 after. 
 
 (4?1.) Assuming these hypothetical principles, 
 Franklin next proceeded to analyse the phenomena of 
 the Leyden jar. His first experiments were directed 
 to establish the fact, that when the jar is charged the 
 inside is electrified positively, and the outside nega- 
 tively. A charged jar was placed on an insulating 
 support, and a metallic wire bent into the form of a 
 circular arc was then placed with one end in con- 
 tact with the outer coating. The other end was 
 capable of being brought into contact with the hook 
 of the wire inserted through the cork, and thereby 
 put in metallic communication with the water con- 
 tained in the jar. This bent wire being supported 
 by a handle of sealing-wax was itself insulated, and 
 no electricity could pass in the experiment, other- 
 wise than between the inside of the jar and the 
 coating on the outside. On bringing the upper ex- 
 tremity of the bent wire into contact with the hook, 
 the jar was instantly discharged, both the inside 
 and the outside being restored to their natural state. 
 Franklin inferred from this, that before the dis- 
 charge the interior of the jar was positively elec- 
 trified, and the exterior coating negatively elec- 
 trified, in an equal degree ; that is to say, that the 
 interior of the jar contained an excess of electricity 
 over and above its natural amount, and the exterior 
 
34 INTRODUCTION. 
 
 coating fell short of its natural amount by a quan- 
 tity equal to that excess. 
 
 (42.) Various other experiments were made to ve- 
 rify this doctrine. Two metallic knobs were placed 
 near each other, one communicating with the ex- 
 ternal coating, and the other with the water within 
 the jar. A small cork ball suspended by a silk thread 
 was placed between those two knobs. The ball was 
 alternately attracted and repelled, " playing inces- 
 santly from one to the other, till the bottle was no 
 longer electrised ; that is, it fetched and carried fire 
 from the top (inside) to the bottom (outside) of 
 the bottle, till equilibrium was restored."* 
 
 It had been observed by electricians in Europe, 
 that a jar could not be charged if its external coat- 
 ing were insulated ; that, in fact, it was a neces- 
 sary condition that a communication between that 
 coating and the ground should be provided and 
 maintained by some conducting matter, such as a 
 metallic wire. Franklin assumes, that no electricity 
 can be conveyed to the inside without causing the 
 expulsion of an equal quantity from the outside ; 
 but if the jar be insulated, no means of escape 
 being left for the electricity on the outside, no ac- 
 cumulation can take place on the inside, f 
 
 (43.) In these experiments, we find also a de- 
 scription of the method of charging a series of jars, 
 now called the charge by cascade. " Suspend two or 
 more phials on the prime conductor, one hanging 
 on the tail of another, and a wire from the last to 
 the floor. An equal number of turns of the wheel 
 
 * Franklin's Works (Letters), vol. v. p. 192. Boston, 1837 
 f Ibid. p. 190. 
 
THEORY OF THE LEYDEN JAB. 35 
 
 will charge them all equally, and every one as much 
 as one alone would have been ; what is drawn out 
 of the tail of the first serving to charge (the inside 
 of) the second ; what is driven out of the seconcj 
 charging the third, and so on." * 
 
 (4-4-.) In this way he constructed an electrical 
 battery. After charging a series of jars he separated 
 them, putting the insides in metallic communication 
 with each other, and the outsides, in like manner, in 
 metallic communication. By such means he ob- 
 tained discharges sufficiently powerful to kill the 
 smaller animals. 
 
 (4-5.) But the experiment which appeared to be 
 most conclusive in the support it gave to his hypo- 
 thesis, of the transfer of the electricity from the ex- 
 terior to the interior of the jar in the process of 
 charging it, was the following: A jar was suspended 
 by its hook on the prime conductor of the machine, 
 so that a metallic communication was maintained 
 between the conductor and the inside of the jar. 
 Meanwhile, the rubber was insulated. On working 
 the machine, the jar was found to receive no charge. 
 A metallic wire was now rolled round the outer 
 coating of the jar, and carried from thence to the 
 rubber, so as to make a communication between 
 them, both being still, in other respects, insulated. 
 The jar was now charged with ease, which was 
 explained by the supposition that the electric fluid 
 passed from the outside coating by the wire to the 
 rubber, and thence by the glass globe and prime 
 conductor to the inside of the jar.f 
 
 * Letters, p. 199. t Ibid - P- 253. 
 
36 INTRODUCTION. 
 
 (46.) According to the hypothesis above stated, 
 there is no essential distinction, so far as relates to 
 the charge, between the external coating and the 
 internal contents of the jar; the one ought to be as 
 easily charged as the other. This was accordingly 
 found to be the case. A jar was placed on an in- 
 sulating support, and while the external coating was 
 put in communication with the prime conductor of 
 the machine, the wire extending from the interior 
 was put in communication with the rubber. The 
 electricity of the outer coating was now positive, 
 and that of the inside negative ; and the jar was 
 discharged, and produced the same effects as before. 
 
 (4?7.) The next important investigation was as to 
 the place in which the electricity of the jar was con- 
 tained. To determine this, Franklin charged a jar, 
 and insulated it. He then removed the cork, and 
 the wire by which the electricity was conveyed 
 from the machine to the inside of the jar. On 
 examining these, he found them free from electri- 
 city. He next carefully decanted the water from 
 the charged jar into another insulated vessel On 
 examining this it was found to be free from electri- 
 city. Other water in its natural state was fiow in- 
 troduced into the charged jar to replace that which 
 had been decanted ; and on placing one hand on 
 the outside coating, and the other in the water, he 
 received the shock as forcibly as if no change had 
 been made in the jar since it was first charged.* 
 
 A piece of glass was then placed between two 
 plates of lead extending nearly to its edge on every 
 
 * Letters, p. 201. 
 
THEORY OF THE LEYDEN JAB. 3J 
 
 side. One of these plates of lead being touched by 
 the hand, the other was charged with electricity as 
 usual. The plates were then removed from the 
 glass, and, being examined, were found to be in 
 their natural state. On presenting the finger to the 
 glass where the lead had covered it, little sparks 
 were received; and on displacing the lead, and 
 touching it at both surfaces, a violent shock was 
 received. 
 
 From this he inferred that the glass was the sub- 
 stance in which the electricity was deposited; and the 
 metallic coating, or the water, or other conductor, 
 applied to it, " served only, like the armature of the 
 loadstone, to unite the forces of the several parts, 
 and bring them at once to any point desired ; it 
 being the property of a non-electric (conductor), 
 that the whole body instantly receives, or gives, 
 what electrical fire is given to, or taken from, any 
 one of its parts." * 
 
 (4-8.) From a very early period of the progress 
 of electrical observations, the analogy between elec- 
 tricity and lightning had been noticed, and conjec- 
 tures as to their identity were expressed ; and in 
 some cases distinct predictions hazarded, that the 
 time would arrive which would fully establish their 
 identity. Dr. Wall, in a paper published in the 
 " Philosophical Transactions/' speaking of the elec- 
 tricity of amber, said that he had no doubt, " that 
 by using a longer and larger piece of amber, both 
 the cracklings and the light would be much greater. 
 
 * Letters, p. 202. 
 D 3 
 
INTRODUCTION. 
 
 This light and crackling seems in some degree to 
 represent thunder and lightning." * 
 
 (49.) Mr. Grey, whose experiments have been al- 
 ready referred to, says, speaking of electrical effects, 
 tf These are at present but in minimis. It is pro- 
 bable that, in time, there may be found out a way to 
 collect a greater quantity of electric fire, and conse- 
 quently to increase the force of that power, which, 
 by several of these experiments, si licet magnis 
 componere parva, seems to be of the same nature 
 with that of thunder and lightning" 
 
 (50.) But of all the anticipations which are pre- 
 tended to of the grand discovery of the philosopher of 
 Philadelphia, that which is by far the most remark- 
 able proceeded from his contemporary and compe- 
 titor, the Abbe Nollet. Immediately after the first 
 exhibition of the experiments proving the identity 
 of electricity and lightning, the Abbe urged his 
 claim to a share of the merit of having suggested 
 them. In a paper, dated Paris, June 6. 1 752, the 
 Abbe, after noticing the experiments, observes that 
 he (i is more interested than any one to come at 
 the facts, which prove a true analogy between 
 lightning and electricity, since these experiments 
 establish incontestably a truth which he had con- 
 ceived, and which he ventured to lay before the 
 public more than four years ago." 
 
 In the fourth volume of his Lemons de Phy- 
 sique is found the following passage : "If any 
 one should undertake to prove, as a clear conse- 
 quence of the phenomenon, that thunder is, in the 
 
 * Priestley, History of Electricity, p. 11. 
 
THEORY OF LIGHTNING. 3Q 
 
 hands of Nature, what electricity is in ours that 
 those wonders which we dispose at our pleasure 
 are only imitations on a small scale of those grand 
 effects which terrify us, and that both depend on 
 the same mechanical agents if it were made ma- 
 nifest that a cloud prepared by the effects of the 
 wind, by heat, by a mixture of exhalations, &c. is 
 in relation to a terrestrial object what an electrified 
 body is in relation to a body near it not electrified, 
 I confess that this idea, well supported, would 
 please me much ; and to support it, how numerous 
 and specious are the reasons which present them- 
 selves to a mind conversant with electricity. The 
 universality of the electric matter, the readiness of 
 its action, its instrumentality and its activity in 
 giving fire to other bodies ; its property of striking 
 bodies externally and internally, even to their small- 
 est parts (the remarkable example we have of 
 this effect even in the Leyden jar experiment, 
 the idea which we might truly adopt in supposing 
 a greater degree of electric power) ; all these points 
 of analogy which I have been for some time medi- 
 tating, begin to make me believe that one might, 
 by taking electricity for the model, form to oneself, 
 in regard to thunder and lightning, more perfect 
 and more probable ideas than any hitherto pro- 
 posed." * 
 
 The volume containing this passage was printed 
 and published towards the close of the year 1748, 
 as appears by the register of the Academy of 
 Sciences, in which the order to print it bears date 
 
 * Nollet, Le9ons de Physique, torn. iv. p. 315., Sme. edi- 
 tion. 
 
 D 4} 
 
40 INTRODUCTION. 
 
 on the 9th of August in that year. It will presently 
 appear that Franklin's first publication of the same 
 views was in a letter addressed to Mr. Collinson, 
 despatched in 1749. So far, therefore, as relates to 
 these speculations, the priority of publication must 
 be conceded to Nollet. It seems, however, impro- 
 bable that Franklin, residing at Philadelphia, could 
 have seen Nollet's volume between the date of its 
 publication and the despatch of his letter, an inter- 
 val not exceeding a few months ; and the probability 
 is, therefore, that these views occurred simulta- 
 neously to the American and the French philosopher. 
 (51.) From the moment that Franklin first engaged 
 ill electrical inquiries, his views were constantly bent 
 on the discovery of some useful purpose to which 
 the science could be applied. Cui bono 9 was a ques- 
 tion never absent from his thoughts.* This crav- 
 ing after utility was the great characteristic of his 
 
 * After he had succeeded in making the discoveries which 
 have been already explained, and besides inventing a little 
 moving power, which he called an electrical jack, he expressed 
 to Mr. Collinson, in his usual playful manner, his disappoint- 
 ment at being unable to find any application of the science 
 beneficial to mankind. " Chagrined a little that we have 
 hitherto been able to produce nothing in this way of use to 
 mankind, and the hot weather coming on when electrical ex- 
 periments are not so agreeable, it is proposed to put an end to 
 them for this season, somewhat humorously, in a party of 
 pleasure on the banks of the Schuylkill. Spirits, at the same 
 time, are to be fired by a spark sent from side to side through 
 the river without any other conductor than the water ; an ex- 
 periment which we some time since performed to the amaze- 
 ment of many. A turkey is to be killed for dinner by the 
 electrical shock, and roasted by the electrical jack, before a fire 
 kindled by the electrified bottle, when the healths of all the 
 famous electricians of England, Holland, France, and Ger- 
 many are to be drunk in electrified bumpers, under the dis- 
 charge of guns from the electrical battery." Letters, p. 210. 
 
CI 
 
 CHARACTER OF FRANKLINS GENIUS. 41 
 
 mind, and might be regarded as being carried 
 almost to a fault. To bring the properties of mat- 
 ter and the phenomena of Nature into subjection 
 to the uses of civilised life, is undoubtedly one of 
 the great incentives to the investigation of the laws 
 of the material world ; but it is assuredly a great 
 error to regard it either as the only or the principal 
 motive to such inquiries. There is in the percep- 
 tion of Truth itself in the contemplation of con- 
 nected propositions, leading by the mere operation 
 of the intellectual faculties, exercised on individual 
 physical facts, to the development of those great 
 general laws by which the universe is maintained 
 an exalted pleasure, compared with which the mere 
 attainment of convenience and utility in the eco- 
 nomy of life is poor and mean. There is a noble- 
 ness in the power which the natural philosopher 
 derives from the discovery of these laws, of raising 
 the curtain of futurity, and displaying the decrees 
 of Nature, so far as they affect the physical universe 
 for countless ages to come, which is independent of 
 all utility. There is a lofty and disinterested plea.- 
 sure in the mere contemplation of the harmony and 
 order of Nature, which is above and beyond mere 
 utility. While, however, we thus claim for truth and 
 knowledge all the consideration to which, on their 
 own account, they are entitled, let us not be mis- 
 understood as disparaging the great benefactors of 
 the human race, who have drawn from them those 
 benefits which so much tend to the wellbeing of 
 man. When we express the enjoyment which 
 arises from the beauty and fragrance of the flower, 
 we do not the less prize the honey which is ex- 
 
42 INTRODUCTION. 
 
 tracted from it, or the medicinal virtues it yields. 
 That Franklin was accessible to such feelings, the 
 enthusiasm with which he expresses himself through- 
 out his writings in regard to natural phenomena 
 abundantly proves. Nevertheless, useful application 
 was, undoubtedly, ever uppermost in his thoughts ; 
 and he probably never witnessed any physical fact, 
 or considered for a moment any law of nature, with- 
 out inwardly proposing to himself the question, " In 
 what way can this be made beneficial in the eco- 
 nomy of life ? " 
 
 (52.) The analogy and probable identity of light- 
 ning and electricity were first suggested and demon- 
 strated by Franklin in a letter addressed to Collinson, 
 which appears without a date, and which has by 
 some been referred to the date (1750) of that which 
 immediately follows it in the published collection 
 of letters. It appears, however, by a subsequent 
 letter*, addressed to the same gentleman in 1753, 
 that he was occupied in the investigation of this 
 question from 1747 to 174-9 ; that the paper now 
 referred to was first written in the former year, but 
 that it was enlarged and improved and sent to 
 England in 1749, which must, therefore, be taken 
 as its date. In this letter he enters very fully into 
 his reasons for considering the cause of electricity 
 and lightning to be the same physical agent, differ- 
 ent in nothing save the intensity of its action ; and 
 he truly observes, that the difference in degree, 
 however enormous, is no argument against the 
 
 * " In my former paper on this subject, written first in 1747, 
 enlarged and sent to England in 1 749, I considered the sea 
 as the great source of lightning," &c. Letters, p. 300. 
 
LIGHTNING AND ELECTRICITY IDENTIFIED. 43 
 
 identity of the agents, but that, on the contrary, 
 an almost infinite difference might be naturally 
 looked for. " When a gun-barrel in electrical ex- 
 periments has but little electrical fire in it, you 
 must approach it very near with your knuckle 
 before you can draw a spark. Give it more fire, 
 and it will give a spark at greater distance. Two 
 gun-barrels united, and as highly electrified, will 
 give a spark at a still greater distance. But if two 
 gun-barrels electrified will strike at two inches dis- 
 tance, and make a loud snap, to what a great dis- 
 tance may ten thousand acres of electrified cloud 
 strike and give its fire, and how loud must be that 
 crack I " * 
 
 (53.) The analogies which he stated as affording 
 presumptive evidence of the identity of lightning and 
 electricity may be briefly enumerated. The elec- 
 trical spark is zigzag, and not straight ; so is light- 
 ning. Pointed bodies attract electricity ; lightning 
 strikes mountains, trees, spires, masts, and chim- 
 neys. When different paths are offered to the 
 escape of electricity, it chooses the best conductor ; 
 so does lightning. Electricity fires combustibles ; 
 so does lightning. Electricity fuses metals ; so 
 does lightning. Lightning rends bad conductors 
 when it strikes them ; so does electricity when 
 rendered sufficiently strong. Lightning reverses 
 the poles of a magnet ; he proved by direct expe- 
 riment that electricity had the same effect. A 
 stroke of lightning, when it does not kill, often 
 produces blindness ; he rendered a pigeon blind by 
 a shock of electricity intended to kill it. Light- 
 
 * Letters, p. 218. 
 
44 INTRODUCTION. 
 
 ning destroys animal life ; he killed a hen and a 
 turkey by electrical shocks. 
 
 (54.) Having ascertained by experiment the pro- 
 perty of points in attracting and discharging elec- 
 tricity, Franklin, acknowledging his inability to give 
 
 satisfactory theory of this effect, set himself to 
 inquire how " this power of points might possibly 
 be of some use to mankind/' To discover this, he 
 suspended a large conductor, by silk lines, from the 
 ceiling, and charged it with electricity, so as to 
 enable it to give a spark at the distance of two 
 inches, " strong enough to make one's knuckle 
 ache." Under these circumstances, he found that 
 if a person presented the point of a needle to the 
 conductor at more than a foot distance, no elec- 
 tricity could be retained upon it, all passing off by 
 the needle as fast as it was supplied. He also 
 found, that if, after it was strongly electrified, the 
 -needle was presented at the same distance, the 
 conductor would instantly lose its electricity. That 
 the electricity, in this case, really passed off by the 
 point, he ascertained by observing that, in the 
 dark, the light was visible on the point of the 
 needle ; and also because, when the person pre- 
 senting the needle was himself insulated, or stuck 
 the needle in a bundle of sealing wax, the electricity 
 no longer escaped. 
 
 (55.) The next experiment is so remarkable in 
 itself, and so characteristic of the mind of Franklin, 
 that we shall give it in his own words : 
 
 " Take a pair of large brass scales, of two or 
 more feet beam, the cords of the scales being silk. 
 Suspend the beam by a packthread from the ceil- 
 
INVENTION OP PARATONNERRES. 45 
 
 ing, so that the bottom of the scales may be about 
 a foot from the floor ; the scales will move round 
 in a circle by the untwisting of the packthread. 
 Let an iron punch (a silversmith's iron punch, an 
 inch thick, is what I use) be put on the end upon 
 the floor, in such a place as that the scales may 
 pass over it in making their circle ; then electrify 
 one scale by applying the wire of a charged phial 
 to it. As they move round, you see that scale draw 
 nigher to the floor, and dip more when it comes 
 over the punch; and, if that be placed at a proper 
 distance, the scale will snap, and discharge its fire 
 into it. But if a needle be stuck on the end of 
 the punch, its point upwards, the scale, instead of 
 drawing nigh to the punch and snapping, dis- 
 charges its fire silently through the point, and rises 
 higher from the punch. Nay, even if the needle be 
 placed upon the floor near the punch, its point 
 upwards, the end of the punch, though so much 
 higher than the needle, will not attract the scale 
 and receive its fire ; for the needle will get it, and 
 convey it away, before it comes nigh enough for 
 the punch to act. 
 
 " Now, if electricity and lightning be the same, 
 the conductor and scales may represent electrified 
 clouds. If a tube (conductor) of only ten feet 
 long will strike and discharge its fire on the punch 
 at two or three inches distance, an electrified cloud 
 of perhaps ten thousand acres may strike and 
 discharge on the earth at a proportionally greater 
 distance. The horizontal motion of the scales over 
 the floor may represent the motion of the clouds 
 over the earth, and the erect iron punch a hill or 
 
46 INTRODUCTION. 
 
 high building ; and then we see how electrified 
 clouds, passing over hills or high buildings at too 
 great a height to strike, may be attracted lower till 
 within their striking distance. And, lastly, if a 
 needle fixed on the punch with its point upright, or 
 even on the floor below the punch, will draw the 
 fire from the scale silently at a much greater than 
 the striking distance, and so prevent its descending 
 towards the punch ; or if in its course it would 
 have come nigh enough to strike, yet, beipg first 
 deprived of its fire, it cannot, and the punch is 
 thereby secured from the stroke ; / say, if these 
 things are so, may not the knowledge of this power 
 of points be of use to mankind in preserving houses, 
 churches, ships, fyc. from the stroke of lightning, by 
 directing us to fix, on the highest parts of those 
 edifices, upright rods of iron, made sharp as a 
 needle, and gilt, to prevent rusting ; and, from the 
 foot of those rods, a wire down the outside of the 
 building into the ground, or down round one of the 
 shrouds of a ship, and down her side till it reaches 
 the water 9 Would not these pointed rods probably 
 draw the electrical fire silently out of a cloud before 
 it came nigh enough to strike, and thereby secure us 
 from that most sudden and terrible mischief? 
 
 (56.) " To determine this question, whether the 
 clouds that contain lightning be electrified or not, I 
 would propose an experiment to be tried, where it 
 may be done conveniently. On the top of some high 
 tower or steeple, place a kind of sentry-box, big 
 enough to contain a man and an electrical stand. 
 From the middle of the stand let an iron rod rise, 
 and pass, bending, out of the door, and then upright 
 
LIGHTNING AND ELECTRICITY IDENTIFIED. 4fJ 
 
 twenty or thirty feet, pointed very sharp at the 
 end. If the electrical stand be kept clear and dry, 
 a man standing on it, when such clouds are passing 
 low, might be electrified, and afford sparks, the rod 
 drawing fire to him from a cloud. If any danger to 
 the man be apprehended, let him stand on the floor 
 of his box, and now and then bring near to the rod 
 the loop of a wire that has one end fastened to the 
 leads, he holding it by a wax handle ; so the sparks, 
 if the rod is electrified, will strike from the rod to 
 the wire, and not affect him." * 
 
 When this and other papers by Franklin, illus- 
 trating similar views, were sent to London, and 
 read before the Royal Society, they are said to 
 have been considered so wild and absurd that they 
 were received with laughter, and were not con- 
 sidered worthy of so much notice as to be admitted 
 to a place in the " Philosophical Transactions."-)- 
 They were, however, shown to Dr. Fothergill, who 
 considered them of too much value to be thus 
 stifled ; and he wrote a preface to them, and pub- 
 lished them in London. They subsequently went 
 through five editions. 
 
 After the publication of these remarkable letters, 
 and when public opinion in all parts of Europe had 
 been expressed upon them, an abridgment or ab- 
 stract of them was read to the Society on the 6th 
 of June, 1751. It is a remarkable circumstance 
 that, in this notice, no mention whatever occurs of 
 Franklin's project of drawing lightning from the 
 clouds. Possibly this was the part which had 
 
 Letters, p. 235. 
 
 t Franklin's Works (Memoirs), vol. i. p. 299. 
 
48 INTRODUCTION. 
 
 before excited laughter, and was omitted to avoid 
 ridicule. 
 
 (57.) Franklin was under an impression that a 
 pointed rod could not be expected to attract the 
 lightning, unless it were placed at a very great height 
 in the atmosphere; and to render the result of his 
 projected experiment more certain, he determined to 
 wait for the completion of a spire then being 
 erected in Philadelphia. Meanwhile, however, a 
 different and more promising expedient occurred to 
 him ; which was, to send up the pointed wire upon 
 a kite, by the string of which the lightning might 
 be brought within his reach. He soon succeeded 
 in realizing this, the most bold arid grand concep- 
 tion which ever presented itself to the imagination 
 of an experimental philosopher. 
 
 He prepared his kite by making a small cross of 
 two light strips of cedar, the arms of sufficient 
 length to extend to the four corners of a large silk 
 handkerchief stretched upon them. To the extre- 
 mities of the arms of the cross he tied the corners 
 of the handkerchief. This being properly supplied 
 with a tail, loop, and string, could be raised in the 
 air like a common paper kite, and, being made of 
 silk, was more capable of bearing rain and wind. 
 To the upright arm of the cross was attached an 
 iron point, the lower end of which was in contact 
 with the string by which the kite was raised, which 
 was a hempen cord. At the lower extremity of 
 this cord, near the observer, a key was fastened ; 
 and, in order to intercept the electricity in its de- 
 scent, and prevent it from reaching the person who 
 held the kite, a silk ribbon was tied to the ring of 
 
LIGHTNING DRAWN FROM THE CLOUDS. 49 
 
 the key, and continued to the hand by which the 
 kite was held. 
 
 Furnished with this apparatus, on the approach 
 of a storm, he went out upon the commons near 
 Philadelphia, accompanied by his son, to whom 
 alone he communicated his intentions, well knowing 
 the ridicule which would have attended the report 
 of such an attempt, should it prove to be unsuc- 
 cessful. Having raised the kite, he placed himself 
 under a shed, that the ribbon by which it was held 
 might be kept dry,, as it would become a conductor 
 of electricity when wetted by rain, and so fail to 
 afford that protection for which it was provided. 
 A cloud, apparently charged with thunder, soon 
 passed directly over the kite. He observed the 
 hempen cord ; but no bristling of its fibres was ap- 
 parent, such as was wont to take place when it was 
 electrified. He presented his knuckle to the key, 
 but not the smallest spark was perceptible. The 
 agony of his expectation and suspense can be ade- 
 quately felt by those only who have entered into 
 the spirit of such experimental researches. After 
 the lapse of some time, he saw that the fibres of 
 the cord near the key bristled, and stood on end. 
 He presented his knuckle to the key, and received 
 a strong bright spark. It was lightning. The dis- 
 covery was complete, and Franklin felt that he was 
 immortal. 
 
 A shower now fell, and wetting the cord of the 
 kite, improved its conducting power. Sparks in 
 rapid succession were drawn from the key, a Leyden 
 jar was charged by it, and a shock given ; and, in 
 fine, all the experiments which were wont to be 
 
50 INTRODUCTION. 
 
 made by electricity were reproduced identical in 
 all their concomitant circumstances. 
 
 (58.) This experiment was performed in the month 
 of June, 1752. It will be remembered that Franklin's 
 letters to Mr. Collinson had been previously pub- 
 lished, translated, and widely circulated in different 
 languages throughout Europe ; and in these letters, 
 not only the object of the experiment and the prin- 
 ciple it was designed to establish were fully ex- 
 plained, but minute and circumstantial directions 
 were given as to the manner of executing it. Per- 
 sons engaged in physical inquiries in different parts 
 of Europe were invited, and prepared to submit it 
 to a trial when convenient opportunities offered. 
 Among these was a French electrician, M. Dalibard, 
 who, in the spring of 1752, prepared means of 
 making the experiment at Marly-la-Ville, a place 
 situate about six leagues from Paris. He succeeded 
 on the 10th of May, about a month before the ex- 
 periment of Franklin, and made a report of his 
 proceedings to the Academy of Sciences at Paris 
 on the 13th, in which he states that the experiment 
 had been made at the suggestion and according to 
 the method laid down by Franklin.* The experi- 
 ment of Franklin, in June, was made before he 
 could have been informed of that of Dalibard. 
 The same experiment was repeated on the 18th of 
 May by M. de Lor, at his house in the Estrapade, 
 at Paris ; and an account of it, as well as that 
 of M. Dalibard, was communicated to the Royal 
 
 * " En suivant la route que M. Franklin nous a tracee, 
 j'ai obtenu une satisfaction complete." Memoir de M. 
 Dalibard, quoted in Franklin's Works, vol. v. p. 288. 
 
FRANKLIN'S CLAIM DISPUTED. 51 
 
 Society of London by the Abbe Mazeas, in a letter 
 dated 20th May, two days after the latter experi- 
 ment, in which the Abbe ascribes all the credit of 
 the experiment to Franklin.* 
 
 (59.) The right of Franklin to the credit of having 
 established the identity of lightning and electricity 
 has been denied, and the honour claimed for the 
 French philosophers Nollet and Dalibard. This 
 claim was advanced, not when Europe from east to 
 west, and from north to south, was filled with 
 amazement and admiration at the philosophic bold- 
 ness of the " Philadelphian experiment " (as it was 
 universally called), or the profound sagacity with 
 which it was conceived, with which its minute de- 
 tails were prescribed, and its results foretold not 
 when its illustrious author was elected by acclama- 
 tion a member of the learned societies of Europe, 
 and received the academical degree from the most 
 ancient and honoured of universities but after 
 the lapse of nearly a century, after the story of 
 Franklin's kite had passed from the transactions of 
 philosophical societies, and the memoirs of insti- 
 tutes of sciences, into the primers of children. In 
 short, it was so recently as the year 1831, that, in 
 his admirable Eloge of Volta, M. ARAGO, taking a 
 retrospect of electrical discovery, maintained that 
 after the conjecture of Nollet, on the identity of 
 lightning and electricity, an experiment to ascer- 
 tain the fact was almost useless. And the reasons 
 he assigned for such inutility were, that the ex- 
 periment had been first made when flame appeared 
 
 * See Phil. Trans, vol. xvii. 1752. 
 E 2 
 
52 INTRODUCTION. 
 
 on the spears of soldiers, and the masts of ships * ; 
 but that, if any credit be claimed for the actual 
 exhibition of the fact by immediate experiment, 
 that credit is due to M. Dalibard. 
 
 (6jO.) If such a statement, supported by such a rea- 
 son, had proceeded from a quarter less entitled to re- 
 spect than the "perpetual secretary of the Academy 
 of Sciences," the astronomer royal of France, the 
 man who stands, if not first, incontestably in the 
 first rank of living meteorologists in a word, than 
 M. ARAGO no one would think it entitled to a 
 serious answer. It would be classed among those 
 strange obliquities of historic vision which have led 
 some persons to see in Richard and Macbeth, not 
 tyrants and murderers, but mild and virtuous 
 
 * " Les premieres vues de Franklin sur 1'analogie de 1'elec- 
 tricite et du tonnerre n'^taient, comme les idees anterieures de 
 Nollet que de simples conjectures. Toute la difference, entre 
 les deux physiciens, se reduisait alors a un projet d'experience, 
 
 dont Nollet n'avait pas parler Sans porter atteint 
 
 a la gloire de Franklin, je dois remarquer que 1'experience 
 proposee etait presque inutile. Les soldats de la cinquieme 
 legion Romaine 1'avaient dejafaite pendant la guerre d' A fri que, 
 le jour oii, comme Cesar le rapporte, le fer de tous' les javelots 
 parut en feu a la suite d'un orage. II en est de meme des 
 nombreux navigateurs a qui Castor et Pollux s'etaient montres, 
 soit aux pointes metalliques des mats ou des vergues, soit sur 
 
 d'autres parties saillantes de leurs navires Au reste, 
 
 soit que plusieurs de ces circonstances fussent ignorees, soit 
 qu'on ne les trouvat pas demonstratives, des essais directs 
 semblerent necessaires, et c'est a Dalibard, notre compatriote, 
 que la science en a etc redevable. Le 10 Mai, 1752, pendant 
 un orage, la grande tige de metal pointue qu'il avait etablie 
 dans un jardin de Marly-la- Ville donnait de petites etincelles, 
 comme le fait le conducteur de la machine electrique ordi- 
 naire, quand on en approche un fil de fer. Franklin ne 
 realisa cette meme experience aux Etats-Unis, a 1'aide d'un 
 cerf volant, qu'un mois plus tard." Eloge de Volta, p. 1 2. 
 
FRANKLIN S CLAIM VINDICATED. 53 
 
 princes, cruelly wronged by the calumnies of tra- 
 dition. 
 
 Nollet conjectured the probable identity of light- 
 ning and electricity, but gave not the most distant 
 hint of any possible method by which the proba- 
 bility could be experimentally tested. Franklin 
 boldly maintained the identity of these agents, gave* 
 numerous and cogent reasons to support that posi 
 tion, and moreover prescribed with minute details 
 two distinct methods by which lightning could be 
 brought into the hands of the observer, and sub- 
 mitted to the same experimental examination as 
 electricity had undergone. One of these two me- 
 thods was, in scrupulous accordance with his direc- 
 tions, applied in France ; and the other, within a 
 few weeks, was adopted by himself in America. 
 The results of both were precisely what Franklin 
 had foretold. Both were completely successful. 
 
 But, rejoins M. Arago, the whole affair of the 
 experiment was useless, for it had already been 
 effected. The flame on the javelins of the Roman 
 sentinels of the fifth legion was sufficient as an ex- 
 periment, not to mention Castor and Pollux, so 
 often seen by sailors on their mast-tops ! What 
 would so severe a reasoner as M. Arago say to 
 another who should maintain, without further ex- 
 periment, that either of these luminous appear- 
 ances was identical with lightning? and if that 
 were conceded, where would have been found the 
 proof that these meteors, and the lightning with 
 which they would be granted to be identified, were 
 due to the same physical agent as that manifested 
 by the friction of glass and resin ? 
 E 3 
 
54< INTRODUCTION. 
 
 If however, says M. Arago again, the experiment 
 were necessary or useful, science owes it to M. Da- 
 libard, who executed it at Marly-la- Ville a month 
 before Franklin, with his kite, made it at Phila- 
 delphia. This statement is not attended with the 
 circumstantial accuracy which M. Arago is accus- 
 tomed to observe. The fact, as stated by M. Da- 
 libard himself, was, that he took Franklin's printed 
 directions as to the manner of performing his 
 (Franklin's) projected experiment, and followed 
 them to the letter in preparing his apparatus at 
 Marly-la- Ville. Having accomplished this, he put 
 the directions for making the observation into the 
 hands of one COIFFIER, an old retired soldier, who 
 followed the trade of a carpenter, and who pro- 
 bably also erected the apparatus itself, and desired 
 Coiffier to make the experiment in the manner pre- 
 scribed by Franklin, if a storm should occur at a 
 time when he (Dalibard) was absent. The first storm 
 did occur when Dalibard was at Paris. Coiffier 
 presented a piece of metal to the rod, and received 
 several sparks. He then ran for the cur, who, 
 with him, repeated the experiment, and imme- 
 diately wrote a full description of it, with which he 
 despatched Coiffier himself to Paris to M. Da- 
 libard. 
 
 Thus it appears that so far from science being 
 indebted to M. Dalibard for the earliest exhibition 
 of this capital experiment, that philosopher had no 
 other share in it, save that of having caused the 
 erection of the conducting rod and other apparatus 
 according to Franklin's directions. In the actual 
 
FRANKLIN'S CLAIM VINDICATED. 55 
 
 performance of the first experiment, he had no 
 share whatever. 
 
 Let us now see how the account of credit stands 
 on the score of this memorable discovery : 
 
 In 1708, Dr. Wall mentions a resemblance of elec- 
 tricity to thunder and lightning. 
 
 In J 735, Mr. Grey conjectures their identity, and 
 that they differ only in degree. 
 
 In 1748, the Abbe Nollet reproduces the con- 
 jecture of Grey, attended with more circum- 
 stantial reasons. 
 
 In 1749, Franklin strongly maintains their iden- 
 tity, and accurately describes two ways of ex- 
 perimentally testing it, and sends his instruc- 
 tions to Europe, to enable others with better 
 local opportunities than he possessed to try it. 
 
 In 1 752, MM. Dalibard and Delor, in France-. 
 make the preparations prescribed according to 
 one of Franklin's methods; and Franklin makes 
 in Philadelphia preparations according to the 
 other method. 
 
 On 10th May, 1752, Coiffier and the curate 
 make the experiment as directed by Franklin, 
 and obtain the results foretold by Franklin. 
 
 In June, 1752, Franklin makes the same ex- 
 periment in Philadelphia, according to the 
 other method, with like results. 
 
 If the credit of the discovery is due to him who 
 first conjectured the identity of lightning and electri- 
 city, then it is due to Mr. STEPHEN GREY. 
 
 If it be due to him who showed the method of 
 making the capital experiment by which the iden- 
 
56 INTRODUCTION. 
 
 tity must be either established pr refuted, it belongs 
 to FRANKLIN. 
 
 If it be due to the persons at whose expense 
 Franklin's apparatus was first constructed, it must 
 be shared between FRANKLIN, DALIBARD, and 
 DELOR. 
 
 If it be due to him who first, in person, per- 
 formed the experiment proposed by Franklin, it 
 must be accorded to the carpenter and dragoon 
 COIFFIER. 
 
 We shall now dismiss this matter, to which more 
 space has been allotted than it is entitled to, merely 
 observing, that much as living philosophers must 
 be surprised at the claim advanced in favour of 
 M. Dalibard, that electrician himself, could he rise 
 from his tomb, would see with infinitely more asto- 
 nishment an- honour sought for him to which he 
 never himself aspired, or supposed he had the 
 slightest title. 
 
 (61.) Franklin having established, beyond the 
 possibility of dispute, the identity of lightning and 
 electricity, proceeded, in accordance with that cha- 
 racteristic attribute of his mind already noticed, to 
 turn this discovery to the benefit of mankind, and 
 proposed the general adoption of those pointed 
 metallic rods now so commonly erected at the sum- 
 mits of buildings to protect them from the effects 
 of lightning. The principle of this apparatus, as 
 now constructed for edifices and ships, differs in 
 nothing essential from that proposed by its cele- 
 brated inventor. 
 
 (62.) This part of the labours of Franklin in elec- 
 tricity cannot be dismissed without a passing notice 
 
FRANKLIN AND GEORGE III. 57 
 
 of the dispute which was maintained in England re- 
 specting the comparative advantages of conductors 
 with pointed ends as proposed by Franklin, or with 
 round or blunted ends as suggested by some others. 
 It were for the honour of science that this discredit- 
 able controversy had never taken place. It forms 
 a rare, if not a solitary example, of the prostitution 
 of philosophy to gratify the meanest passions of an 
 obstinate and imbecile prince. The persevering 
 tenacity with which the British monarch fastened 
 his last grasp on his American subjects about to 
 wrest themselves from his power, and assert their 
 independence, is well known. By his pursuit of 
 that object, after all reasonable hope of securing it 
 had expired, the treasures of his kingdom were 
 lavished, and the blood of his people flowed in 
 mutual slaughter. Bad as were these consequences, 
 they were nevertheless the ordinary consequences 
 of war. But the vindictive spirit of the court passed 
 from the field and council-board to the peaceful 
 halls of science ; and because Franklin, the agent, 
 representative, and counsellor of the American 
 people, had proposed the use of pointed conductors, 
 a party of parasites was found, who, to gratify 
 George III., advocated blunt conductors; and, to 
 crown this most egregious absurdity, blunt con- 
 ductors were actually erected upon the royal 
 palace ! * 
 
 * " The king's changing his pointed conductors for Hunt ones 
 is a matter of small importance to me. If I had a wish about 
 them, it would be, that he would reject them altogether as in- 
 effectual. For it is only since he thought himself and his 
 family safe from the thunder of heaven that he has dared to 
 use his own thunder in destroying his innocent subjects." 
 Franklin's Works, viii. 227. 
 
58 INTRODUCTION. 
 
 (63.) Franklin next directed his inquiries to the 
 quantity and nature of the electricity with which 
 the clouds in various states of the atmosphere were 
 charged. To facilitate his experimental inquiries 
 on this subject, he erected in his house in Phila- 
 delphia a pointed iron rod, which he was enabled to 
 insulate at pleasure. This rod was put in commu- 
 nication with a system of bells, which alternately 
 attracted and repelled their hammers when elec- 
 trified. Whenever a cloud charged with electricity 
 passed over the house within such a distance as to 
 affect the conductor, these bells would ring and in- 
 form him of the opportunity of prosecuting his 
 experiments. 
 
 (64.) Having satisfied himself that the clouds were 
 frequently in an electrified state when there was 
 no thunder or lightning, his next inquiry was, 
 whether they were electrified positively or nega- 
 tively. This was a question of more interest to 
 him, because, according to his theory, if their elec- 
 tricity were negative, the earth, " in thunder-strokes, 
 would strike into the clouds, and not the clouds into 
 the earth." To determine this, he " took two phials 
 and charged one of them with lightning from the iron 
 rod, and gave the other an equal charge (of elec- 
 tricity) from the prime conductor. When charged, 
 he placed them on a table within three or four 
 inches of each other, asmall cork ball being suspended 
 by a fine silk thread from the ceiling, so as to play 
 between the wires. If both bottles then were elec- 
 trified positively, the ball being attracted and then 
 repelled by the one must be repelled by the other. 
 If the one positively and the other negatively, then 
 
DEATH OF RICHMANN. 59 
 
 I would be attracted and repelled by each, 
 and continue to play between them, so long as any 
 considerable charge remained." * 
 
 From experiments with this apparatus, he con- 
 cluded that clouds were sometimes positively and 
 sometimes negatively electrified, but oftener nega- 
 tively. Electrical instruments had not yet, how- 
 ever, advanced to such a state of improvement as to 
 enable a mind, even acute as his, to make much 
 further discovery in atmospheric electricity ; and 
 although the details of his experiments and his theo- 
 retical speculations regarding them must always be 
 read with profound interest, yet no further prin- 
 ciples of importance appear to have been evolved 
 from them. 
 
 (65.) If it be true that the Royal Society laughed at 
 his speculations and refused to them a place in their 
 Transactions, they were not slow to retract and re- 
 pair their error. They conferred upon him their 
 highest honour (the Copley medal), and unanimously 
 elected him an honorary member of their society, 
 in 1753. 
 
 (66.) An experiment so remarkable as the attraction 
 of lightning from the clouds, could not fail to be veri- 
 fied and repeated by many enthusiastic lovers of 
 science. One of the first instances of this zeal was 
 rendered memorable by its fatal result. Professor 
 George William Richmann of St. Petersburg was 
 preparing an essay on electricity; and in order to 
 obtain the most certain and accurate knowledge of 
 the phenomena, he placed a conductor on his 
 house, making a metallic communication between 
 * Letters, p. 302. 
 
60 
 
 INTRODUCTION. 
 
 it and his study, where he provided means for re- 
 peating Franklin's experiments. On the 6th of 
 August, 1753, while Richmann attended a meeting 
 of the Petersburg Academy of Science, distant thun- 
 der was heard, on which he went to his house, ac- 
 companied by Sokolow the engraver, who being 
 engaged to illustrate his work, desired to see those 
 electrical appearances which he would have to re- 
 present in the plates. While Richmann was de- 
 scribing to Sokolow the nature of the apparatus, a 
 thunder-clap was heard louder and more violent 
 than any which had been remembered at St. Peters- 
 burg. Richmann stooped towards the electrometer 
 of the apparatus to observe the force of the elec- 
 tricity, and " as he stood in that posture, a great 
 white and bluish fire appeared between the rod of the 
 electrometer and his head. At the same time a sort 
 of steam or vapour arose which entirely benumbed 
 the engraver, and made him sink on the ground." 
 Several parts of the apparatus were broken in pieces 
 and scattered about. The doors of the room were 
 torn from their hinges, and the house shaken in 
 every part. The wife of the professor, alarmed by 
 the shock, ran to the room, and found her husband 
 sitting on a chest, which happened to be behind 
 him when he was struck, and leaning against the 
 wall. He appeared to have been instantly struck 
 dead.* 
 
 (67) During 1752 and the succeeding years the 
 subject of atmospheric electricity engaged the atten- 
 tion of persons devoted to physical science in different 
 parts of Europe. The climate of England being less 
 
 * Phil. Trans, vol. xlix. p. 61. 
 
BECCAK1A. 61 
 
 favourable to such researches than more southern 
 latitudes, fewer opportunities of observation were of- 
 fered; nevertheless, Canton, Wilson, and Bevis soon 
 repeated and verified the Philadelphia experiments. 
 Canton showed that the clouds were electrified, 
 sometimes negatively and sometimes positively, and 
 carried such observations further than Franklin. 
 
 (68.) But the most acute and indefatigable follower 
 of Franklin at this time, in atmospheric electricity, 
 was Beccaria, who, in 1753, published a treatise on 
 electricity at Turin, and a series of letters on the 
 same subject at Bologna in 1758. He erected nu- 
 merous conducting rods in different places of ob- 
 servation, and elevated kites according to Franklin's 
 method. By raising these to various heights, he 
 observed the electricity of different atmospheric 
 strata, and he improved this mode of observation by 
 interlacing the strings with metallic wire. To keep 
 his kites constantly insulated, and, at the same time, 
 to give them more or less string, he rolled the 
 string upon a reel, which was supported by pillars 
 of glass, and his conductors were placed in metallic 
 communication with this reel. 
 
 (69.) This profound philosopher, and acute and ac- 
 curate observer, has left in the history of electricity 
 traces of his genius second only to those with which 
 Franklin and Volta impressed it. Beccaria was the 
 first who diligently studied and recorded the circum- 
 stances attending the phenomena of a thunder-storm. 
 He observes that the first appearance of a thunder- 
 storm (which generally happens when there is little 
 or no wind) is one dense cloud or more, increasing 
 rapidly in magnitude, and ascending into the higher 
 
62 INTRODUCTION. 
 
 regions of the atmosphere. The lower edge is black 
 and nearly horizontal, but the upper is finely arched 
 and well defined. Many of these clouds often seem 
 piled one upon the other, all arched in the same 
 manner; but they keep constantly uniting, swelling, 
 and extending their arches. When such clouds 
 rise, the firmament is usually sprinkled over with 
 a great number of separate clouds of odd and bizarre 
 forms, which keep quite motionless. When the 
 thunder-cloud ascends, these are drawn towards it ; 
 and as they approach they become more uniform 
 and regular in their shapes, till, coming close to the 
 thunder-cloud, their limbs stretch mutually towards 
 one another, finally coalesce, and form one uniform 
 mass. But sometimes the thunder-cloud will swell 
 and increase without the addition of these smaller 
 adscititious clouds. Some of the latter appear like 
 white fringes at the skirts of the thunder-cloud or 
 under the body of it, but they continually grow 
 darker and darker as they approach it. 
 
 When the thunder-cloud, thus augmented, has 
 attained a great magnitude, its lower surface is 
 often ragged, particular parts being detached to- 
 wards the earth, but still connected with the rest. 
 Sometimes the lower surface swells into large pro- 
 tuberances, tending uniformly towards the earth ; 
 and sometimes one whole side of the cloud will 
 have an inclination to the earth, which the ex- 
 tremity of it will nearly touch. When the observer 
 is under the thunder-cloud after it has grown large 
 and is well formed, it is seen to sink lower and to 
 darken prodigiously, and, at the same time, a great 
 number of small clouds are observed in rapid mo- 
 
PRECURSORS OF A STORM. 63 
 
 tion, driven about in irregular directions below it. 
 While these clouds are agitated with the most rapid 
 motions, the rain generally falls in abundance ; and 
 if the agitation be very great, it hails. 
 
 While the thunder-cloud is swelling and ex- 
 tending itself over a large tract of country, the 
 lightning is seen to dart from one part of it to 
 another, and often to illuminate its whole mass. 
 When the cloud has acquired a sufficient extent, 
 the lightning strikes between the cloud and the 
 earth in two opposite places, the path of the light- 
 ning lying through the whole body of the cloud and 
 its branches. The longer this lightning continues, 
 the rarer does the cloud grow, and the less dark in 
 its appearance, till it breaks in different places and 
 shows a clear sky. When the thunder is thus dis- 
 persed, those parts which occupy the upper regions 
 of the atmosphere are spread thinly and equally, 
 and those that are beneath are black and thin also, 
 but they vanish gradually without being driven away 
 by the wind. 
 
 The instruments for electrical observation used 
 by Beccaria never failed to give indications corre- 
 sponding to the successive changes in progress in the 
 atmosphere above his observatory. The stream of 
 fire from his conductor was generally uninterrupted 
 while the thunder-cloud was directly above it. The 
 same cloud in its passage electrified his conductor 
 alternately with positive and negative electricity. 
 The electricity of the conductor continued to be of 
 the same kind so long as the thunder-cloud was 
 simple and uniform in its direction ; but when the 
 lightning changed its place, a change in the species 
 
64 INTRODUCTION. 
 
 of electricity ensued. A sudden change of this 
 kind would also happen after a violent flash of light- 
 ning ; but the change would be gradual when the 
 lightning was moderate, and the progress of the 
 thunder-cloud slow.* 
 
 (70.) But among the labours of this philosopher, 
 that rendered by modern discoveries most memorable 
 was one which by his contemporaries and their im- 
 mediate successors was regarded as an ingenious 
 and over-refined conjecture, rather than what it 
 afterwards proved to be, the distant shadow of a 
 coming discovery detected by the far-sighted mind 
 of this acute and extraordinary man. Franklin had 
 been the first to magnetise fine sewing needles by 
 the electric spark. Dalibard observed that the ex- 
 tremity of the needle at which the spark from the 
 excited glass entered had northern polarity, and 
 both Franklin and Dalibard discovered that a spark 
 of equal force given to the other end of the needle 
 deprived it of the magnetic virtue. From these 
 and from similar experiments made by himself, 
 Beccaria inferred that the polarity of the magnetic 
 needle was determined by the direction in which the 
 electric current had passed through it. He as- 
 sumed the magnetic polarity acquired by ferrugi- 
 nous bodies which had been struck by lightning, 
 as a test of the direction of the electric current in 
 passing through them, and thence inferred the 
 species of electricity with which the thunder-cloud 
 had been charged.! 
 
 * Beccaria, Lettere dell' Elettricismo. Bologna, 1758, 
 p. 146. et seq. 
 
 f " I poli del mattone teste descritto, provano chc anche in 
 
BECCABIA'S BESEABCHES. 65 
 
 (71.) Extending this analogy to the earth itself, 
 Beccaria conjectured that terrestrial magnetism was, 
 like that of the needle magnetised by Franklin and 
 Dalibard, the mere effect of permanent currents of 
 natural electricity, established and maintained upon 
 its surface by various physical causes ; that, as a 
 violent current, like that which attends the exhibi- 
 tion of lightning, produces instantaneous and power- 
 ful magnetism in substances capable of receiving 
 that quality, so may a more gentle, regular, and 
 constant circulation of the electric fluid upon the 
 earth impress the same virtue on all such bodies as 
 are capable of it. Observation proves that a vast 
 quantity of this fluid circulates between different 
 parts uf the atmosphere in storms ; that a quantity 
 not inconsiderable circulates in the time of ordinary 
 rain ; and that even when the weather is serene and 
 the heavens unclouded, some quantity is still observ- 
 able. " Of such fluid, thus ever present," observes 
 Beccaria, " I think that some portion is constantly 
 passing through all bodies situate on the earth, 
 especially those which are metallic and ferruginous ; 
 and I imagine it must be those currents which im- 
 press on fire-irons, and other similar things, the 
 power which they are known to acquire of directing 
 themselves according to the magnetic meridian 
 when they are properly balanced." * 
 
 certi corpi che abbiano certa porzione di ferro, il fulmine tm- 
 prime un segno permanente delta sua direzione." Beccaria, 
 Lettere, p. 261. 
 
 * " Di tale ftioco, io penso che alcuna parte perpetuamente 
 discorra per tutti i corpi situati sopra la terra, massimamente 
 per i metallic! e ferigni. Penso che esso sia, il quale attra- 
 
 VOL I. F 
 
INTRODUCTION. 
 
 (72.) He observed, that to say we are insensible to 
 this current around us,, is no good argument against 
 its existence ; for that its uniformity, constancy, and 
 universality would necessarily render it impercep- 
 tible, since all bodies must partake of it in common, 
 His hypothesis to account for the variation and 
 dip is not the least remarkable part of this extra- 
 ordinary anticipation. He considers that the electro- 
 magnetic currents have not all a common centre, 
 but may have several situate in our northern he- 
 misphere. The aberration of their common centre 
 from the true terrestrial pole may probably be the 
 cause of the variation of the compass. The periodi- 
 cal change to which the position of this common 
 centre is subject would correspond with and cause 
 the periodical change of that variation, and the 
 obliquity of these currents may be the cause of 
 the dip.* 
 
 versando le padelle, le molle, le palette ed altri si fatti bis- 
 lunghi ferri, i quali d'ordinario pendono o posano verticalmente, 
 imprima loro la virtu di situarsi nella meridiana magnetica, 
 allora che sono convenientemente bilicati." Letter^ p. 266. 
 
 * " Questa sistematica elettrico-magnetica circolazione, se- 
 condo me, non procederebbe da un solo punto settentrionale, 
 ma avrebbe infinite sorgenti in diversi punti del nostro set- 
 tentrionale emisfero, forse successivamente, piu folte ne luoghi 
 piu vicini ad alcun punto settentrionale ; e la frequenza, la 
 posizione, o piuttosto la direzione del corso loro mi si rap- 
 presenterebbono dalla posizione, frequenza, e diverzione, con 
 che si dispongono intorno alii emisferi di una sferica calamita 
 le ordinatissime filze della limitura di ferro. E giusta una 
 tale ipotesi, 1' aberrazione del centre comune di tutte le varit 
 sorgenti, che estenderebbono la loro azionead una data ragione, 
 dal vero punto settentrionale mi spiegherebbe 1' aberrazione 
 della calamita ; il periodo di quella aberrazione mi spieghe- 
 rebbe il periodo di qutsta declinazione j 1' obbliquita, con che 
 
BECCARIA/S RESEARCHES. 6? 
 
 That the anticipation of the fundamental prin- 
 ciple of electro-magnetism, arid terrestrial magnetism, 
 should have been complete in all its details, could 
 scarcely have happened at that epoch without 
 something approaching to inspiration ; but it will 
 be readily admitted that these guesses of BECCARIA, 
 when compared with the discovery of OERSTED 
 and the theory of AMPERE, form one of the most 
 striking episodes in the history of science. 
 
 (73.) The analogy between lightning and the elec- 
 tric spark, arising from the peculiar noise or explosion 
 with which each was attended, had been noticed 
 by many electricians. Beccaria, however, inves- 
 tigated and demonstrated its cause, by showing 
 that it proceeded from a pulsation produced in the 
 air by the sudden displacement of that portion of 
 it through which the electric fluid passes. This 
 displacement being transmitted through the atmo- 
 sphere in exactly the same manner as vibrations are 
 produced by a sonorous body, the sound accom- 
 panying an electric discharge, and the thunder 
 which attends the atmospheric electricity, ensue. 
 Beccaria verified this hypothesis by experiment. 
 He constructed a glass siphon, in one leg of which 
 air was inclosed above a column of mercury, and 
 compressed by the column in the other leg of the 
 siphon. On discharging a Leyden jar through the 
 air thus inclosed, the column of mercury in the 
 other leg was suddenly elevated, and recovered its 
 
 quelle sorgenti spiccierebbono da terra, e si direggerebbono 
 verso mezzo di, mi spiegherebbe e la inclinazione degli agbi, 
 e la particolare facilita con che si calamitano i ferri si fattu- 
 uiente inclinati.'* Lettere, p. 268. 
 
INTRODUCTION. 
 
 position after several oscillations.* This fact was 
 also noticed by Kinnersley, the friend and associate 
 of Franklin, but not until a later period. 
 
 (74.) This was afterwards corroborated by Bouguer 
 and De la Condamine, when they encountered a vio- 
 lent thunder-storm on one of the highest mountains 
 of Peru. The cloud from which the thunder pro- 
 ceeded was placed at but a small distance above 
 their heads. The thunder heard by them consisted 
 only of single cracks, or explosions, like those which 
 attend the discharge of electric batteries ; an effect 
 manifestly produced by the proximity of the cause 
 of the sound, and the highly rarefied state of the 
 air at that great elevation. 
 
 (75.) Contemporaneously with Beccaria, Franklin, 
 and Canton, the subject of atmospheric electricity 
 engaged the attention of Lemonnier, who erected an 
 apparatus according to Franklin's method at St. 
 Germain-en-Laye, with which he showed that sparks 
 were received from the conductor not only in times 
 of storm, but also when the heavens were cloud- 
 less. He also first showed that the electricity of the 
 air underwent every twenty-four hours periodical 
 variations of intensity. 
 
 (76.) Beccaria determined the law of these varia- 
 tions, and was the first who demonstrated that at 
 all seasons, at all heights, and in every state of the 
 wind, the electricity of an unclouded atmosphere is 
 positive. He found no indications of electricity 
 in the air in high winds, when the firmament was 
 covered with black and scattered clouds, having a 
 slow motion in a humid state of the air ; but in the 
 * Beccaria, Elettricismo Artificiale. Turin, 1753, p. 227. 
 
CANTON'S EXPERIMENTS. 69 
 
 absence of actual rain, he found that in change- 
 able squally weather, attended with occasional 
 showers of snow, hail, or rain, the electricity was 
 very variable, both as to its quantity and quality 
 being sometimes feeble and sometimes intense^ 
 sometimes positive and sometimes negative. 
 
 (77.) Contemporaneously with Beccaria in Italy, 
 Canton prosecuted inquiries in many respects similar 
 in England, and in various matters of minor import- 
 ance these philosophers arrived at the same results. 
 The most considerable discovery due to Canton 
 was, that the electricity developed in the friction of 
 the same substance is not always of the same kind. 
 It will be remembered that Dufaye gave the names 
 vitreous and resinous to the two fluids, on the sup- 
 position that each was invariably produced by the 
 friction of the classes of bodies signified by these 
 terms. Canton, however, showed that glass itself 
 was capable of being electrified negatively, and 
 would be always so electrified, if the rubber used 
 were the fur of a cat. Canton also (as well as 
 Beccaria) proved that a volume of air in a quiescent 
 state might be charged with electricity. To Canton 
 is also due the discovery of the virtue of the amal- 
 gam of tin and mercury, still used with so much effect 
 to augment the development of electricity on glass. 
 
 The progress of the science had now attained 
 a point at which the great principle of induction 
 could scarcely fail to force itself upon the notice 
 of those engaged in electrical researches. A na- 
 tural law of the highest order, embracing within 
 the range of its application nearly the whole do- 
 main of electrical phenomena, its discovery and 
 F 3 
 
70 INTRODUCTION. 
 
 development, forms an epoch in the history of the 
 science, scarcely second in importance even to that 
 by which Franklin brought meteorology within the 
 legislation of electricity. How much, then, will 
 the veneration in which the memory of the philo- 
 sopher of the West is held be increased, if it can 
 be demonstrated, contrary to what has been gene- 
 rally maintained by the historians of the science, 
 that to him is justly owing the honour of the dis- 
 covery of this physical principle! 
 
 (78.) Some of the more obvious phenomena of 
 induction were noticed so early in the progress of 
 electrical science, as the researches of Mr. Grey; and 
 many other effects proceeding from it presented 
 themselves to subsequent experimental inquiries, 
 but attracted no attention, and led to no conse- 
 quences. The first series of experiments, conducted 
 so as to develop in an unequivocal manner this 
 principle, were laid before the Royal Society by 
 Canton, on the 6th of December, 1753.* They 
 consisted chiefly in rendering insulated conductors 
 electrical, by bringing near to one end an excited 
 glass tube, or stick of wax, and exhibiting the 
 varying state of cork-balls suspended on the con- 
 ductor by the alternate approach and removal of 
 the excited electric. 
 
 (79.) These experiments having been communi- 
 cated to Franklin, he pursued the inquiry, and suc- 
 ceeded in expressing, in clear and unequivocal terms, 
 the principle of induction ; that is to say, in demon- 
 strating that a body charged with either kind of elec- 
 
 * Phil. Trans, vol. xlviii, p. 350. 
 
FRANKLIN DISCOVERS INDUCTION. 71 
 
 tricity will, on approaching a conductor in its natural 
 state, render that part of such conductor which is 
 nearest to it electrical ; that its electricity will be 
 contrary to that of the approaching electrified body; 
 that on removing the electrified body the conduc- 
 tor would be restored to its natural state ; all which 
 effects Franklin showed would follow from his 
 theory, by assuming that the electric fluid is self- 
 repulsive, and attracted by the matter of the con- 
 ductor. 
 
 (80.) The experiments and reasoning which ap- 
 pear to establish Franklin's right to the honour of 
 this discovery are so concise, that they may be 
 stated here nearly in his own words. 
 
 Let a metallic conductor, about five feet long 
 and four inches in diameter, be suspended by dry 
 silk lines, so as to be insulated. From one end of 
 it suspend a tassel consisting of fifteen or twenty 
 threads in a damp state, so as to give them a con- 
 ducting power. Present an electrified glass tube 
 within five or six inches of the opposite end, and 
 keep it in that position for a few seconds. The 
 threads of the tassel will diverge, and when the 
 tube is withdrawn they will collapse. 
 
 While the tube is held near the opposite end of 
 the conductor and the threads are divergent, pre- 
 sent the finger to the end of the conductor at which 
 the tassel is suspended. A spark will be received, 
 and the threads of the tassel will collapse. 
 
 Let the tube be then removed. The threads of 
 the tassel will again diverge. 
 
 Let the tube be again presented as before. The 
 threads will again collapse, and so on. 
 F 4 
 
72 INTRODUCTION. 
 
 Finally, let the tube be presented to the tassel. 
 The divergence of the threads will immediately 
 increase, and continue to increase, as the tube is 
 brought nearer to the tassel. 
 
 These phenomena are accounted for by Franklin 
 in the following manner : " By taking the spark 
 from the end of the conductor, you rob it of part 
 of its natural quantity of electrical matter, which 
 part so taken away is not supplied by the glass 
 tube, and the conductor remains negatively elec- 
 trified. On withdrawing the tube, the electric 
 matter on the conductor recovers its equilibrium, or 
 equal diffusion ; and the conductor, having lost some 
 of its natural electricity, the threads connected 
 with it lose part of theirs, and so are electrified 
 negatively, and repel each other. 
 
 "When the tube is again presented to the opposite 
 end of the conductor, the part of the natural elec- 
 tricity which the threads had lost is again restored 
 to them by the repulsion of the tube forcing the 
 electric fluid towards them from other parts of the 
 conductor, and thus restoring them to their natural 
 state. When the tube is once more withdrawn, the 
 fluid is again equally diffused, and the threads, as 
 before, are negatively electrified. 
 
 "Finally, when the tube is presented to the threads 
 already diverging with negative electricity,, still 
 more of their natural electricity is repelled by the 
 excited tube, and the threads are more strongly 
 negative than before, and their divergence is con- 
 sequently augmented." 
 
 Pursuing the principle thus developed still fur- 
 ther, Franklin now having restored the conductor 
 
DISCOVERY OF INDUCTION. 73 
 
 to its natural state, presented the excited glass 
 tube to the tassel. The threads immediately di- 
 verged. 
 
 Maintaining the tube in that position with one 
 hand, he presented the finger of the other to the 
 tassel. The threads receded from the finger as if 
 repelled by it. 
 
 This was explained on the same principle. When 
 the excited tube is presented to the tassel, part of 
 the natural electricity of the threads is driven out 
 of them into the conductor, and they are negatively 
 electrified, and therefore repel each other. When 
 the finger is presented to the tassel (being then 
 close to the glass tube), part of its natural electri- 
 city is driven back through the hand and body, 
 and the finger becomes, as well as the threads, 
 negatively electrified, and so repels, and is repelled 
 by them. To confirm this, hold a slender light lock 
 of cotton, two or three inches long, near a conduc- 
 tor positively electrified. You will see the cotton 
 stretch itself out towards the conductor. Attempt 
 to touch it with the finger of the other hand, and 
 it will be repelled by the finger. Approach it with 
 a positively charged wire of a bottle, and it will fly 
 to the wire. Bring it near a negatively charged 
 wire of a bottle, it will recede from that wire in 
 the same manner that it did from the finger, which 
 demonstrates that the finger was negatively electri- 
 fied as well as the cotton.* 
 
 (81.) The great principle thus thrown before the 
 scientific world by Franklin, was immediately taken 
 
 * Letters, p. 341. Also see Phil. Trans, vol. xlix. p. 300. 
 
74 INTRODUCTION. 
 
 up and pursued through its consequences by WILKE 
 and ^EPINUS, who carried on their researches to- 
 gether at Berlin. The most important result of 
 their combined labours was the invention of the 
 instrument, which, as subsequently improved under 
 the hands of VOLTA, became the CONDENSER now 
 so useful in electroscopical investigations. 
 
 In applying the principle of induction to the 
 phenomena of the Leyden jar, and to the same 
 effect as exhibited by the oppositely electrified sur- 
 faces of a coated plate of glass, these philosophers 
 saw that the negative state of one surface of the 
 glass was, according to the Franklinian theory, the 
 necessary consequence of the positive state of the 
 other. This contrary state of the electricities could 
 only be maintained on the supposition that glass 
 was impermeable by the electric fluid ; and Wilke 
 and ^Epinus reasoned, that to whatever extent air 
 or any other body might be similarly impermeable, 
 to the same extent might it be charged on its oppo- 
 site surfaces. To realize this conception with a 
 plate of air, they coated two large boards of equal 
 size with tin-foil, and suspended them one over the 
 other, leaving a space of about an inch in thickness 
 between them. This space was, in fact, a plate of 
 air, of which the upper and lower surfaces were in 
 contact with the metallic coating of the boards. 
 The lower board communicated with the ground, 
 and a charge of positive electricity was given to the 
 upper one. The lower one then became charged 
 with negative electricity ; and when a person touched 
 at the same time the coating of the two boards, the 
 equilibrium was re-established, and he received the 
 
INVENTION OF THE CONDENSER. 75 
 
 shock produced by the passage of the electric fluid 
 from the one to the other. 
 
 Many curious experiments were exhibited with 
 this apparatus. They found that the two boards, 
 when electrified, strongly attracted each other, and 
 would have rushed together if they had not been 
 prevented by the strings. Sometimes, when the 
 charge was strong, the intervening plate of air was 
 not sufficiently impermeable to resist the mutual 
 attraction of the opposite electricities, and a spon- 
 taneous discharge would take place through it. 
 They considered these two plates to represent the 
 state of the clouds and the earth during a thunder- 
 storm ; the clouds being always charged with one 
 kind of electricity, and the earth with the other, 
 while the body of atmosphere between them was 
 analogous to the stratum of air between the two 
 boards. When the charges of the earth and clouds 
 become so strong that the air can no longer resist 
 the passage of the electric fluid through it, a spon- 
 taneous discharge ensues, the fluid is seen in its 
 passage by the light it evolves, and the violent dis- 
 placement of the air produced in its passage causes 
 the thunder. 
 
 From these experiments, ^pinus inferred that 
 the phenomena of the Leyden jar was not owing, as 
 Franklin supposed, to any peculiar attraction of the 
 glass for the electric fluid ; for, since a plate of air 
 might be charged as well as a plate of glass, that 
 property must be common to them, and was not 
 peculiar to the glass. He inferred, therefore, that 
 this impermeability was a property of all non- 
 conductors; and, since they can all receive elec- 
 
76 INTRODUCTION. 
 
 tricity to a certain degree, it must consist in the 
 difficulty and slowness with which the electric fluid 
 moves in their pores, whereas, in perfect conductors, 
 it meets with no obstruction at all.* 
 
 (82.) ^pinus brought to the investigation of the 
 Franklinian theory of electricity those mathemati- 
 cal attainments in which its illustrious founder was 
 deficient. The manner in which that theory had 
 been assailed by its opponents, and defended by its 
 partisans, was such as might have allowed inter- 
 minable controversy. ^Epinus first reduced its 
 principles to exact mathematical statement, with 
 a view to ascertain whether the consequences de- 
 ducible from them, by rigorous calculation, should 
 be in accordance with the observed phenomena, 
 not only in their general character, but in their 
 numerical quantity. He assumed, according to 
 Franklin's hypothesis, that the molecules of the 
 electric fluid were self- repulsive, and that they 
 were attracted by those of the bodies on which 
 they were diffused. He found, however, that the 
 phenomena could not be explained on these suppo- 
 sitions, unless it were also assumed that between 
 the matter composing the masses of different bodies 
 there existed a mutually repulsive force, acting at 
 sensible distances. At first he recoiled from an 
 assumption in direct opposition to the known pro- 
 perties of matter ; but the necessity for its admis- 
 sion, in order to give consistency and validity to 
 the Franklinian theory, appears at length to have 
 reconciled him to it, 
 
 * JEpini Tentamen, &c. Petersburg, 1759, p. 82, 83. 
 
RESEARCHES OF uEPlNUS. 77 
 
 (83.) The investigation of the physical relation 
 between the principle of heat and that of electricity, 
 had attracted the attention of experimental philoso- 
 phers at a very early period in the history of elec- 
 trical research. Beccaria suspected that heat might 
 itself be an immediate means for the development 
 of electricity, and made some experiments to illustrate 
 this. He soon, however, relinquished the inquiry, 
 concluding that, in cases where the appearance of 
 electricity followed the application of heat, the 
 effect was due to evaporation, or other physical 
 agents, which ensued. Priestley observed that heat 
 had some relation to the conducting power of 
 bodies, since, by the elevation of temperature, that 
 quality was improved. 
 
 (84.) A mineral substance, brought from the East 
 by the Dutch navigators, called by the natives of 
 Ceylon, where chiefly it was found, Tournamal, 
 and since known as Tourmaline, exhibited, under 
 certain circumstances, a property similar to that of 
 amber, and other electrics. But the power was 
 excited in it by mere elevation of temperature. 
 Lemery, the Due de Noia, Wilson, Priestley, and 
 others, made experiments on this mineral, and pub- 
 lished results, in which there were much discordance 
 and contradiction. .ZEpinus first showed that the 
 attraction and repulsion exerted by this gem when 
 exposed to heat was owing to the development of 
 electricity upon it ; and that, when so excited, its 
 opposite sides or ends had contrary kinds of elec- 
 tricity, one being always negative, and the other 
 positive. This was the first case of the distinct 
 exhibition of electrical polarity. Canton observed 
 
78 INTRODUCTION". 
 
 that the development of the electric fluid upon it 
 was produced only by change of temperature, and 
 that whenever the gem was broken each fragment 
 exhibited the same electrical polarity. 
 
 (85.) At this period effects were observed, which, 
 if chemical science had attained a sufficiently ad- 
 vanced state, could not fail to have led to the dis- 
 covery of electro-chemistry. Beccaria, by the 
 electric spark, decomposed the sulphuret of mercury, 
 and recovered the metals, in some instances, from 
 their oxides.* Watson found that an electric dis- 
 charge passing through fine wire rendered it incan- 
 descent, and that it was even fused and burned. 
 Canton, repeating these experiments with brass 
 wire, found that, after the fusion by electricity, 
 drops of copper only were found, the zinc having 
 apparently evaporated. Beccaria observed that 
 when the electric spark was transmitted through 
 water, bubbles of gas rose from the liquid, the 
 nature or origin of which he was unable to deter- 
 mine. Had he suspected that water was not what 
 it was then supposed to be, a simple elementary 
 substance, the discovery of its composition could 
 scarcely have eluded his sagacity. 
 
 After general laws have once been developed, 
 and their application to particular phenomena has 
 become familiar, it appears wonderful that even 
 quick-sighted and acute observers should have had 
 such effects continually reproduced under their 
 eyes, without even making an approach to the dis- 
 covery of their causes. Franklin found that the 
 
 * Lettere del Elettricismo, 341 . p. 282. 
 
INVENTION OF ELECTROMETERS. 79 
 
 frequent application of the electric spark had eaten 
 away iron ; on which Priestley observed, that it 
 must be the effect of some acid, and suggested 
 the inquiry, whether electricity might not probably 
 redden vegetable blues 9 Priestley also observed 
 that in transmitting electricity through a copper 
 chain, a black dust was left on the paper which 
 supported the chain at the points where the links 
 touched it ; and, on examining this dust, he found 
 it to contain copper. 
 
 (86.) Some years after the invention of the Leyden 
 jar, when the necessity of some sufficient indicator 
 of the presence of electricity, and some visible 
 measure of its power became apparent, the inven- 
 tion of electrometers engaged the attention of 
 electricians. After several abortive attempts on 
 the part of others, the Abbe Nollet proposed the 
 simple expedient of suspending two threads, which, 
 when electrified, would separate by their mutual 
 repulsion. Cavallo afterwards improved upon this, 
 by substituting two pith balls, suspended in contact 
 by fine metallic wires, an apparatus still used. 
 After this, various forms of electroscopic instru- 
 ments were suggested and constructed by Volta, 
 Saussure, and others, all depending on the principle 
 that the intensity of the electric fluid was mani- 
 fested by the force of its attraction or repulsion 
 exerted upon light substances to which it was im- 
 parted. 
 
 (87.) The principle of induction applied to the air- 
 condenser by Wilke and ^Epinus, was taken up 
 by Volta, and applied, first, to the construction of 
 the ELECTROPHORUS, and subsequently to the 
 
80 INTRODUCTION. 
 
 common CONDENSER, which, combined with the 
 electroscope, became in electricity an instrument of 
 investigation analogous in its character and im- 
 portance to the compound microscope in optics. 
 
 (88.) The manner in which the electrified fluid is 
 distributed upon insulated electrified conductors next 
 became the subject of inquiry. Beccaria showed 
 that its distribution is superficial, and that the in- 
 ternal parts of the electrified body are in their 
 natural state. It was shown that, whether the elec- 
 trified conductor were hollow or solid, the electricity 
 contained on it was the same. Lemonnier first 
 showed that the form of the conductor had an in- 
 fluence on the quantity and the distribution of the 
 fluids. 
 
 (89.) In 1778 Volta published a memoir on this 
 subject, in which he proved, that of two cylinders of 
 equal superficial dimensions, that which had the 
 greater length would receive, cceteris paribus, the 
 stronger charge, and inferred that great advantage 
 would arise from the substitution of a system of 
 small cylinders for the large conductors of elec- 
 trical machines. About the same period, he showed 
 how inflammable gases could be ignited in close 
 glass receivers by the electric spark, the apparatus 
 for which purpose soon grew into his eudiometer, 
 for the analysis of gases. Soon after this, the same 
 apparatus supplied the means of inflaming a mix- 
 ture of oxygen and hydrogen gas, which led to 
 the discovery of the composition of water. 
 
 (90.) In the year 1759 appeared, in the "Philoso- 
 phical Transactions/' a series of papers by Mr. Ro- 
 bert Symmer, which are entitled to be recorded in the 
 
VOLT A. SYMMER. 81 
 
 history of electricity ; not so much on account of 
 what they describe, as for the theoretical views 
 developed in them. The experiments of Symmer 
 consisted chiefly in exhibiting, by striking examples, 
 the effect of the mutual attraction of bodies elec- 
 trified by opposite kinds of electricity. These re- 
 sults led him to doubt the sufficiency of the Frank- 
 linian theory, then and long afterwards universally 
 received, to explain satisfactorily the phenomena ; 
 and he was led to consider whether the hypothesis 
 of Dufaye might not be so modified as to explain 
 them more adequately. Dufaye, as has been already 
 stated, assumed the existence of two independent 
 electric fluids, which he supposed to be latent in two 
 distinct classes of bodies, the one in bodies of a 
 vitreous, and the other in bodies of a resinous na- 
 ture; and that these fluids, while they were eacli 
 self-repulsive, were mutually attractive of each 
 other. 
 
 (91.) It was obvious that such an hypothesis was 
 quite inconsistent with the known phenomena of elec- 
 tricity, even limited as they were in variety at the 
 period now referred to. Symmer retained the sup- 
 position of Dufaye so far as regarded the assumed 
 existence of two distinct fluids mutually attractive, 
 but he maintained that these fluids were not inde- 
 pendent of each other. On the contrary, he as- 
 sumed that they were always co-existent in bodies 
 not electrified ; that, by their mutual attraction, 
 they held each other in subjection ; that every 
 body in its natural state contained equal quantities 
 of these fluids, each molecule of the vitreous fluid 
 being combined with a molecule of the resinous 
 
82 INTRODUCTION. 
 
 fluid, the compound molecule thus formed exciting 
 neither attraction nor repulsion on the other parts 
 of the natural fluid. 
 
 This theory of two fluids was left by its author 
 unsupported by any extensive application to the 
 phenomena which could be expected to shake the 
 confidence then generally given to the hypothesis 
 of Franklin ; and although it is noticed at some 
 length in his History of Electricity by Dr. Priestley, 
 it obtained no countenance or support until further 
 advances in electrical experiments rendered appa- 
 rent the defects of the theory of a single fluid. 
 It may be here observed, that the French writers 
 generally ascribe the theory of two fluids to Dufaye, 
 and are silent as to Symmer's share in it ; with what 
 justice will be apparent from what has been above 
 stated. 
 
 (92.) In the year 1770, Dr. Priestley published his 
 works on electricity. This philosopher did not con- 
 tribute materially to the advancement of the science 
 by the development of any new facts ; but in his 
 History of Electricity he collected and arranged 
 much useful information respecting the progress of 
 the science. At this period the Honourable Henry 
 Cavendish, whose name has been distinguished in 
 other departments of physics, engaged in some ori- 
 ginal investigations respecting electricity. The 
 discovery of the composition of water, by trans- 
 mitting an electric spark through a mixture of 
 oxygen arid hydrogen gases, has been generally 
 ascribed to him.* Cavendish conceived the notion 
 
 * This claim has been recently called in question. --- See 
 Lardner on the Steam-engine. Seventh edition, p. 303. 
 
CAVENDISH. COULOMB. 83 
 
 of reducing the phenomena of electricity to mathe- 
 matical analysis, and had proceeded with a memoir 
 on that subject, which was completed before he 
 learned that ^Epinus had produced a work with the 
 same object. On comparing his own paper with 
 the Tentamen of JEpinus, he found that they were 
 nearly similar. Nevertheless, Cavendish published 
 his memoir. 
 
 (93.) The year 1785 formed an important epoch 
 in the history of electrical science, marking, as it did, 
 the commencement of those labours by which Cou 
 LOME laid the foundations of ELECTRO-STATICS 
 This great experimental philosopher was the first 
 who really brought the phenomena of electricity 
 within the reach of numerical calculation, and 
 thereby prepared the way for his followers in the 
 same field to reduce this most subtle of all physical 
 agents to the rigorous sway of mathematics. It is 
 to COULOMB we owe it that statical electricity is 
 now a branch of mathematical physics. 
 
 The immediate instrument by which this vast 
 object was attained was the balance of torsion, 
 which he had already used with signal success in 
 other delicate physical inquiries. This apparatus, 
 which will be fully explained in the following work, 
 consisted of a needle suspended in a horizontal po- 
 sition by an extremely fine wire or filament of silk 
 attached to its centre of gravity. The attraction, 
 or other force of which the intensity is to be mea- 
 sured, is made to act on one end of this needle, so as 
 to twist the filament by which it is suspended; and it 
 is resisted in its effort to effect this by the reaction 
 proceeding from the torsion so produced. This 
 G 2 
 
INTRODUCTION. 
 
 reaction, and therefore the force which produces 
 it, and is in equilibrium with it, was proved by 
 Coulomb to be proportionate to the angle described 
 by the needle round its centre of motion. Such 
 was the sensibility of this exquisite instrument, that 
 it was found to be perceptibly affected by a force 
 not exceeding the twenty-millionth part of a grain. 
 
 (94.) With this instrument Coulomb measured 
 the force with which electrified bodies attract and 
 repel each other ; and the first result of his investi- 
 gation was the discovery, that the law of this attrac- 
 tion and repulsion was the same which Newton 
 showed to prevail among the great bodies of the uni- 
 verse. In fact, he showed that two bodies,, oppositely 
 electrified, attract each other with a force which, 
 cceteris paribus, is the same at equal distances, and 
 which augments in the same proportion as that in 
 which the square of the distance is diminished. 
 Also if two bodies be similarly electrified, they will 
 repel each other by a force which increases accord- 
 ing to the same proportion when the distance be- 
 tween them is diminished. 
 
 (95.) By attaching a very small circular disc of 
 paper coated with metallic foil to an insulating handle, 
 Coulomb found that by touching with the face of 
 the disc an electrified surface, and then submitting 
 the disc itself thus electrified by contact to the test 
 of the balance of torsion, he could determine the 
 depth of the electric fluid on the surface touched by 
 , the disc. In this manner was he enabled to gauge 
 or sound the electricity on the surface of bodies, so 
 as to compare numerically its depth on different 
 bodies, or on different parts of the same body. 
 
RESEARCHES OF COULOMB. 85 
 
 With this instrument he measured the proportion 
 in which electricity was shared between insulated 
 conductors when brought into contact, and also 
 the law according to which its depth varied on 
 different parts of the same insulated conductor. 
 These results acquired, at a later period, still greater 
 importance, supplying, as they did, tests by which 
 the mathematical analysis of the science could be 
 tried. 
 
 (96.) The same apparatus supplied the means of 
 investigating the law according to which an insulated 
 electrified conductor had its charge gradually di- 
 minished by dissipation in the surrounding air, and 
 by the escape of the fluid by the imperfect insula- 
 tion of the supports. 
 
 The results of the observations of Coulomb on 
 the distribution of the electric fluid on the sur- 
 faces of conductors illustrated satisfactorily the 
 doctrine of points, which formed so prominent a part 
 of Franklin's researches. The theoretical solution 
 of this problem was not, however, effected till a 
 later period. 
 
 (97.) The demonstration of the identity of light- 
 ning and electricity naturally directed the attention of 
 philosophers to the solution of other meteorological 
 phenomena by means of the same agency. The 
 explanation of the aurora borealis had long exer- 
 cised the sagacity and baffled the attempts of those 
 devoted to physical researches. Some ascribed this 
 appearance to solar light refracted in the higher 
 regions of the air, others assigned it to the agency 
 of the magnetic fluid. Euler imagined it to pro- 
 ceed from the same ether which formed the tails of 
 r, 3 
 
86 INTRODUCTION. 
 
 comets : Mairan conceived it to arise from the mix- 
 ture of the atmosphere of the sun with that of the 
 earth ; but when the properties of electric light 
 became known, and when its appearance in raretied 
 air had been observed, all these hypotheses were 
 by common consent abandoned, and no doubt was 
 entertained that, whatever might be the details of 
 the natural process by which it was produced, the 
 aurora borealis was an effect of atmospheric elec- 
 tricity. EBERHART, professor at Halle, and PAUL 
 FRISI at Pisa, were the first who proposed an ex- 
 planation of it, founded on the following facts : 
 " ]. Electricity transmitted through rarefied air 
 exhibits a luminous appearance, precisely similar to 
 that of the aurora borealis." "2. The strata of 
 atmospheric air become rarefied as their altitude 
 above the surface of the earth is increased." Hence 
 they argued that the aurora is nothing more than 
 electrical discharges transmitted through parts of the 
 upper regions of the atmosphere, so rarefied as to 
 produce that peculiar luminous appearance which 
 they exhibit. This theory, which was embraced 
 and improved in its details by Canton, Beccaria, 
 Wilke, Franklin, and other contemporary electri- 
 cians, has received further countenance from more 
 recent researches. 
 
 Attempts were also made to explain on electrical 
 principles other meteorological effects ; such as 
 waterspouts, whirlwinds, rain, fogs, hail, &c., but 
 no satisfactory conclusions resulted from these in- 
 vestigations, and the discussion of such phenomena 
 forms a part of the meteorological inquiry of the 
 present time. 
 
VOLTA'S METEOROLOGICAL RESEARCHES. 87 
 
 (98.) While the series of experimental researches 
 which have just been related were in progress, many 
 attempts were made to trace electricity in the phe- 
 nomena of vegetable and animal life, and more 
 especially to apply it as a medical agent in cases 
 of organic disease in the animal system. None of 
 these attempts, however, led to any consequences 
 sufficiently important to entitle them to attention 
 in this brief sketch. 
 
 (99.) After electroscopes had been much im- 
 proved, and in their application to atmospheric elec- 
 tricity had derived great power from the addition of 
 a long pointed conductor, extending from the di- 
 verging balls to a height of several feet, Volta en- 
 gaged in the investigation of the electric state of 
 the air. He substituted for the suspended balls 
 two blades of dry straw, hanging in contact and 
 communicating with the lower end of the con- 
 ducting rod. In addition to this, he had recourse 
 to another apparently strange and unusual expe- 
 dient. He placed on the point of the rod a taper, 
 so as to cause this conductor to terminate in 
 a flame. He contended that the flame attracted 
 to the point of the conductor three or four times 
 as much electricity as would be collected in its ab- 
 sence. This was explained by the effect of the 
 vertical current of air which the flame maintained 
 directly over it, which established a better commu- 
 nication between the metallic conductor and the 
 strata of air above it. 
 
 (100.) Assuming this property of flame, Volta 
 argued, that since fires robbed the atmosphere above 
 them of electricity faster and more effectually than 
 4 
 
88 
 
 INTRODUCTION. 
 
 metallic points, it must follow that to prevent coming 
 storms, or to mitigate their force, the best expedient 
 would be to light enormous fires in the middle of 
 extensive plains, or, better still, on elevated stations. 
 If the effects of the lamp on the atmospheric elec- 
 trometer were admitted, there would be nothing un- 
 reasonable in the supposition that large fires may, 
 in a short interval of time, rob immense volumes of 
 air and vapour of their electricity. 
 
 (101.) Volta wished to submit this theory to an 
 experiment on a large scale, but was not able to 
 carry the design into effect. M. Arago suggested, 
 that by making suitable meteorological observations 
 in those parts of Staffordshire and other English 
 counties which abound in vast iron furnaces, where 
 fires of extraordinary magnitude are maintained 
 night and day, and comparing the results with 
 similar observations made in adjoining agricultural 
 districts, the conjecture of Volta might be tested.* 
 
 Observations of this kind have accordingly been 
 recently made both in England and in certain parts 
 of Italy, the results of which will be explained at 
 the proper place in these volumes. 
 
 (102.) It has been already stated, that direct ob- 
 servations proved that the atmosphere, in its ordinary 
 condition, is always charged with positive electricity. 
 The beginning of the year 1780 was signalised by 
 a capital experiment, by which it was proved that 
 he source from whence this vast amount of the 
 electric fluid was derived, or, to speak more cor- 
 rectly, the cause of the disturbance of the general 
 equilibrium of the globe, which gives a surplus of 
 * Eloge de Volta, p. 18. 
 
ATMOSPHERIC ELECTRICITY. 89 
 
 the positive fluid to the air, and leaves the earth sur- 
 charged with negative fluid, and which, in its effects? 
 assumes all the terrific forms of the tempest and the 
 hurricane., and probably of many other violent con- 
 vulsions which are occasionally exhibited in the war 
 of the elements, is to be found in the process of natu- 
 ral evaporation, which continually maintains its silent 
 and imperceptible progress upon the surfaces of 
 ocean, lake, and river, and even upon those of organ- 
 ized bodies. That heat passes off in a latent form 
 by such means, and equalizes and moderates the ge- 
 neral temperature around us, was well known ; but 
 it was not suspected that the elements of the storm, 
 the coruscations of meteoric light, and the splen- 
 dours of the aurora were due to the same cause. 
 
 Volta states, that in the year 1778 this idea oc- 
 curred to him, and that he conceived the notion of 
 an experiment by which it might be brought to an 
 immediate trial. Let a metallic dish filled with 
 water be placed on an insulating support, and ex- 
 posed in the open air until it evaporates, the dish being 
 maintained in communication with a sufficiently 
 sensible condensing electroscope. If, in evaporating, 
 the positive fluid be carried off, the dish will, after 
 the evaporation, be negatively electrical, and the 
 electroscope will show it ; if not, the electroscope 
 will give no sign. Various circumstances prevented 
 Volta from trying this experiment until the month 
 of March, 1780, when, being in Paris, he succeeded, 
 in company with some members of the Academy of 
 Sciences. There appears, nevertheless, to remain 
 some doubt as to the share which Volta really had 
 in this famous experiment, since, in the account of 
 it published by Lavoisier and Laplace, it is related 
 
90 INTRODUCTION. 
 
 as performed by them, and Volta is mentioned inci- 
 dentally as being present on the occasion.* 
 
 (103.) After the phenomena of electricity had, by 
 the labours of Coulomb, been reduced to exact 
 numerical estimation, this branch of physics was in a 
 state to permit its being brought within the pale of 
 mixed mathematics. To accomplish this it was ne- 
 cessary to express, by mathematical formulae, the 
 intensity of the electric fluid on different parts of 
 insulated conductors of given forms, placed either 
 separately, or in such a position as to exercise an 
 electrical influence upon each other without con- 
 tact, or, finally, when placed in actual contact. To 
 establish such formulae, it was necessary to as- 
 sume some definite hypothesis as the law of elec^ 
 trical action. The Franklinian theory of a single 
 fluid appeared to be incapable of affording the 
 means of explaining, with numerical precision, the 
 state of such bodies. It is true, that this long- 
 received hypothesis was sufficient to account, in a 
 general way, for the electrical state of bodies under 
 the ordinary circumstances of their mutual action ; 
 but when rigorous numerical accuracy was demand- 
 ed, when not merely the general circumstances 
 of the dense accumulation of electricity in one part 
 of the surface, its more feeble intensity at another, 
 its total absence from a third place, or the presence 
 of negative electricity on a certain side of a con- 
 ductor, and positive electricity on another, were 
 severally demanded ; but when it was required to 
 determine the exact numerical measure of the depth 
 of the fluid at each particular spot on a given insu- 
 lated conductor, placed under given conditions with 
 * Eloge de Volta, p. 21. 
 
LABOURS OF POISSON. 91 
 
 reference to others, so that such numerical measure, 
 so obtained by calculation, might be compared with 
 the actual depth observed by the instruments in- 
 vented and applied by Coulomb, then this theory 
 appeared to fail ; at least, none of its advocates pro- 
 duced any such calculations. La Place investigated, 
 on mathematical principles, the distribution of elec- 
 tricity on ellipsoids of revolution, assuming, as the 
 basis of his reasoning, the hypothesis of two fluids. 
 Biot also investigated the same problem applied to 
 spheroids of small eccentricity ; but the general 
 subjugation of this portion of electrical science to 
 mathematical analysis is due to Poisson. 
 
 This illustrious analyst took as the basis of his 
 investigations the theory of two fluids proposed 
 by Symmer and Dufaye, with such modifications 
 and additions as were suggested by the researches 
 of Coulomb. He regarded the mutual attractions 
 and repulsions exhibited by electrified bodies, not 
 as real forces exercised by those bodies, but as 
 altogether due to the electric fluids with which they 
 are charged. The laws of attraction and repulsion 
 developed by Coulomb are therefore assumed as 
 those of the electric fluids. The particles of each 
 of these fluids are assumed to repel each other with 
 a force varying according to that law, while tne 
 particles of each fluid attract those of the contrary 
 fluid by a force governed by the same law. These 
 conditions are sufficient to supply the mathematical 
 formuke necessary to the determination of the depth 
 and quality of the electric fluid on every part of the 
 surface of a body of given figure placed under any 
 given electrical conditions. The electric fluids of 
 either kind would, by virtue of their self-expansive 
 
92 INTRODUCTION. 
 
 property, escape from the surface of the body on 
 which they rest ; but this is prevented by the pres- 
 sure of the surrounding air, which retains them in 
 their position so long as their expansive force is less 
 than that pressure. On bodies of elongated forms, 
 or those which have edges, corners, or points, it 
 is shown, as a consequence of this theory, that the 
 electric fluid accumulates in greater depths about 
 the ends, edges, corners, or points, than in other 
 places. Its expansive force at such parts is there- 
 fore greater than elsewhere, and will exceed the 
 atmospheric pressure, and escape when at other 
 parts of the surface it is retained. 
 
 This theory will be explained in the present work, 
 as far as its development is consistent with the ob- 
 ject of these volumes. It will not, therefore, be 
 needful to enlarge upon it further in this place. It 
 may, however, be asked why it is, seeing that the 
 theory of two fluids is sufficient for the explanation 
 of all the phenomena to which it has yet been ap- 
 plied, and that, on the other hand, the theory of a 
 single fluid fails to afford any satisfactory or accu- 
 rate explanation of so many phenomena, the latter 
 theory nevertheless still has followers, and that even 
 among electricians, whose opinions cannot be re- 
 garded otherwise than with sentiments of respect, 
 it is still clung to as the hypothesis best entitled to 
 reception and confidence ? It is not easy to assign 
 any sufficient reason for this, unless one can be 
 found in the profound and abstruse nature of the 
 mathematical principles by the aid of which alone 
 the effects are capable of being expressed. When 
 it is remembered that, until very recently, electricity 
 was regarded as exclusively a part of experimental 
 
MATHEMATICAL ANALYSIS 9^ 
 
 physics ; that researches in it were chiefly carried 
 on by persons engaged in chemical investigations ; 
 that, from the nature of their studies and pursuits, 
 such persons rarely cultivated even the elements of 
 mathematics, and almost never pursued analytical 
 science into those more profound parts which are 
 now indispensable for the solution of the class of pro- 
 blems which electricity has presented, it cannot be 
 matter of much surprise that reasoning which is 
 incapable of being expressed save by symbols, of 
 which the force and import must be unintelligible 
 to the great mass of such persons, should fail to 
 carry conviction to their understanding. To arrive 
 at such conviction, they must either commence their 
 education anew, or be content to receive those new 
 doctrines on their faith in the assurance of those who 
 are capable of investigating them. Either side of 
 such an alternative is never very willingly embraced. 
 Having now followed the progress of discovery 
 in this part of electrical science to that point at 
 which all subsequent researches must be regarded 
 as the labour of our contemporaries, the province 
 of the historian ceases. Whatever has been effected 
 more recently will properly form a part of the sub- 
 ject matter of the volumes here presented to the 
 reader, of which it is hoped that a brief exposition 
 and analysis of the researches of contemporary 
 philosophers will form not the least interesting and 
 useful portion. 
 
 II. ELECTRO-DYNAMICS. 
 (104.) The investigation of the mechanical pheno- 
 mena of material substances has been, in modern 
 
94- INTRODUCTION. 
 
 works, conducted by resolving these effects into two 
 principal divisions ; those in which the bodies exhi- 
 biting them are at rest, and those in which they 
 are in motion. As applied to solid bodies, these 
 divisions have been respectively denominated STA- 
 TICS and DYNAMICS*; and, as applied to fluids, 
 HYDROSTATICS and HYDRODYNAMICS. Electricity 
 being assumed to be a physical agent, having the 
 properties of an elastic fluid, and capable, like the 
 grosser solids and fluids, of being maintained in a 
 state of equilibrium by the mutual action and re- 
 action of antagonist forces, or of moving in definite 
 directions, and forming currents of greater or less 
 intensity, the analysis of its effects would naturally 
 be conducted by means of the same classification ; 
 and, accordingly, that division of the science in 
 which the electric fluid is considered in a state of 
 equilibrium or repose, and in which the physical 
 conditions on which such equilibrium depends are 
 investigated, would be denominated ELECTRO-STA- 
 TICS, while that in which the effects of currents of 
 electricity are considered would be called ELECTRO- 
 DYNAMICS. 
 
 REST being in its nature more simple than MO- 
 TION, and the cases of forces mutually destructive 
 of each other's influence, and therefore productive 
 of equilibrium, being more simple than those in 
 which motion ensues from the combined action of 
 forces differing from each other in various respects, 
 it was natural that, in every part of physics, the 
 principles of statics should be first established and 
 understood. Such has been accordingly the course 
 
 * The terms STEREO- STATICS and STEREO-DYNAMICS would 
 be preferable. 
 
ELECTRO-DYNAMICS. 5 
 
 which the progress of discovery has taken in other 
 branches of natural philosophy, and electricity is not 
 an exception to it. All the phenomena which have 
 been hitherto adverted to in this notice belong pro- 
 perly to ELECTRO-STATICS. In all of them the elec- 
 tric fluid is contemplated in a state of equilibrium ; 
 or if its motion be occasionally considered, it is 
 only in sudden and momentary changes from one 
 state of equilibrium to another. Thus, when a 
 Leyden jar is charged, the positive electricity ac- 
 cumulated on the inner surface of the glass is main- 
 tained there, in spite of the tendency it has to 
 escape in virtue of its self-expansive property, by 
 the attraction of the negative electricity accu- 
 mulated on the external surface. When a com- 
 munication is made between the internal and 
 external surfaces by a metallic wire, this state of 
 equilibrium ceases ; the positive fluid of the inner 
 surface runs along the wire in one direction, and the 
 negative fluid of the external surface runs along it 
 in the other direction, until each neutralises the 
 other, and a new state of equilibrium is established 
 by the actual combination of the two fluids. If this 
 change occupied a sensible interval of time, and it 
 were required to investigate the effects which would 
 be produced during that interval either on the jar 
 and wire, or on any bodies which might be within 
 their influence, the question would properly belong 
 to ELECTRO-DYNAMICS ; but in fact the discharge, 
 as it is called, or the transition from the one state 
 of equilibrium to the other, is instantaneous, and 
 the same may be said of all the phenomena which 
 form the subject of the preceding pages. 
 
96 INTRODUCTION. 
 
 In the commencement of this notice, the frequent 
 influence of circumstances, apparently fortuitous, 
 on the progress of discovery in the sciences has 
 been mentioned. It would be difficult, either in the 
 history of the sciences or of the political growth of 
 states, to find a more signal example of this than 
 was offered by the discovery of that powerful 
 instrument of physical investigation, the VOLTAIC 
 PILE. " It may be proved," says M. Arago, " that 
 this immortal discovery arose in the most immediate 
 and direct manner from a slight cold with which a 
 Bolognese lady was attacked in 1790, for which 
 her physician prescribed the use of frog-broth" 
 
 Galvani was professor of anatomy at Bologna. 
 At the period just mentioned, it happened that 
 several frogs, divested of their skins and preparec 
 for cooking the broth prescribed for Madame 
 Galvani, lay upon a table in the laboratory of the 
 professor, near which at the moment stood an elec- 
 trical machine. One of the professor's assistants, 
 being employed in some process in which the ma- 
 chine was necessary, took sparks occasionally from 
 the conductor, when Madame Galvani was asto- 
 nished to see the limbs of the dead frogs convulsed 
 with movements resembling vital action. She 
 called the attention of her husband to the fact, who 
 repeated the experiment, and found the motions re- 
 produced as often as a spark was taken from the 
 conductor. This was the first, but not the only or 
 chief part played by chance in this great discovery. 
 (105.) Galvani was not familiar with electricity. 
 Had he been so, he would have seen in the con- 
 vulsions of the frog evidence of nothing more than a 
 
DISCOVERY OF GALVANISM. 97 
 
 high electroscopic sensibility in the nerves of that ani- 
 mal, and an interesting example of the known prin- 
 ciple of electrical induction. But luckily for the pro- 
 gress of science, he was more an anatomist than an 
 electrician, and beheld with sentiments of unmixed 
 wonder the manifestation of what he believed to be 
 a new principle in the animal economy, and, fired 
 with the notion of bringing to light the proximate 
 cause of vitality, engaged with ardent enthusiasm 
 in a course of experiments on the effects of electri- 
 city on the animal system. It is rarely that .an ex- 
 ample is found of the progress of science being 
 favoured by the ignorance of its professors. 
 
 Chance now again came upon the stage. In the 
 course of his researches he had occasion to separate 
 .e legs, thighs, and lower part of the body of the 
 rog from the remainder, so as lay bare the lumbar 
 nerves. Having the members of several frogs thus 
 dissected, he passed copper hooks through part of 
 the dorsal column which remained above the junc- 
 tion of the thighs, for the convenience of hanging 
 them up till they might be required for the purposes 
 of experiment. In this manner he happened to 
 suspend several upon the iron balcony in front of 
 his laboratory, when, to his inexpressible astonish- 
 ment, the limbs were thrown into strong convul- 
 sions. No electrical machine was now present to 
 exert any influence. 
 
 If the supply of capital facts be occasionally due 
 to chance, or to the BEING by whom what is mis- 
 called chance is directed, it is to the operation of 
 the faculties of exalted minds that the develop- 
 
98 INTRODUCTION. 
 
 ment of the laws of nature is due : if rude lumpe* 
 of the natural ore of science be now and then 
 thrown under the feet of philosophy, the dis- 
 covery of the vein itself, its depth and direc- 
 tion, its quality and value, the separation of the 
 precious metal it contains from its baser elements, 
 the demonstration of its connection with the phe- 
 nomena of nature, and its adaptation to the uses of 
 life, are all and severally the work of that noble 
 faculty of intellect, that image of his own essence, 
 which the Creator of the universe has impressed 
 upon man, and which is never more worthily ex- 
 ercised than in the investigation of those laws of 
 the material world, in all of which, whether they 
 affect the vast bodies of the universe, or the imper- 
 ceptible molecules of those around us, there is ever 
 conspicuous a provident care for the wellbeing of 
 his creatures. 
 
 In the convulsions of the frog, suspended by a 
 copper wire on an iron rail, Galvani saw &new fact, 
 and soon discovered that the circumstance on which 
 it depended was the simultaneous contact of the 
 metals with the nerves and muscles of the animal. 
 He found that the effects were reproduced when- 
 ever the muscles touched the iron while the nerves 
 touched the copper, but that contact with the 
 copper alone did not produce them. He next placed 
 the body of the animal upon a plate of iron, and 
 touching the plate with one end of a copper wire, 
 brought the other end into contact with the lumbar 
 nerves. The convulsions followed as before. Gal- 
 vani inferred from these and other similar experi- 
 ments and observations, that the conditions under 
 
GALVANl's THEORY. 99 
 
 which the phenomenon was produced were, that a 
 connection should be made between the nerves of 
 the animal and the muscles with which those nerves 
 were united by a continued line or circuit com- 
 posed of two different metals ; and he explained this 
 singular effect by assuming, hypothetically, that, in 
 the animal economy, there exists a natural source of 
 electricity ; that, at the junction of the nerves and 
 muscles, the natural electricity is decomposed ; that 
 the positive fluid goes to the nerve, and the negative 
 to the muscle ; that the nerve and muscle are there- 
 fore analogous to the internal and external coating 
 of a charged Leyden jar ; that the metallic connec- 
 tion made between the nerve and the muscle in the 
 experiments above mentioned serves as a conductor 
 between these opposite electricities ; and that, on 
 making the connection, the same discharge takes 
 place as in the Leyden experiment. 
 
 This theory fascinated for a time the physiologists. 
 The phenomena of animal life had been ascribed to 
 an hypothetical agent, which passed under the name 
 of the " nervous fluid." The Galvanic theory con- 
 signed this term to the obsolete list ; and electricity 
 was now the great vital principle, by which the 
 decrees of the understanding, and the dictates of 
 the will, were conveyed from the organs of the brain 
 to the obedient members of the body. Those who 
 know how passionate is the love of a theory which 
 appears to give a satisfactory account of effects 
 otherwise mysterious, and how much more grati- 
 fying to the amour-propre it is to be able to con- 
 nect effects with supposed causes, than to be corn- 
 H 2 
 
100 INTRODUCTION. 
 
 pelled to view the former as the real limits of our 
 knowledge, will understand the reluctance with 
 which the Bolognese school and its distinguished 
 leader would surrender a theory so dazzling as 
 animal electricity ; nevertheless, it was doomed 
 soon to fall under the irresistible assaults of phy- 
 sical truth directed against it by a giant intellect, 
 which, though located in a little village of the 
 Milanese, belonged to mankind. 
 
 (106.) VOLT A, professor of natural philosophy at 
 Como, and subsequently at Pavia, had been already 
 known for his researches in different parts of physics, 
 but more especially in electricity. The Bolognese 
 experiments naturally engaged his attention, and it 
 \vas not long before his superior sagacity enabled 
 him to perceive that the theory of Galvani was- 
 destitute of any sound foundation. Indeed, a single 
 experiment was sufficient to overturn it, though not 
 to carry conviction of it& futility to the minds of its 
 partisans. Volta applied the metals in contact with 
 each other to the muscle alone without touching 
 the nerves, and the convulsions nevertheless ensued. 
 The analogy of the muscle and nerve to the Ley den 
 phial was no longer tenable. Volta transferred 
 this analogy to the two rnetals, and contended that 
 the mutual contact of two dissimilar metals must 
 be regarded as the source of the electricity ; that by 
 the contact the natural electricity was decomposed, 
 and the positive fluid passed to one metal, and the 
 negative one to the other ; and that the muscle 
 merely played the part of a conductor in carrying 
 off one of the fluids thus developed. 
 
VOLTA REFUTES GALVANT S* THEORY. K) 1 
 
 (107.) To this Galvani replied by showing that, 
 when a single metal was used to connect the nerves 
 and muscles the convulsions ensued, and that there- 
 fore the contact of dissimilar metals could not be 
 the source of the electricity. Volta rejoined, that 
 it was impossible to be assured of the perfect ho- 
 mogeneity of the metal, and that any the least hete- 
 rogeneous matter contained in it would be sufficient 
 for his hypothesis. Also, that when a single metal 
 was used, the convulsions were uncertain, and 
 never produced, except in cases where the organs 
 were in the highest state of excitability ; whereas, on 
 the contrary, they happened invariably, and were 
 long continued, when the connection was made by 
 two dissimilar metals. 
 
 Tenacious of the cherished theory to the last, 
 Doctor Valli, a partisan of Galvani, confounded 
 the advocates of the school of Pavia, by showing 
 that, by merely bringing the muscles themselves 
 into contact with the nerves, without the interven- 
 tion of any metal whatever, the convulsions ensued. 
 To this, the expiring effort of the Bolognese party, 
 Volta readily and triumphantly replied, that the 
 success of the experiments of Valli required two 
 conditions; 1st, that the parts of the animal brought 
 into contact should be as heterogeneous as possible ; 
 and, 2dly, the interposition of a third substance be- 
 tween these organs. This, so far from overturning 
 the theory of Volta, only gave it increased gene- 
 rality, showing, as it did, that electricity was deve- 
 loped, not alone by the contact of two dissimilar 
 metals, but also by the contact of dissimilar sub- 
 stances not metallic. 
 
 H 3 
 
I Q.I , INTRODUCTION. 
 
 From this time, the partisans of animal electri- 
 city gradually diminished, and no effort worth re- 
 cording to revive Galvani's theory was made. 
 Meanwhile, the hypothesis of Volta was, as yet, re- 
 garded only as the conjecture of a powerful and 
 sagacious mind, requiring nevertheless much more 
 cogent and direct experimental verification. This 
 experimental proof he soon supplied. 
 
 (108.) The first analogy which Volta produced 
 in support of his theory of contact was derived from 
 the well-known experiment of Sulzer. If two pieces 
 of dissimilar metal, such as lead and silver, be 
 placed one above and the other below the tongue, 
 no particular effect will be perceived so long as they 
 are not in contact with each other ; but if their 
 outer edges be brought to touch each other, a pecu- 
 liar taste will be felt. If the metals be applied 
 in one order, the taste will be acidulous. If the 
 order be inverted, it will be alkaline. Now, if the 
 tongue be applied to the conductor of a common 
 electrical machine, an acidulous or alkaline taste 
 will be perceived, according as the conductor is 
 electrified positively or negatively. Volta contended, 
 therefore, that the identity of the cause should be 
 inferred from the identity of the effects ; that, as 
 positive electricity produced an acid savour, and 
 negative electricity an alkaline, on the conductor of 
 the machine, the same effects on the organs of taste 
 produced by the metals ought to be ascribed to the 
 same cause. 
 
 (109.) However sufficient this analogy might 
 seem to the understanding of Volta, it was insuffi- 
 cient for the rigid canons of the logic of modern phy- 
 
.. 
 
 VOLTA'S EXPERIMENTS. 103 
 
 sics, and he accordingly sought and obtained more 
 direct and unequivocal proof of his hypothesis. 
 Two discs, one of copper, and the other of zinc, were 
 attached to insulating handles, by means of which 
 they were carefully brought into contact, and sud- 
 denly separated without friction. They were then 
 presented severally to a powerful condensing elec- 
 troscope. The usual indications of electricity were 
 obtained, and it was shown that this electricity was 
 positive on the zinc, and negative on the copper. 
 By repeating the contact, and collecting the electri- 
 city by means of the condenser, sparks were pro- 
 duced, and the demonstration was complete. 
 
 That the contact of dissimilar metals was followed 
 by the evolution of electricity, could therefore no 
 longer be doubted. It will, however, hereafter 
 appear that philosophers are not even yet agreed 
 that the contact is the immediate or the only cause 
 of the disengagement of electricity in such cases. 
 Chemical agency is now known to be one of the 
 sources of electricity ; and its operation is so subtle, 
 often so imperceptible, and generally so inevitable, 
 when heterogeneous molecules come into con- 
 tact, that doubts have been entertained whether, in 
 every case where electricity seems to proceed from 
 contact, it has not really its origin in feeble and 
 imperceptible chemical action. 
 
 Although the complete development of this last- 
 mentioned idea belongs to a much more recent 
 epoch in the progress of electrical discovery, yet 
 the chemical origin of electricity did not altogether 
 escape notice even at the period to which we now 
 refer. 
 
104 INTRODUCTION. 
 
 ( 1 1 0.) Of the numerous philosophers in every part 
 of Europe who took part in the discussions, and 
 varied and repeated the experiments connected with 
 these questions, one of those to whom attention is 
 more especially due was Fabroni, who, in the year 
 1792* 5 two years after the discovery of Galvani, 
 communicated his researches to the Florentine Aca- 
 demy. In this paper is found the first suggestion 
 of the chemical origin of Galvanic electricity. 
 
 Fabroni observes, that in the mutual contact of 
 heterogeneous metals there is a reciprocal action 
 which favours chemical change ; that to this action 
 must be ascribed many well-known phenomena, 
 such as the more rapid oxidation of certain metals 
 when combined, or in mere contact with other 
 metals. According to him, a metal, like all che- 
 mical re-agents, has a tendency to combination with 
 another metal when they are brought into contact ; 
 that this effect is only prevented by the superior 
 force of cohesion which prevails among the par- 
 ticles of each. This cohesive force will, however, 
 be lessened in its energy by the antagonism of the 
 attraction of the molecules of the two metals to- 
 wards each other, just in the same manner as it 
 would be lessened by the action of heat. Being 
 thus lessened, its opposition to the tendency which 
 the. particles of either metal have to combine with 
 oxygen, taken either from the atmosphere, or ob- 
 tained from the decomposition of water, would be 
 proportionally diminished, and such oxidation would 
 
 * The date of the researches of this philosopher is generally, 
 but erroneously, assigned to the year 17G9. 
 
FABRONl's RESEARCHES. 105 
 
 accordingly be promoted. In this way Fabroni 
 accounted for the tendency of certain alloys of metal 
 to oxidation, and for the well-known fact that iron 
 nails, then used in attaching the copper sheathing to 
 vessels, were rendered so liable to rust by their 
 contact with the copper, that they became soon too 
 small for the holes in which they were inserted. 
 He supposed, therefore, that in the experiments of 
 Galvani and Volta, in which the convulsions of the 
 limbs of animals were produced, a chemical change 
 was made by the contact of one of these metals 
 with the liquid matter always found on the parts of 
 the animal body ; and that the immediate cause of 
 the convulsions was not, as supposed by Galvani, 
 due to animal electricity, nor, as assumed by 
 Volta, to a current of electricity emanating from 
 the surface of contact of the two metals, but to 
 the decomposition of the fluid upon the animal 
 substance, and the transition of oxygen from a state 
 of combination with it to combination with the 
 metal. The electricity produced in the experi- 
 ments Fabroni ascribed entirely to the chemical 
 changes, it being then known that chemical pro- 
 cesses were generally attended with sensible signs 
 of electricity. He maintained that the convulsions 
 were chiefly due to the chemical changes, and not 
 to the electricity incidental to them, which, if it 
 operated at all, he considered to do so in a second- 
 ary way. 
 
 The necessary limits of this notice will not allow 
 of a further analysis of the researches of this philo- 
 sopher ; but if his original papers be referred to, it 
 will be seen that he is entitled to the credit of 
 
106 INTRODUCTION. 
 
 having first distinctly demonstrated the chemical 
 origin of Voltaic electricity. 
 
 (111.) In the year 1800, the attention of the scien- 
 tific world was withdrawn from the controversy re- 
 specting the origin of Galvanic electricity, and all 
 other matters of minor importance, and engrossed 
 by one of those vast discoveries which constitute an 
 epoch in the progress of knowledge, and give a new 
 direction to the sciences. On the 20th March, 1800, 
 Volta addressed a letter to Sir Joseph Banks, then 
 president of the Royal Society, in which he an- 
 nounced to him the discovery of the VOLTAIC PILE, 
 one of the most powerful instruments for the in- 
 vestigation of the laws of nature, as exhibited in 
 the mutual relations of the constituent parts of 
 matter, which ever did honour to the science of any 
 age, or any nation. 
 
 In order to complete the experimental analysis of 
 the effects of Galvanic electricity, Volta felt the 
 necessity of collecting it in much greater quantities 
 than could be obtained in the processes which had 
 then been adopted. According to his theory, when 
 two plates of metal, zinc and copper for example, 
 were brought into contact, two currents of electric 
 fluid originated at their common surface, and 
 moved from that point in opposite directions. The 
 positive fluid passed along the zinc, and the nega- 
 tive along the copper. If the extremities of the 
 two metals most remote from their mutual contact 
 were connected by an arc of conducting matter, 
 these contrary currents would flow along this arc> 
 the positive fluid moving from the zinc towards the 
 copper, and the negative from the copper towards 
 
INVENTION OF THE VOLTAIC PILE. 107 
 
 the zhic ; but the intensity of these currents was 
 supposed to be so feeble that no ordinary electro- 
 scope, whatever might be its sensibility, would be 
 affected by it. In order to bring into operation in 
 this question those instruments which had been ap- 
 plied to common electricity, he therefore sought 
 some expedient by which he could combine, and, 
 as it were, superpose, two or more currents, and thus 
 multiply the intensity, until it should attain such an 
 augmentation as to produce effects analogous to those 
 which had been obtained by ordinary electricity. 
 
 With this object, he conceived the idea of 
 placing alternately, one over the other, discs of differ- 
 ent metals, such as zinc and copper. Let us suppose 
 the lowest disc to be copper, having a disc of zinc 
 upon it. On this disc of zinc let a second copper 
 disc be placed, and over that a second disc of zinc, 
 and so on. According to Volta's theory, currents 
 of electricity would be established at each surface 
 of contact of the two metals, the positive current 
 running along the zinc, and the negative along the 
 copper. With the arrangement above described, 
 there would proceed from the first surface a nega- 
 tive downward, and a positive upward current ; from 
 the second a positive downward, and a negative up- ' 
 ward current ; from the third a negative downward, 
 and a positive upward current, and so on ; the down- 
 ward current being negative, and the upward positive 
 from the upper surface of each copper disc, and the 
 upper current being negative and the downward 
 positive from the lower surface of such disc. It is 
 evident, therefore, that the downward currents would 
 be alternately positive and negative ; and the same 
 
108 INTRODUCTION. 
 
 would be the case with the upward currents. Now, 
 since the surfaces of contact of the metals would be 
 equal, these currents would have equal intensities, 
 and accordingly each positive current would neu- 
 tralize each negative current having the same direc- 
 tion. The result would be, that if the lowest and 
 highest disc of the pile were of the same metal, all 
 the currents neutralizing each other, the pile would 
 evolve no electricity whatever ; and if they were of 
 different metals, all the downward currents, except 
 one, would neutralize each other, and that one 
 would be positive. The effect of the pile would 
 therefore be the same as if it consisted of only two 
 discs, one of copper, and the other of zinc. 
 
 Volta therefore saw the necessity of adopting 
 some expedient by which all the currents in the 
 same direction should be of the same kind ; so that, 
 for example, all the decending currents should be 
 negative, and all the ascending currents positive. 
 If this could be accomplished, the current issuing 
 from the bottom of the pile would be a negative 
 current as many times more intense than one pro- 
 ceeding from a single pair of discs as there were 
 surfaces of contact supplying currents, and the 
 same would be true of the positive current issuing 
 from the top of the pile. 
 
 To effect this, it was necessary to destroy the 
 Galvanic action at all those surfaces from which 
 descending positive and ascending negative currents 
 would proceed ; that is, the lower surfaces of the 
 copper discs and the upper surfaces of the zinc 
 discs. But while this was effected, it was also essen- 
 tial that the progress of the descending negative 
 
INVENTION OF THE PILE. 109 
 
 and ascending positive currents should still be un- 
 interrupted, The interposition of any substance 
 which would have no sensible Galvanic action on 
 either of the metals between each disc of copper 
 and the disc of zinc immediately below it would 
 attain one of these ends, since the action of all 
 the surfaces in which ascending negative or de- 
 scending positive currents could originate would 
 thus be prevented. But in order to allow the free 
 progress of the remaining currents in each direc- 
 tion, such substance must be a sufficiently free con- 
 ductor of electricity. Volta selected, as the fittest 
 means of fulfilling these conditions, discs of wet 
 cloth. They would be free from any sensible Gal- 
 vanic action on the metal, and their moisture would 
 give them sufficient conducting power. 
 
 (112.) Having discovered the principles by which 
 this species of electricity can be accumulated in 
 quantity and strong currents obtained, he varied its 
 form, and contrived the apparatus which is known 
 by the name of La Couronne de Tasses. This ar- 
 rangement, which Volta himself most commonly 
 used in his experiments, consisted of a circle of 
 cups filled with warm water, or a solution of sea- 
 salt. He immersed in each cup a plate of zinc 
 and one of silver, not in contact, and then es- 
 tablished a metallic communication by means of 
 wire between the zinc of one cup and the silver of 
 the adjacent one. The positive fluid was found to 
 proceed from the. extreme zinc plate, and the nega- 
 tive from the extreme silver one, and a continuous 
 current was obtained by connecting these by any 
 conductors of electricity. 
 
110 INTBOD UCTION. 
 
 (113.) Profoundly impressed with the importance 
 of the results likely to arise from the application of 
 the powers of the pile in physical inquiries, and 
 doubtless animated by the desire for which he 
 was honourably distinguished to extend all possible 
 encouragement and advantage to those engaged in 
 the natural sciences, Napoleon, then first consul, 
 and surrounded by the splendour of his southern 
 triumphs, invited Volta to visit Paris ; and there, at 
 the Institute, before the elite of European philoso- 
 phers, to explain personally his great invention, and 
 expound his views as to its probable uses and powers 
 as an instrument of scientific research. Volta ac- 
 cepted the proffered honour, and, in 1801, attended at 
 three meetings of the Academy of Sciences, at which 
 he explained his theory of contact, and developed 
 his views respecting the voltaic, or, as he called it, 
 electro-motive, action of different metals upon each 
 other. Among the audience at these memorable 
 meetings was Napoleon himself, and none present 
 appeared to appreciate more justly the vastness of 
 the power which was on that occasion placed in the 
 hands of the experimental philosopher. 
 
 (114.) When the report of the committee on the 
 subject was read, the FIRST CONSUL proposed that 
 the rules of the Academy, which produced some 
 delay in conferring its honours, be suspended, and 
 that the gold medal be immediately awarded to Volta, 
 as a testimony of the gratitude of the philosophers 
 of France for his discovery. This proposition being 
 carried by acclamation, the hero of an hundred fields, 
 who never did things by halves, and who was filled 
 with a prophetic enthusiasm as to the powers of the 
 
NAPOLEON INVITES VOLTA. Ill 
 
 pile, ordered two thousand crowns to be sent to 
 Volta the same day from the public treasury, to 
 defray the expenses of his journey.* He also 
 founded an annual medal, of the value of three thou- 
 sand francs, for the best experiment on the electric 
 fluid ; and a prize of sixty thousand francs to him 
 who should give electricity or magnetism, by his 
 researches, an impulse comparable to that which 
 it received from the discoveries of Franklin and 
 Volta. 
 
 (115.) The relation in which the voltaic pile stood 
 in reference to theLeyden jar and electrical machines 
 now began to be perceived. In the latter apparatus 
 a great quantity of electricity is accumulated on 
 the surfaces of the jar, and held there in equili- 
 brium, the positive fluid on one side of the glass, and 
 the negative on the other. When the communi- 
 cation is made between the two surfaces, a torrent 
 of the fluid precipitates itself instantaneously along 
 the line of communication, and the electrical equili- 
 brium is re-established in an interval of time so short 
 as to be inappreciable. A sudden, instantaneous, 
 and violent effect is produced on whatever bodies 
 may be exposed to the transit of this electric fluid. 
 On the other hand, the Voltaic pile is a generator of 
 electricity, which supplies to its opposite poles the 
 two fluids, the positive and the negative electricity, 
 in a continued, gentle, and regulated current. It dis- 
 charges it not suddenly or instantaneously, or with 
 uncontrollable and irresistible violence, but with 
 gentle, moderate, continued, and regulated action. 
 
 * Arago, Eloge de Volta, p. 42. 
 
INTRODUCTION. 
 
 What takes place in the Leyden jar in an interval 
 so brief as to render observation of its progress, or 
 examination of its successive effects impossible, is 
 with the pile spread over as long an interval as the ob- 
 server may desire. Besides this, the effects themselves 
 consequent on the two modes of action are different. 
 That which in mechanical phenomena is effected by 
 a violent blow or concussion is not more different 
 from the effects of the long-continued action of an 
 uniform accelerating force or a constant pressure, 
 than are the effects of the common electrical dis- 
 charge from those of the currents of electricity pro- 
 pagated between the poles of the pile. 
 
 (J 16.) The physiological effects of electricity ex- 
 hibited under these different forms, differ in a manner 
 which might be anticipated from these modifications 
 in the transmission of the electric fluid. If the wires 
 proceeding from the opposite poles, and conducting 
 the contrary currents of fluid, be taken in the hands, 
 the sudden and violent shock of the Leyden jar is 
 no longer felt. It is replaced by a continued con- 
 vulsion in the arms and shoulders, which does not 
 cease so long as the wires are held. 
 
 If a metallic plate, in connection with the positive 
 pole, be applied to the tongue, and another con- 
 nected with the negative pole to any other part, a 
 strong acidulous savour is perceived. If the plate 
 applied to the tongue be connected with the nega- 
 tive pole, a strong alkaline savour is felt. 
 
 It is not the organs of taste only which are sen- 
 sible to the influence of this instrument. The sense 
 of sight is susceptible of its operation in a manner 
 even more wonderful. Let a metallic surface con- 
 
PHYSIOLOGICAL ACTION OF THE PILE. 113 
 
 nected with one of the poles be applied to the fore- 
 head, the cheek, the nose, the chin, or the throat 
 and, at the same time, let the patient take in hii 
 hand the wire connected with the other pole. Im- 
 mediately a light will be perceived, even though the 
 eyes be closed, the splendour and appearance of 
 which will vary with the part of the face in con- 
 tact with the metallic plate. By similar means, 
 the perception of sound will be perceived in the 
 ears. 
 
 The action of the pile on the animal body after 
 the vital principle is destroyed is so well known, 
 that it is scarcely necessary to mention it here. 
 The trunk of a decapitated body will rise from its 
 recumbent posture ; the arms will move and strike 
 objects near them ; the legs will elevate themselves 
 with a force sufficient to raise considerable weights ; 
 the breast will heave as if respiration were restored ; 
 and, in fine, all the vital actions will be manifested 
 with terrific and revolting precision. 
 
 In the hands of the entomologist, the pile affords 
 results not less interesting. The glow-worm, sub- 
 mitted to the electric current, shines with increased 
 splendour ; the grasshopper chirps, as if under the 
 action of a stimulant.* 
 
 (117.) The physiological action of the pile was 
 strongly suggestive of a mysterious connection be- 
 tween the electric fluid and the proximate principle 
 of vitality. When some of these effects were ex- 
 hibited to Napoleon, the emperor turned to Corvisart, 
 his physician, and said, " Docteur, voila 1'image de 
 
 * Eloge, p. 33. 
 
 VOL I. I 
 
114 INTRODUCTION. 
 
 la vie : la colonne vertebrale est la pile ; le foie, 
 le pole negatif; la vessie, le pole positif."* 
 
 (118.) The invention of the pile had been scarcely 
 more than hinted at, when that course of electro- 
 chemical investigations began which soon led to the 
 magnificent discoveries of Davy, and the series of 
 experimental researches which have been continued 
 to the present time with results so remarkable by 
 those who succeeded him. The first four pages 
 only of the letter of Volta to Sir Joseph Banks 
 were despatched on the 20th of March, 1 800 ; and 
 as these were not produced in public till the receipt 
 of the remainder, the letter was not read at the 
 Royal Society, or published until the 26th of June 
 following. The first portion of the letter, in which 
 was described generally the formation of the pile, 
 was shown in the latter end of April by Sir Joseph 
 Banks to some scientific men, and among others to 
 Sir Anthony (then Mr.) Carlisle, who was engaged 
 
 * This anecdote was told by Chaptal, who was present on the 
 occasion, to Bequerel ; and the latter relates it in the first 
 volume of his work on electricity, published in 1834. The 
 idea that electricity is the immediate principle of vitality has 
 occurred to other minds. Sir John Herschel, in his Prelimi- 
 nary Discourse published in the CABINET CYCLOPAEDIA in 1830, 
 without any knowledge of the above anecdote, says (p. 343.), 
 " If the brain be an electric pile constantly in action, it may 
 be conceived to discharge itself at regular intervals, when the 
 tension of the electricity developed reaches a certain point, 
 along the nerves which communicate with the heart, and thus 
 to excite the pulsation of that organ. This idea is forcibly 
 suggested by the view of that elegant apparatus, the dry pile 
 cf De Luc, in which the successive accumulations of electricity 
 are carried off by a suspended ball, which is kept by the dis- 
 charge in a state of regular pulsation for any length of time." 
 A similar idea occurred to Dr. Arnott, and is mentioned in 
 his PHYSICS. 
 
CHEMICAL ACTION OP THE PILE. 115 
 
 at the time in certain physiological inquiries. Mr. 
 W.Nicholson, the conductor of the scientincjournal 
 known as Nicholsons Journal, and Carlisle con- 
 structed a pile of seventeen silver half-crown pieces 
 alternated with equal discs of copper and cloth 
 soaked in a weak solution of common salt, with 
 which on the 30th of April they commenced their 
 experiments. It happened that a drop of water was 
 used to make good the contact of the conducting 
 wire with a plate to which the electricity was to be 
 transmitted ; Carlisle observed a disengagement of 
 gas in this water, and Nicholson recognised the 
 odour of hydrogen proceeding from it. In order 
 to observe this effect with more advantage, a small 
 glass tube, open at both ends, was stopped at one 
 end by a cork, and being then filled with water was 
 similarly stopped at the other end. Through both 
 corks pieces of brass wire were inserted, the points 
 of which were adjusted at a distance of an inch and 
 three quarters asunder in the water. When these 
 wires were put in communication with the opposite 
 ends of the pile, bubbles of gas were evolved from 
 the point of the negative wire, and the end of the 
 positive wire became tarnished. The gas evolved 
 appeared on examination to be hydrogen, and the 
 tarnish was found to proceed from the oxydation of 
 the positive wire. It was inferred that the process 
 in which these effects were produced was the de- 
 composition of water. This took place on the 2d of 
 May, shortly after the receipt of the first portion 
 of Volta's letter. 
 
 To ascertain whether the oxydation of the po- 
 sitive wire was an effect incidental to the experiment, 
 i 2 
 
116 
 
 INTRODUCTION. 
 
 or had an influence in producing the decomposition, 
 Nicholson determined to try the effect of wires 
 formed of metal more difficult of oxydation. Wires 
 of platinum were accordingly inserted through the 
 corks, and the experiment repeated. Bubbles of 
 gas were now evolved from both wires. Two pla- 
 tinum wires were next inserted at the closed ends 
 of two separate tubes, which, being open at the 
 other ends and filled with water, were inserted in 
 the same vessel of water. Being placed side by 
 side close together, and the wires being continued 
 to the lower ends of the tubes, so that the distance 
 between their points was not more than two inches, 
 their upper extremities were put in connection with 
 the ends of the pile. Gas was evolved from the 
 points of both wires, and, ascending through the 
 water, was collected separately in the two tubes. 
 These gases being examined, proved to be hydrogen 
 from the negative, and oxygen from the positive 
 wire, nearly in the proportion known to constitute 
 water. * 
 
 Thus was the decomposing power of the pile es- 
 tablished within a few weeks after the first intimation 
 of the invention of that instrument had been re- 
 ceived in England, and before any description of it 
 had been published. It seemed proper to give these 
 details here, not only on account of the great im- 
 portance of the discovery, but because it has been 
 sought to depreciate the merit of it by ascribing it 
 altogether to chance. It is probably impossible to 
 exclude chance altogether from such investiga- 
 
 * Nicholson's Journal, vol. iv. p. 179. 1800. 
 
CRUICKSHANK S EXPERIMENTS. 
 
 tions, but in this there was as little as is gene 
 found. 
 
 (119.) When these experiments became knownV 
 Mr. W. Cruickshank of Woolwich repeated them, 
 and obtained similar results ; but observed that when 
 the distilled water was tinged with litmus, the effects 
 of an acid were produced at the positive and those of 
 an alkali at the negative wire. Led by this indi- 
 cation, he tried the effects of the wires on solutions 
 of acetate of lead, sulphate of copper, and nitrate of 
 silver. In each case he found the metallic base 
 deposited at the negative pole, and the acid mani- 
 fested at the positive pole. Muriate of ammonia 
 and nitrate of magnesia were next decomposed, the 
 acid as before going to the positive, and the alkali 
 to the negative pole. These experiments of Mr. 
 Cruickshank were made as early as June, 1800.* 
 
 In the September following, Mr. Cruickshank 
 published the continuation of his researches-)-, in 
 which he corroborated the results of his former ex- 
 periments, showing more generally the tendency of 
 oxygen and the acids in Voltaic decomposition to 
 collect round the positive wire, and hydrogen, 
 metals, alkalies, &c. round the negative pole. 
 
 ( 1 20.) The investigations of which the pile became 
 the instrument now began to assume an importance 
 which rendered it necessary to give it considerably 
 augmented power, either by increasing its height 
 or enlarging its component plates. In either case, 
 inconveniences were encountered which imposed a 
 
 * Nicholson's Journal, vol. iv. p. 187. 1800. 
 t Ibid. p. 2o'4. 
 
 I 3 
 
118 INTRODUCTION. 
 
 practical limit on the increase of its power. When 
 the number or magnitude of the metallic discs was 
 considerable, the incumbent pressure discharged 
 the liquid from the intermediate discs of cloth or 
 card. The trouble of refilling it whenever its use 
 was required, and of wetting the cloth or card, was 
 very great. Mr. Cruickshank, adopting the prin- 
 ciple of Volta's couronne des tasses, proposed, as a 
 more convenient form for the apparatus, an arrange- 
 ment consisting of a trough of baked wood, which 
 is a non-conductor of electricity, divided by parallel 
 partitions into a series of cells. Into these cells the 
 liquid to be interposed between the successive pairs 
 of metallic plates was poured. A series of rectan- 
 gular plates of metal, alternately zinc and copper, 
 were arranged so as to be parallel to each other, and 
 at such a distance as to allow the partitions of the 
 trough to pass between each pair of plates. This 
 modification rendered the Voltaic apparatus capable 
 of having its power increased without practical 
 limit. 
 
 (121.) While these investigations were proceeding, 
 Hitter, afterwards so distinguished for his experi- 
 mental researches, but then young and unknown, 
 made various experiments at Jena on the effects of the 
 pile; and, apparently without knowingwhat had been 
 done in England, discovered its property of decom- 
 posing water and saline compounds, and of collecting 
 oxygen and the acids at the positive, and hydrogen 
 and the bases at the negative pole. He also showed 
 that the decomposing power in the case of water 
 could be transmitted through sulphuric acid, the 
 oxygen being evolved from a portion of water on 
 
SIR HUMPHRY DAVY. 119 
 
 one side of the acid, while the hydrogen was pro- 
 duced from another separate portion of water on 
 the other side of it.* 
 
 (122.) When the chemical powers of the pile be- 
 came known in England, Sir Humphry (then Mr.) 
 Davy was commencing those labours in chemical 
 science which subsequently surrounded his name 
 with so much lustre, and left traces of his genius in 
 the history of scientific discovery which must remain 
 as long as the knowledge of the laws of nature is 
 valued by mankind. The circumstance attending 
 the decompositions effected between the poles of 
 the pile which caused the greatest surprise, was the 
 production of one element of the compound at one 
 pole, and the other element at the other pole, without 
 any discoverable transfer of either of the disengaged 
 elements between the wires. If the decomposition 
 was conceived to take place at the positive wire, 
 the constituent appearing at the negative wire must 
 be presumed to travel through the fluid in the se- 
 parated state from the positive to the negative 
 point ; and if it was conceived to take place at the 
 negative wire, a similar transfer must be imagined 
 in the opposite direction. Thus, if water be de- 
 composed, and the decomposition be conceived to 
 proceed at the positive wire where the oxygen is 
 visibly evolved, the hydrogen from which that 
 oxygen is separated must be supposed to travel 
 through the water to the negative wire, and only to 
 become visible when it meets the point of that wire; 
 and if, on the other hand, the decomposition be 
 
 * Nicholson's Journal, vol. iv. p. 511. 
 i 4 
 
120 INTRODUCTION. 
 
 imagined to take place at the negative wire where 
 the hydrogen is visibly evolved, the oxygen must 
 be supposed to pass invisibly through the water to 
 the point of the positive wire, and there become 
 visible. But what appeared still more unaccount- 
 able was, that in the experiment of Ritter it would 
 seem that one or other of the elements of the water 
 must have passed through the intervening sulphuric 
 acid. So impossible did such an invisible transfer 
 appear to Ritter, that at that time he regarded his 
 experiment as proving that one portion of the water 
 acted on was wholly converted into oxygen, and 
 the other portion into hydrogen. * 
 
 (123.) This point was the first to attract the 
 attention of Davy, and it occurred to him to try if de- 
 composition could be produced in quantities of water 
 contained in separate vessels united by a conducting 
 substance, placing the positive wire in one vessel 
 and the negative in the other. For this purpose, 
 the positive and negative wires were immersed in 
 two separate glasses of pure water. So long as the 
 glasses remained unconnected, no effect was pro- 
 duced ; but when Davy put a finger of the right 
 hand in one glass and of the left hand in the other, 
 decomposition was immediately manifested. The 
 same experiment was afterwards repeated, making 
 the communication between the two glasses by a 
 chain of three persons. If any material principle 
 passed between the wires in these cases, it must 
 have been transmitted through the bodies of the 
 persons forming the line of communication between 
 the glasses. 
 
 * Nicholson's Journal, vol. iv. p. 51 2. 
 
DAVY'S RESEARCHES. 121 
 
 The use of the living animal body as a line of 
 communication being inconvenient where expe- 
 riments of long continuance were desired, Davy 
 substituted fresh muscular animal fibre, the con- 
 ducting power of which, though inferior to that of 
 the living animal, was sufficient. When the two 
 glasses were connected by this substance, decom- 
 position accordingly went on as before, but more 
 slowly. 
 
 To ascertain whether metallic communication be- 
 tween the liquid decomposed and the pile was 
 essential, he now placed lines of muscular fibre 
 between the ends of the pile and the glasses of 
 water respectively, and at the same time connected 
 the two glasses with each other by means of a me- 
 tallic wire. He was surprised to find oxygen evolved 
 in the negative, and hydrogen in the positive glass, 
 contrary to what had occurred when the pile was 
 connected with the glasses by wires. In none of 
 these cases did he observe the disengagement of gas 
 either from the muscular fibre or from the living 
 hand immersed in the water. 
 
 (124.) In October, 1800, after many experiments 
 on the chemical effects of the pile, Davy commenced 
 an investigation of the relation which its power had 
 to the chemical action of the liquid conductor on the 
 more oxydable of its metallic elements. The in- 
 fluence of chemical decomposition in evolving the 
 Voltaic electricity originally maintained by Fabroni, 
 was again brought under inquiry by Colonel Hal- 
 dane. Davy showed that at common temperatures 
 zinc, connected with silver, suffers no oxydation in 
 water which is well purged of air and free from 
 
122 INTRODUCTION. 
 
 acids ; and that with such water as a liquid con- 
 ductor, the pile is incapable of evolving any quan- 
 tity of electricity which can be rendered sensible 
 either by the shock or by the decomposition of 
 water ; but that if the water used as a liquid con- 
 ductor hold in combination oxygen or acid, then 
 oxydation of the zinc takes place, and electricity is 
 sensibly evolved. In fine, he concluded that the 
 power of the pile appeared to be, in great measure, 
 proportional to the power of the liquid between the 
 plates to oxydate the zinc.* 
 
 He inferred from these results that although the 
 exact mode of operation could not be accounted 
 for, the oxydation of the zinc in the pile, and the 
 chemical changes connected with it, were somehow 
 the cause of its electrical effects. 
 
 (125.) To ascertain whether a liquid solution ca* 
 pable of conducting the electric current between the 
 positive and negative wires of a Voltaic pile, but not 
 capable of producing any chemical action on its 
 metallic elements, would, when used between its 
 plates, evolve electricity, Davy constructed a pile 
 in which the liquid was a solution of sulphuret of 
 strontia. When the current from an active pile 
 was transmitted through the liquid, the shock was 
 as sensible as if the communication had been made 
 through water ; but, on the other hand, solutions of 
 the sulphurets were incapable of acting chemically 
 on the zinc. If, therefore, chemical action on the 
 zinc be a necessary condition to ensure the activity 
 of the pile, such an arrangement must be inactive. 
 
 * Nicholson's Journal, vol. iv. p. 337. 
 
DAVY'S RESEARCHES. 123 
 
 Twenty-five pairs of silver and zinc plates, erected 
 with cloths moistened in solution of sulphuret of 
 strontia, produced no sensible action, though the 
 moment the sides of the pile were moistened with 
 nitrous acid, the ends gave shocks as powerful as 
 those of a similar pile constructed in the usual 
 manner. 
 
 The next question brought to the test of experi- 
 ment was, whether the chemical action which takes 
 place between the liquid and the plates of the pile 
 is of the same kind as that which is manifested 
 when water is decomposed by its extreme wires ; 
 that is, whether, when the oxygen is freed upon the 
 surface of the zinc, the remaining constituent of 
 the solution decomposed is also liberated at the sur- 
 face of the zinc, as in ordinary oxydation ; or is 
 transmitted invisibly through the fluid to the sur- 
 face of the silver, and there deposited, or otherwise 
 liberated, as in the decomposition between the po- 
 sitive and negative wires. An arrangement of zinc 
 and copper plates, in the form of the couronne des 
 tasses, was formed, and charged with spring water. 
 The general result of these experiments showed 
 that the hydrogen liberated by the zinc was 
 manifested not at the zinc, but at the silver sur- 
 face ; and therefore that the action in the cells is 
 similar to the decomposition of water at the ex- 
 treme wires of the pile. The phenomena were, how- 
 ever, rendered less decisive of the question by the 
 modifications produced by the azote of the common 
 air combined with the water, and also by saline 
 matter which it held in solution, effects which were 
 then imperfectly understood. 
 
1 24 INTRODUCTION. 
 
 (126.) The inventor of the pile maintained that, 
 among the metals, those which held the extreme 
 places in the scale of electro-motive power were silver 
 and zinc; and that, consequently, these metals, paired 
 in a pile, would be more powerful, ceteris paribus, 
 than any other. But as he also showed that pure 
 charcoal was a good conductor of the electric cur- 
 rent, and that the electro-motive virtue depended 
 on the different conducting powers of the metallic 
 elements, it was consistent with analogy that char- 
 coal, combined with another substance of different 
 conducting power, would produce Voltaic action. 
 Dr. Wells accordingly showed that a combination 
 of charcoal and zinc produced sensible convulsions 
 in the frog ; and Davy, adopting this principle, 
 constructed a couronne des tasses, consisting of a 
 series of eight glasses, with small pieces of well- 
 burned charcoal connected with zinc by pieces of 
 silver wire, using a solution of red sulphate of iron 
 as the liquid conductor. This series gave sensible 
 shocks, and rapidly decomposed water. Compared 
 with an equal and similar series of silver and zinc, 
 its effects were much stronger. Hence he inferred 
 that charcoal and zinc formed a combination equal, 
 if not superior, to any of the metals. 
 
 (127.) Volta was understood to refer the electro- 
 motive power of the metallic elements of the pile to 
 the difference of their powers as conductors of elec- 
 tricity. The experiments of Davy induced him to 
 connect the electro-motive power with the amount 
 of chemical action on the more oxydable metal 
 These two principles might, nevertheless, be con> 
 patible, if it could be shown that the oxydation wai 
 
DAVY'S RESEARCHES. 125 
 
 dependent on, and proportional to, the difference 
 of conducting power of the metals. To test this, it 
 was only necessary to construct a pile with metals 
 of nearly equal conducting power. With this view, 
 Davy constructed a pile with gold and silver plates, 
 these metals being supposed to differ very little in 
 their power of conducting electricity, interposing 
 discs of cloth moistened with dilute nitric acid. 
 Voltaic action was produced. A similar pile, 
 formed of plates of silver and copper, and a so- 
 lution of nitrate of mercury, acted powerfully. The 
 conducting powers of these several metals were 
 then considered as nearly equal.* 
 
 (128.) In considering the various arrangements 
 and combinations in which Voltaic action had been 
 manifested, Davy observed, as a common character, 
 that, in every case, one of the two metallic elements 
 was oxydated, arid the other not. Did the produc- 
 tion of the electric current, then, depend merely on 
 the presence of two metallic surfaces, one under- 
 going oxydation, separated by a conductor of elec- 
 tricity ? and, if so, might not a Voltaic arrange- 
 ment be made by one metal only, if its opposite 
 surfaces were placed in contact with two different 
 liquids, one of which would oxydate it, and the 
 other transmit electricity without producing oxyda- 
 tion ? To reduce this to the test of experiment 
 with a single metallic plate would have been easy ; 
 but in constituting a series or pile, the two liquids, 
 the oxydating and the non-oxydating, must be in 
 
 * The relative conducting power of the metals has not even 
 yet been satisfactorily established. 
 
126 INTRODUCTION. 
 
 contact, and subject to intermixture. To overcome 
 this difficulty, different expedients were resorted to, 
 with more or less success ; but the most convenient 
 and effectual method of attaining the desired end 
 was suggested to Davy by Count Rumford. Let an 
 oblong trough be formed, similar to that suggested 
 by Cruickshank, as a substitute for the pile ; and let 
 grooves be made in it such as to allow of the inser- 
 tion of a number of plates, by which the trough 
 may be divided into a series of water-tight cells* 
 Let plates of the metal of which the apparatus is to 
 be constructed be made to fit these grooves ; and let 
 as many plates of glass or other non-conducting 
 material, of the same form and magnitude, be pro- 
 vided. Let the metallic plates be inserted in alter- 
 nate grooves of the trough, and the glass plates in 
 the intermediate grooves, so as to divide the 
 trough into a succession of separate cells, each cell 
 having on one side metal, and on the other glass. 
 Let such an arrangement be represented in fig. 1., 
 
 where the metallic plates are represented at M, the 
 intermediate plates being glass. Let the alternate 
 cells O be filled with the oxydating liquid, and the 
 intermediate cells L with the liquid which con- 
 ducts without oxydating. Let slips of moistened 
 cloth be hung over the edge of each of the glass 
 
DAVY'S RESEARCHES. 12? 
 
 tubes, so that its ends shall dip into the liquids in 
 the adjacent cells. This cloth, or rather the liquid 
 it imbibes, will conduct the electric current from 
 cell to cell, without permitting the intermixture of 
 the liquids. 
 
 In the first arrangements made on this principle, 
 the most oxydable metals, such as zinc, tin, and 
 some others, were tried. The oxydating liquid O 
 was dilute nitric acid, and the liquid L was water. 
 In a combination consisting of twenty plates of 
 metal, sensible but weak effects were produced on 
 the organs of sense, and water was decomposed 
 slowly by wires from the extremities. The wire 
 from the end towards which the oxydating surfaces 
 were directed evolved hydrogen, and the other 
 oxygen. 
 
 To determine whether the evolution of the elec- 
 tric current was dependent on the production of 
 oxydation, or would attend other chemical effects 
 producible by the action of substances in solution 
 upon metal, the oxydating liquid was now replaced 
 by solutions of the sulphurets, and metallic plates 
 were selected on which these solutions would exert 
 a chemical action. Silver, copper, and lead were 
 tried in this way. Solution of sulphuret of potash 
 was used in the cells O, and pure water in L. A 
 series of eight metallic plates produced sensible ef- 
 fects. Copper was the most active of the metals 
 tried, and lead the least so. In these cases, the 
 terminal wires produced, in the usual manner, 
 the decomposition of water, the wire from which 
 hydrogen was evolved being that which was con- 
 nected with the end of the series to which the 
 
128 INTRODUCTION. 
 
 surfaces of the metal not chemically acted on were 
 presented. 
 
 It will be observed that in this case the direction 
 of the electric current relatively to the surfaces of 
 the metallic plates was the reverse of the former. 
 When oxydation was produced, the oxydating sides 
 of the plates looked towards the negative end of 
 the series. Comparing these two effects, Davy was 
 led by analogy to suspect that if the cells O were 
 filled with an oxydating solution, while the cells L 
 were filled with a solution of sulphuret, or any 
 other which would produce a like chemical action, 
 the combined effect of the currents proceeding from 
 the two distinct chemical processes would be ob- 
 tained. This was accordingly tried, and the results 
 were as foreseen. The acid solution was placed in 
 the cells O, and the sulphuret in the cells L. A 
 series, consisting of three plates of copper or silver, 
 arranged in this way, produced sensible effects ; and 
 twelve or thirteen decomposed water rapidly. The 
 oxydating sides of the metal looked to the negative 
 end of the series. 
 
 (129.) As it appeared from former experiments 
 that charcoal possessed, as a Voltaic agent, the same 
 properties as the metals, the next step in this 
 course of experiments was naturally to try whether 
 a Voltaic arrangement could not be constructed 
 without any metallic element, by substituting char- 
 coal for the metallic plates in the series above de- 
 scribed. This was accomplished by means of an 
 arrangement in the form of the couronne des tasses. 
 Pieces of charcoal, made from very dense wood, 
 were formed into arcs ; and the liquids O and L 
 
BIOT AND F. CUVIER. 
 
 129 
 
 were arranged in alternate glasses, as represented 
 in fig. 2. The charcoal arcs C were placed so as 
 
 to have one end immersed in each liquid, ike inte: 
 mediate glasses being connected by slips of bi- 
 bulous paper P. When the liquid O was dilute 
 acid, and L water, a series consisting of twenty 
 pieces of charcoal gave sensible shocks, and decom- 
 posed water. This arrangement also acted, and with 
 increased intensity, when the liquid O was sulphuric 
 acid, and L was solution of sulphuret of potash. 
 
 (130.) The connection of chemical change with the 
 production of electricity in the pile, was too obvious 
 riot to attract the attention of other philosophers. 
 Pepys in England, and MM. Biot and Frederic 
 Cuvier in France, investigated the effect produced 
 by the pile on the atmosphere in which it was 
 placed. The former placed the pile in an atmo- 
 sphere of oxygen, and found that in the course 
 of a night 200 cubic inches of the gas had been 
 absorbed. In an atmosphere of azote the pile had 
 no action. MM. Biot and Cuvier also observed 
 the quantity of oxygen absorbed, and inferred from 
 their experiments that " although, strictly speaking, 
 the evolution of electricity in the pile was produced 
 by oxydation, the share which this had in pro- 
 ducing the effects of the instrument bore no com- 
 parison with that which was due to the contact of 
 
ISO INTRODUCTION. 
 
 the metals, the extremity of the series being in 
 communication with the ground." 
 
 (131.) Dr. Wollaston and Gautherot, on the other 
 hand, reproduced the principle advanced by Fabroni 
 and Creve. Wollaston maintained that chemical 
 action was not only the source of the electricity of 
 the pile, but also of the common electrical machine. 
 He showed that by conveying the electricity of the 
 machine to gold wires terminated in extremely fine 
 points the decomposition of water could be effected, 
 and that the phenomenon was the same as when 
 the decomposition was effected by Voltaic wires. 
 He maintained that the friction of the rubber was 
 attended with oxydation, and showed that the ma- 
 chine was ineffective in an atmosphere of dry 
 hydrogen, or any other gas in which chemical action 
 was not produced. 
 
 ( J 32.) If an oblong slip of wet paper have its ex- 
 tremities in contact with the poles of a Voltaic pile, 
 each half of the slip will be electrified; that which is in 
 contact with the positive pole will be positively elec- 
 trified, and that which is in contact with the negative 
 pole will be negatively electrified. If it be removed 
 from contact with the pile by a rod of glass, or other 
 nonconductor, its electric state will continue. This 
 means of producing electrical polarity was observed 
 by Volta, and about the same time by Erhman. 
 
 (133.) This fact suggested to Ritter the idea of 
 his secondary pile, which consisted of a series of 
 discs of a single metal alternated with cloth or card, 
 moistened in a liquid by which the metal would not 
 be affected chemically. If such a pile have its ex- 
 tremities nut in connection by conducting sub- 
 
HITTER'S SECONDARY PILES. 131 
 
 stances with the poles of an insulated Voltaic pile, 
 it will receive a charge of electricity in a manner 
 similar to the band of wet paper, one half taking a 
 positive and the other a negative charge ; and after 
 its connection with the primary pile has been broken, 
 it will retain the charge it has thus received. The 
 secondary pile, while it retains its charge, produces 
 the same physiological and chemical effects as the 
 Voltaic apparatus. 
 
 The polarity which the band of wet paper an, 
 the secondary pile acquire by their temporary con- 
 tact with the ends of a Voltaic apparatus, is a con- 
 sequence of their imperfect conducting power. 
 he electricity of each species appears to force its 
 /ay ^ rough the imperfect conductor till the two 
 opposite currents meet in the centre. 
 
 At the time of the discovery of the secondary 
 piles, it was known that a piece of metallic wire, 
 the ends of which had been placed in contact with 
 the poles of a Voltaic pile, does not instantly recovei 
 its natural state when its contact with the pile is 
 broken. 
 
 (134.) From the experiments of Davy and others, 
 it appeared that if a communication was made be- 
 tween the poles of an insulated pile and two glasses of 
 water, so that the water in the one would be charged 
 with positive, and the other with negative electricity, 
 a metallic wire connecting the two portions of water 
 would evolve oxygen gas at one point, and hydrogen 
 at the other. If, under such circumstances, the 
 connection of the glasses with the pile be suddenly 
 broken, the action of the wire will nevertheless 
 continue for some time, but its effects will be re- 
 K 2 
 
132 INTRODUCTION. 
 
 versed ; the point which before disengaged hydrogen 
 will now disengage oxygen, and vice versa. It ap- 
 pears, therefore, that the sudden suspension of the 
 action of the pile has the effect of reversing the 
 direction of the electric current which passes through 
 the wire.* 
 
 The continuance of the electric state of a wire 
 which had been used to connect the poles of a pile 
 after its separation from the pile was also demon- 
 strated by Oersted, who showed its effect on the or- 
 gans of a frog, f The same effect was produced by a 
 wire through which the current of a powerful elec- 
 trical machine had been transmitted. 
 
 (135.) From the chemical effects of the pile, Davy 
 turned his attention to its calorific powers. The 
 means of experimental investigation placed at his dis- 
 posal were enlarged by the apparatus of the laboratory 
 of the Royal Institution, which was now under his 
 direction. The Voltaic apparatus consisted of a series 
 of 150 pairs of four-inch plates of zinc and copper, 
 and a series of 50 pairs of zinc and silver of the 
 same magnitude. The plates were cemented into 
 four troughs of wood, according to the method pro- 
 posed by Cruickshank. Another apparatus was 
 provided, consisting of a series of twenty pairs of 
 thirteen-inch plates of zinc and copper. 
 
 With the batteries of the smaller plates he re- 
 peated some of the experiments on the production 
 of the spark, and the combustion of the metals 
 which had already been made. When the poles 
 
 * Histoire de Galvanism de Sue, torn. iii. p. 341. 
 f Journ. de Opim. de Van-Mons, No. iv. p. 6' 8. 
 
DAVY'S RESEARCHES. 133 
 
 consisted of two knobs of brass, the spark which 
 attended the discharge was of dazzling brightness, 
 and one eighth of an inch in apparent diameter. 
 Between pieces of charcoal it had a vivid whiteness, 
 and the charcoal remained red hot for some time 
 after the contact was broken, and threw off bright 
 coruscations. The current passing through steel 
 wire -j^oth ^ an ^ nc ^ * n diameter, rendered it white 
 hot, and caused it to burn with great splendour. 
 Gold, silver, copper, tin, lead, and zinc were also 
 burnt Platinum in thin slips was rendered white 
 hot and fused. 
 
 (136.) Fourcroy, Vauquelin, and Thenard had 
 investigated the different effects produced by enlarg- 
 ing the plates of a battery, and by increasing their 
 number. They demonstrated that the power of the 
 apparatus to heat and ignite metallic substances 
 was augmented by enlarging the plates, without in- 
 creasing their number ; but that no increase of 
 power to decompose water, or to produce the shock, 
 ensued. The calorific power, therefore, appeared 
 to depend, ceteris paribus, on the magnitude of the 
 plates, while the chemical and physiological power 
 depended on their number. 
 
 The battery of thirteen-inch plates was tried suc- 
 cessively with pure water, a solution of common 
 salt, and dilute nitric acid. With water its effects 
 were feeble, with the solution of salt they were 
 much more considerable, and were still more en- 
 ergetic with nitric acid. With the last three inches 
 of iron wire, y^oth of an inch in diameter, were 
 rendered white hot, and two inches of the same 
 K 3 
 
134 INTRODUCTION. 
 
 wire were fused. The action of the water, feeble as 
 it was, was ascribed to the air and saline matter it 
 held in solution; and it was judged from analogy 
 that water perfectly purged of air, and free from 
 all saline substances, would have no Voltaic action. 
 A pile of thirty-six pairs of five-inch plates lost its 
 activity in an atmosphere of azote and hydrogen 
 in about two days ; and its power was constantly 
 restored by common air, and rendered more intense 
 by oxygen gas. 
 
 (137.) When two pieces of well-burnt charcoal, or 
 a piece of charcoal and a metallic wire, are connected 
 with the apparatus and immersed in water, on com- 
 pleting the circuit, gas was abundantly evolved, 
 and the points of the charcoal appeared red hot for 
 some time after the contact was made. Sparks were 
 also produced by means of charcoal points immersed 
 in concentrated nitre and sulphuric acids. When two 
 charcoal points acted in water, the gaseous products 
 consisted of one eighth carbonic acid, one eighth 
 oxygen, and one eighth inflammable gas, apparently 
 hydrogen. The gases produced by a similar process 
 from alcohol, ether, and dilute sulphuric acid, were 
 also a mixture of oxygen and hydrogen. In all 
 these cases it appeared that the gases proceeded 
 chiefly from the decomposition of the water con- 
 tained in the several solutions. 
 
 The effects of the ignition of charcoal in muriatic 
 acid confined over mercury were next tried. The 
 charcoal being kept white hot for nearly two hours, 
 the gas was very little reduced in volume, and the 
 charcoal was not sensibly consumed. When the 
 
THEORY OF GROTTHUS. 135 
 
 gas was examined, three fourths of it were absorbed 
 by water, and the remainder was inflammable.* 
 
 (138.) Of the theories proposed at this early pe- 
 riod of the experimental inquiry to explain chemical 
 decomposition by the Voltaic apparatus, that of 
 GROTTHUS was the earliest and most plausible. To 
 simplify the view of this theory, we shall take as an 
 example of its application the decomposition of 
 water. Each molecule of water being composed of 
 a molecule of oxygen and a molecule of hydrogen, 
 their natural electricities are in equilibrium when 
 not exposed to any disturbing force, each possessing 
 equal quantities of the positive and negative fluids. 
 The electricity of the positive wire acting by in- 
 duction on the natural electricities of the contigu- 
 ous molecule of water, attracts the negative and 
 repels the positive fluid. It is further assumed in 
 this theory, that oxygen has a natural attraction for 
 negative, and hydrogen for positive electricity; 
 therefore the positive wire in attracting the negative 
 fluid of the contiguous molecule of water, and re- 
 pelling its positive fluid, attracts its constituent 
 molecule of oxygen, and repels its molecule of hy- 
 drogen. The particle of water, therefore, places 
 itself with its oxygen next the positive wire, and its 
 hydrogen on the opposite side. The positive elec- 
 tricity of the first particle of water thus accumu- 
 lated on its hydrogen molecule, produces the same 
 action on the succeeding molecule of water as the 
 wire did upon the first molecule; and a similar 
 arrangement of the second molecule of water is 
 
 * Davy's Works, vol. ii. p. 214. London, 1839. 
 K 4 
 
136 INTRODUCTION. 
 
 effected. This second molecule acts in like manner 
 on the third, and so on. All the particles of water 
 between the positive and negative wires thus as- 
 sume a polar arrangement, and have their natural 
 electricities decomposed ; the negative poles and 
 oxygen molecules looking towards the -positive wire, 
 and the positive poles and hydrogen molecules look- 
 ing towards the negative wire. The attraction of 
 the positive wire now separates the oxygen mole- 
 cule of the contiguous particle of water from its hy- 
 drogen molecule, neutralizes its negative electricity, 
 and either dismisses it in the gaseous form, or com- 
 bines with it, according to the degree of the affinity 
 of the metal of the wire for oxygen. The hydrogen 
 molecule thus liberated effects in like manner the 
 decomposition of the second particle of water, com- 
 bining with its oxygen, and thus again forming 
 water and dismissing its hydrogen. The latter acts 
 in the same manner on the next particle of water, 
 and so on. Thus, a series of decompositions and re- 
 compositions are supposed to be carried on through 
 the fluid, until the process reaches the particle of 
 water contiguous to the negative wire, and the mole- 
 cule of hydrogen there disengaged gives up. its 
 positive electricity, by which an equal portion of 
 negative electricity proceeding from the wire is neu- 
 tralized, and the molecule of hydrogen escapes in 
 the gaseous form. It is equally compatible with 
 this theory to suppose the series of decompositions 
 and recompositions to commence at the negative 
 and terminate at the positive wire, or to commence 
 simultaneously at both, and terminate at any inter- 
 mediate point by the union of the last molecule of 
 
THEORY OF GBOTTHUS. 137 
 
 oxygen disengaged in the one series with the last 
 molecule of hydrogen disengaged in the other. 
 
 Grotthus illustrated this ingenious hypothesis by 
 comparing the supposed phenomena with the me- 
 chanical effects produced when a number of elastic 
 balls ivory balls for example being suspended,, 
 so that their centres shall be in the same straight 
 line, and their surfaces mutually touch, either of 
 the extreme balls of the series being raised and let 
 fall against the adjacent one, the effect is propa- 
 gated through the series, and the last ball alone 
 recoils in consequence of the impact ; and although 
 the action and reaction are suffered by each ball of 
 the series, and each is instrumental in transmitting 
 the effect, no visible change takes place in any ball 
 except the last, and the effect is continued by the 
 alternate action of the extreme balls until the mo- 
 tion is gradually stopped by the resistance of the air, 
 and other external causes. 
 
 (139.) The experiments of Davy, which have 
 been already mentioned, were only the prelude to 
 a brilliant series of discoveries, the commencement 
 of which burst upon the scientific world in his 
 Bakerian Lecture for the year 1806. As soon as 
 the splendid results detailed in that paper became 
 known in France, the members of the Institute, 
 rising superior to the feelings of national animosity 
 which at that time unhappily prevailed, unani- 
 mously conferred upon its distinguished author the 
 prize which had been established by Napoleon for 
 the best experiments on Voltaic electricity.* 
 
 * It is stated in the Memoirs of Davy by Dr. Paris (p. 168. ), 
 that the prize given to Davy was the annual medal, worth 
 
138 INTRODUCTION. 
 
 The genius, address, and perseverance of him 
 whose vocation is to investigate the laws of nature, 
 are not always confined to the grateful labour of 
 developing truths. The extirpation of error is a 
 task which, while it demands the exercise of equally 
 exalted powers, is never rewarded by that eclat 
 which surrounds the discovery of natural harmonies 
 before unobserved and unsuspected. In the com- 
 mencement of the series of researches now referred 
 to, Davy found it necessary to clear from his path 
 
 3000 francs, which was designed as a reward for the best expe- 
 riments in electricity which should be made in each year. The 
 same statement is made in a note by the editor in the fifth 
 volume of Davy's Works (p. 56.), edited by his brother Dr. 
 John Davy. " The minor prize of 3000 francs, founded by 
 Napoleon when first consul, for the most important result in 
 electrical research during each year, was awarded by the In- 
 stitute to the author for this paper: the principal prize of 
 60,000 francs, of which the preceding was only the interest, in 
 the opinion of the best judges was rather due to him, as it was 
 proposed to be given * a celui, qui par ses experiences et ses 
 decouvertes, fera a faire a 1'electricite et au galvanisme un pas 
 comparable a cela qu'ont fait faire a ces sciences Franklin et 
 Volta.' Thus the writer in the Quarterly Review already re- 
 ferred to remarks, * It was only questioned by those who were 
 capable of appreciating its importance, whether they acted with 
 strict impartiality in assigning to him the annual interest only, 
 when he appeared to have a fair claim to the principal. ' " 
 
 On the other hand, the French writers on electricity claim 
 the merit of having given Davy the higher prize: "Les 
 grandes decouvertes," says Becquerel (torn. i. p. 165.), " dont 
 Davy avait enrichi la science electro-chemique, le placaient 
 hors de ligne avec les autres physiciens qui avaient parcouru 
 la meme carriere depuis Volta; aussi PInstitut lui decerna-t-il 
 le prix de 60,000 f. qui avait ete promis par Napoleon a 1'auteur 
 des plus grandes decouvertes en electricite, comparables a celle 
 de Volta et de Galvani." Whether Davy received the higher 
 or the lower prize (we believe it was the lower), it is evident 
 that the French scientific authorities now think he was entitled 
 to the former. 
 
DAVY'S RESEARCHES. 139 
 
 certain difficulties, and, as he rightly conceived, 
 errors, by which his progress was obstructed. 
 
 (140.) When the decomposing powers of the pile 
 were first exhibited, the excitement attending a 
 discovery so unlocked for prevented the details of 
 the experiments from receiving all the attention to 
 which they were entitled. When the circumstances 
 attending the decomposition of water by the Voltaic 
 wires were submitted to closer examination, it was 
 found that indications of the presence of an acid 
 always existed at the pole where oxygen was evolved, 
 and those of an alkali at the other pole. In cases 
 where the water submitted to decomposition might 
 be supposed to hold saline matter in solution, such 
 effects would create no surprise ; but they were 
 unequivocally manifested when the water used was 
 distilled, and when there was every reason to think 
 it chemically pure. Mr. Cruickshank explained 
 this, by supposing the acid to be nitrous acid, pro- 
 ceeding from the combination of the azote of the 
 common air held in solution by the water with 
 the oxygen evolved at the positive wire ; and the 
 alkali to be ammonia, proceeding from the com- 
 bination of the same principle with the hydrogen 
 evolved at the negative wire. Desormes maintained 
 that the acid was muriatic ; and Brugnatelli that it 
 was an acid sui generis, produced by the combin- 
 ation of positive electricity with one of the con- 
 stituents of water, and called it electric acid. Some 
 maintained that the constituents of the acid and 
 alkali came over from the liquid used in the Voltaic 
 apparatus in some undiscovered manner along the 
 wires, and was thus deposited in the water ; and 
 
140 INTRODUCTION. 
 
 others held that it was generated out of the elements 
 of the water by Voltaic action. An article was 
 published in the " Philosophical Magazine*," by a 
 Mr. Peel of Cambridge, containing an account of 
 an experiment in which the water that remained, 
 after a large portion had been decomposed by the 
 pile, yielded on evaporation muriate of soda, al- 
 though the water used in the experiment had been 
 distilled with every precaution necessary to free it 
 from impurities. On inquiry being made at Cam- 
 bridge, no person corresponding with the name and 
 address of the professed author could be found ; 
 and the statement was concluded to be a mere 
 attempt to practise on the credulity of the scientific 
 world, when the surprise was revived by the publi- 
 cation of experiments actually made by Professor 
 Pacchionif of Pisa, in which the same result was 
 attained as was stated in the pretended Cambridge 
 experiment. Sylvester being led to the same con- 
 clusion, ascribed the supposed effects, in common 
 with Pacchioni, to the oxydation of hydrogen, on 
 the one hand in a higher, and on the other in a 
 lower degree than that which forms water. 
 
 Such were the confusion and obscurity in which 
 the community of science was involved on the sub- 
 ject of the Voltaic decomposition of water, when 
 the question was taken up by Davy. In common 
 with others, he had observed at an early period the 
 presence of an acid and alkali in water under the 
 process of decomposition ; but states, that so early 
 as 1800, he concluded from his experiments that 
 
 * Vol. xxi. p. 279. f Vol. xxii. p. 179. 
 
DAVY S RESEARCHES. 
 
 14J 
 
 Fig. 3. 
 
 the acid proceeded from the animal and vegetable 
 substances which he employed, and that the alkali 
 arose from the corrosion of the glass vessels in 
 which the experiment was conducted. Similar 
 inferences were made by the Galvanic Society of 
 Paris, by MM. Biot and Thenard, and by Dr. Wol- 
 laston ; the last of whom removed one of the 
 sources of these disturbing elements by the happy 
 expedient of connecting the positive and nega- 
 tive portions of water by a piece of well-washed 
 asbestos. 
 
 The investigation now undertaken by Davy was 
 commenced by decomposing distilled water in two 
 small cups of agate P N {fig. 3.), connected by a 
 piece of white transparent 
 amianthus A. The wires of 
 the Voltaic battery of 160 
 pairs of four-inch plates 
 were connected with the 
 water, the positive wire 
 being immersed in the cup 
 P, and the negative wire in 
 N. After the process had 
 been continued for forty- 
 eight hours, the water in 
 the cup P was found to 
 redden litmus paper, and 
 turmeric paper was affected 
 by the water in N. It ap- 
 peared, therefore, and fur- 
 ther experiment confirmed the indication, that acid 
 was present in the positive water, and alkali in the 
 negative. 
 
142 INTRODUCTION. 
 
 This result, after all the precautions which had 
 been taken, was quite unexpected, and, as may be 
 imagined, gave not a little surprise to the experi- 
 menter. Still he did not for a moment entertain 
 any of the speculations of the generation of these 
 substances in the water. His next step was to re- 
 peat the experiment with glass instead of agate 
 cups, using the same quantities of the same water, 
 and exposing them for the same time to the action 
 of the same battery. He argued, that if the cause 
 lay in the water, the effects would be the same; 
 but that if the cups had any share in producing 
 them, they might be expected to be different. The 
 result confirmed his anticipation. The alkali was 
 produced in the cup N in quantity twenty times as 
 great as with the agate cups, but there was no trace 
 of the acid. The experiments were then repeated 
 several times with the agate cups, when the acid 
 and alkali reappeared in quantities, which, when 
 compared with each other and with the result of 
 the experiment with glass cups, left no doubt that 
 the agate cups themselves had been the chief if not 
 the only source of the acid, and, in a considerable 
 degree, of the alkali also. Still it was impossible to 
 ascrfbe the effects altogether to the material of the 
 cups ; and he was impressed with the suspicion that 
 the water itself, notwithstanding its careful distil- 
 lation, must have held more or less alkaline matter 
 in solution. It was known that the usual tests 
 would fail to indicate the presence of alkaline im- 
 purities when their proportion in water was under a 
 certain limit ; and the New River water, which he 
 used, contained animal and vegetable impurities, 
 
Fig. 4. 
 
 DAVY'S RESEARCHES. 143 
 
 which might furnish neutral salts capable of being 
 carried over in the process of distillation. 
 
 The agate cups were now replaced by two conical 
 cups of pure gold (fig. 4.), each containing about 
 twenty-five grains of 
 water. Distilled water 
 in these was exposed 
 to the action of a bat- 
 tery of 100 pairs of 
 six-inch plates. In ten 
 minutes indications of 
 acid and alkali were 
 formed in the cups D 
 and N respectively. 
 The process was con- 
 tinued for fourteen hours, during the whole of 
 which time the acid increased in the cup D. The 
 same increase was not, however, observed in the 
 alkali in the cup N ; on the contrary, it reached its 
 maximum state in a short time, and continued with- 
 out increase afterwards. On heating the cup N, the 
 alkali diminished, but could not be altogether dis- 
 missed. 
 
 These experiments being repeated with similar 
 results, it became apparent that the source of the 
 acid and alkali must exist in the water itself, and 
 must either have arisen from saline matter remaining 
 in solution in the water after distillation, or have 
 been produced by the azote, which exists in minute 
 portions in all water exposed to the air. The latter 
 supposition would not be incompatible with the cir- 
 cumstance of the alkali speedily attaining a maxi- 
 mum, since the continued absorption of azote from 
 
I4t4f INTRODUCTION. 
 
 the atmosphere by the water would be stopped 
 when the latter would become charged with hy- 
 drogen. 
 
 The former supposition was adopted, and it was 
 determined to submit the water which had been used 
 in the last experiments to slow re-distillation. A 
 quart of this water was accordingly evaporated in a 
 silver still at a temperature below 140, and a saline 
 residuum was obtained weighing seven tenths of a 
 grain. 
 
 The gold cups were now again filled with the 
 water thus purified, and exposed to the Voltaic ac- 
 tion. After two hours, the cup N failed to show 
 any alkaline effect on turmeric paper. By very mi- 
 nute observation, its effect on the more delicate test 
 of litmus was perceivable ; but this disappeared by 
 the application of heat, and was therefore ascribed 
 to ammonia produced by the combination of the 
 small quantity of azote contained in the water with 
 the nascent hydrogen. 
 
 Finally, in order to insulate the results from the 
 disturbing effects of the surrounding atmosphere, 
 the gold cups containing the purified water were 
 placed under the receiver of an air-pump, which 
 was exhausted until the gauge stood at half an inch. 
 Hydrogen gas was then introduced under the re- 
 ceiver, which, mixed with the very minute portion 
 of atmospheric air which had remained, was again 
 withdrawn by the pump. Pure hydrogen gas was 
 now once more introduced around the cups, which, 
 being placed in connection with the Voltaic appa- 
 ratus, were suffered to remain under its action for 
 twenty-four hours, at the end of which time neither 
 
DAVY'S RESEARCHES. 145 
 
 of the portions of the water altered in the slightest 
 degree the tint of litmus. 
 
 Thus were dispelled the speculations on the power 
 of electricity to generate new principles in water; and 
 by eliminating the disturbing action of other causes, 
 the decomposing power of the pile upon a binary 
 compound was presented in a manner fitted for 
 theoretical investigation. 
 
 (141.) If chance occasionally deprives the philo- 
 sopher of the merit of discovery by throwing facts 
 under his feet, an ample field for the exercise of his 
 sagacity remains in the due appreciation of the in- 
 numerable effects which are incidental to his expe- 
 rimental researches : to seize which as they arise, to 
 pursue them through their consequences, to strip 
 them of the Protean disguises which they borrow 
 from other phenomena with which they become re- 
 lated, to expand them by comparison and general- 
 ization into comprehensive natural laws, is the 
 province of the highest powers of philosophical 
 inquiry. Never was this felicitous instinct more 
 conspicuous than in the mind of Davy. No effect, 
 however minute or accidental it might apparently 
 be, presenting itself in his experiments, escaped his 
 vigilance, if it offered the least clue to further dis- 
 covery. In the course of the experiments just 
 noticed, he found himself embarrassed by the dis- 
 turbing action of the Voltaic wires on the material 
 of the vessels containing the liquid, which was the 
 immediate object of his attention. One material 
 after another was put aside to get rid of this effect ; 
 but the fact was not overlooked or forgotten. It 
 proved the germ of a vast discovery. 
 
146 
 
 INTRODUCTION. 
 
 The negative wire effected a partial decompo- 
 sition of the glass and agate cups, and brought a 
 portion of their constituents into solution in the 
 water contained in them. Might not a power, which 
 thus subdued affinities so stubborn as those which 
 produce the aggregation of substances so insoluble 
 as agate and glass, be brought to bear on other 
 similar bodies, arid perchance resolve into their 
 components substances now considered simple and 
 elementary ? As a first trial of the decomposition 
 of insoluble or difficultly soluble bodies, cups were 
 formed of wax, resin, marble, argillaceous schist 
 from Cornwall, serpentine from the Lizard, and 
 graywacke. Being filled with purified water * in 
 the same manner as in the experiments above de- 
 scribed, decomposition was in all cases effected and 
 saline matter evolved. 
 
 (142.) Pursuing this investigation, he successively 
 decomposed by the same process sulphate of lime, 
 sulphate of strontia, fluate of lime, sulphate of baryta, 
 and other insoluble salts, and in each case obtained 
 the acid in the positive and the base in the negative 
 cup. Certain mineral substances, such as basalt, 
 zeolite, and vitreous lava from ^Etna, were examined ; 
 and although the saline ingredients in some cases 
 prevailed in extremely minute proportions, their pre- 
 sence was nevertheless distinctly manifested. The 
 soluble compounds, such as sulphate and nitrate of 
 potash, sulphate and phosphate of soda, were easily 
 decomposed, and the results were the same. 
 
 * By purified water in all the following experiments is to 
 be understood water rendered chemically pure by the processes 
 described in p. 144. 
 
DAVYS RESEARCHES. 147 
 
 The metallic salts deposited their metallic ele- 
 ments in crystals on the negative wire, round 
 which the oxide was also deposited, while the acid 
 was collected in the positive cup. 
 
 These, however, were only the first and least im- 
 portant of the consequences of the idea of extend- 
 ing the principle in virtue of which the Voltaic 
 wire corroded the glass. We shall dismiss this for 
 the present to consider the next series of experi- 
 ments in these researches, but shall resume the 
 subject. 
 
 (143.) From many of his own experiments, and 
 some described by Gautherot, Hisinger, Berzelius. 
 and Ritter, it was apparent that the Voltaic influence 
 was capable not only of decomposing compound 
 bodies, but also of transferring, or, if the term 
 may be permitted, decanting their constituents from 
 one vessel to another. The series of experiments 
 which follows next in order in these researches 
 was directed to the examination of the limits of 
 that power, and the effects attending it under con- 
 ditions not before tried. 
 
 When the substance to be decomposed was in- 
 soluble, it was formed into a cup, as in the preced- 
 ing experiments, and water contained in it was 
 exposed to the Voltaic action. Thus, let A, fig. 5., 
 be an agate cup, and S a cup 
 made of the substance to be sub- 
 mitted to Voltaic action. Let 
 them each be filled with purified 
 water and connected by asbestos. 
 If A be connected with the positive and S with 
 the negative wire, it was expected that any acid 
 
148 INTRODUCTION. 
 
 constituent which may be in the substance of 
 which S is formed would pass into A, which would 
 become an acid solution, and appear by the applica- 
 tion of the usual tests. If, on the other hand, A be 
 connected with the negative and S with the posi- 
 tive wire, any alkali which may be in the sub- 
 stance of which S is formed was expected to pass 
 into A, and to be manifested there by the common 
 alkaline tests. 
 
 In the first case in which his method was tried, 
 the cup S was formed of sulphate of lime. The 
 cup A was connected with the negative and S with 
 the positive wire. With a battery of 100 pair of 
 plates, the water in A was in about four hours con- 
 verted into a strong solution of lime, and the liquid 
 in S was converted into sulphuric acid. When the 
 cup A received the positive and S the negative wire, 
 the effects were reversed. In that case the water 
 in A became sulphuric acid, and a solution of lime 
 was found in S. 
 
 Other saline cups were submitted to the same 
 process with like results ; the water in the positive 
 cup always receiving acid, and that in the negative 
 cup alkalL 
 
 Two cups of glass were connected with the poles 
 of the battery. One was filled with distilled water, 
 and the other with a saline solution. In every case 
 the salt was decomposed, the base passing into or 
 remaining in the negative, and the acid in the 
 positive cup. 
 
 The time required for these transmissions ap- 
 peared to increase, ceteris paribus> as the space 
 through which the decomposed elements were to be 
 transmitted increased. 
 
DAVYS RESEARCHES. 
 
 149 
 
 Fig. 6. 
 
 To determine whether the action of the me- 
 tallic wires proceeding from the Voltaic battery was 
 immediately engaged in the production of these de- 
 compositions, the next experiments were arranged 
 so that the electric current should be transmitted to 
 the solution to be decomposed through liquid con- 
 ductors. For this purpose, three cups (P, I, and N, 
 fig. 6.) were provided; the extreme ones P and N 
 receiving the positive 
 and negative wires from 
 the battery, and the cup 
 I connected with each 
 of them by amianthus. 
 The cups P and N were 
 filled with purified wa- 
 ter, and the solution to 
 be decomposed was put 
 into the intermediate 
 cup I. In every case 
 the acid constituent of the solution was decanted 
 into P, and the alkaline into N. Lest the amian- 
 thus siphons should have any mechanical effect on 
 the transference of the solution between the cups, 
 the cups P and N were so filled that the surfaces 
 of the water in them were above that of the solu- 
 tion in I. 
 
 (145.) As it was now abundantly apparent that 
 the elements of the decomposed substance were 
 drawn from cup N through the interstices of the 
 siphons, it was determined to try how far this de- 
 canting power could be carried by breaking the con- 
 tinuity of the siphons, and rendering it impossible 
 for the decomposed element to reach its destination 
 L 3 
 
150 INTRODUCTION. 
 
 without passing through an intermediate liquid. 
 For this purpose, the three cups being arranged as 
 before, two of them, P and I, were filled with 
 distilled water, the water in I being tinged with 
 litmus ; and the negative cup N was filled with a 
 solution of the sulphate of potash. If the energy 
 of the attraction of the positive wire for the acid 
 constituent of the salt were sufficiently strong to 
 cause it to pass from N to P, through the liquid in 
 I, it was naturally expected that, on its route, its 
 presence in I would be rendered manifest by the 
 usual effect of reddening the litmus. The acid 
 passed from N to P through I, but without being 
 manifested in I by any redness of the solution. 
 
 When the saline solution was put in the positive 
 cup P, and the purified water in the negative cup 
 N, the water in I being tinged with turmeric, the 
 alkali passed in like manner from P to N without 
 producing any effect on the colour of the liquid 
 in I. 
 
 (146.) As the transmission of acid or alkali by 
 means of the electric current through water tinged 
 with vegetable colours was effected without pro- 
 ducing any sensible change in these delicate tests of 
 their presence, it was conjectured that, while in this 
 state of transition, or electrical progression, the che- 
 mical elements were deprived of their wonted pro- 
 perties, and that therefore they would equally pass 
 through solutions of substances for which, under or- 
 dinary circumstances, they exhibit a strong affinity, 
 that affinity being rendered dormant, or counteracted, 
 by the predominating influence of the electrical at- 
 traction. To reduce this conjecture to the test oi 
 
DAVY'S RESEARCHES. 151 
 
 experiment, the water tinged with vegetable colours 
 in the intermediate cup I was replaced by a weak 
 solution of ammonia, purified water was put into 
 the cup P, and a solution of the sulphate of potash 
 in the cup N. The sulphuric acid, attracted by the 
 positive wire, could only reach the cup P by pass- 
 ing through the solution of ammonia. With a 
 battery of 150 pairs, the presence of the acid in P 
 was manifested in five minutes by litmus paper. 
 In half an hour, the solution in P became sour to 
 the taste, and precipitated solution of nitrate of ba- 
 ryta. Thus the sulphuric acid passed through the 
 solution of ammonia in I without producing upon it 
 any chemical change. Solutions of lime, potash, 
 and soda were successively substituted, with similar 
 results. 
 
 Muriate of soda and nitrate of potash were suc- 
 cessively placed in the cup N, and the muriatic 
 and nitric acids made to pass through concentrated 
 alkaline menstrua in I without any chemical effects. 
 
 The neutral salts of lime, potash, soda, ammonia, 
 and magnesia in solution, were successively placed 
 in the cup P, distilled water in N, and sulphuric, 
 nitric, and muriatic acids successively in the inter- 
 mediate cup I. The alkaline elements of the salts 
 were thus drawn through the acids, and decanted 
 into N, without undergoing any change themselves, 
 or causing any change in the acids. 
 
 (147.) Strontia and baryta passed freely by a si- 
 milar process through muriatic and nitric acids, and 
 reciprocally these acids passed with equal facility 
 through solutions of strontia and baryta. The uni- 
 formity of this series of phenomena was, however, 
 L 4 
 
152 INTRODUCTION. 
 
 broken when it was attempted to transmit the same 
 alkalies through sulphuric acid, or to pass sulphuric 
 acid through them. A new order of effects was 
 here evolved. 
 
 A solution of sulphate of potash was placed in 
 the cup N, distilled water in P, and a solution of 
 baryta in I. The sulphate of potash was decomposed 
 as before, and sulphuric acid passed from the ne- 
 gative cup on its route towards the positive wire; 
 but its progress was arrested in the intermediate 
 cup, where it was seized by the baryta and precipi- 
 tated. It appeared, however, that this obstruction 
 to the progress of the acid was not absolutely com- 
 plete ; for when the process was continued for 
 several days, traces of acid were found in the posi- 
 tive cup. When a solution of strontia was substi- 
 tuted for the baryta in the intermediate cup, the 
 effects were similar. 
 
 When the muriate of baryta was put in the posi- 
 tive cup, sulphuric acid in the intermediate cup I, 
 and water in the negative cup N, no alkali passed 
 to the cup N, all being arrested in I, where the sul- 
 phate of baryta was manifest, and muriatic acid 
 remained in the cup P. 
 
 It appeared, therefore, that the exception to the 
 transmission of the elements of bodies through 
 menstrua for which they have an affinity, includes 
 the cases in which the result of that affinity would 
 be an insoluble compound. The sulphates of stron- 
 tia and baryta are insoluble in water ; and sulphuric 
 acid cannot be transmitted, by the electric current, 
 through strontia or baryta, nor the latter through 
 the former. 
 
DAVY'S RESEARCHES. 153 
 
 The operation of these principles was very beau- 
 tifully illustrated by the following experiment: The 
 cups P and N were filled with solution of muriate 
 of soda, and the cup I with solution of sulphate of 
 silver. The cup P was connected with I by a slip 
 of wet turmeric paper, arid the cup N was con- 
 nected with I by a slip of wet litmus paper. When 
 the operation of the battery commenced, the pre- 
 sence of soda in a free state was manifested in the 
 cup N, and muriatic acid in the cup P. The muri- 
 atic acid drawn from the cup N, through the litmus 
 paper, was seen to form a dense precipitate in the 
 cup I, and the soda passing through the turmeric 
 paper from the cup P was observed in the cup I 
 forming a more diffused and lighter precipitate. 
 But neither the acid in passing through the litmus 
 paper, nor the alkali in passing through the tur- 
 meric paper, produced any change in the colour of 
 these tests. 
 
 (148.) When salts having metallic oxides as bases 
 were placed in the cup P, acid solutions being put in 
 I, the oxides passed through the acids; but their 
 progress was much slower than that of the alkalies. 
 When a solution of the green sulphate of iron was 
 placed in P, and muriatic acid in I, the green oxide 
 of iron began to appear in about ten hours on the 
 amianthus connecting N and I ; and it took three 
 days to collect any considerable quantity of it in 
 the cup N. The results were similar when solutions 
 of sulphate of copper, nitrate of lead, and nitro- 
 muriate of tin were placed in the cup P. 
 
 (149.) The transmission of the constituents of 
 salts through solutions of the neutral salts was next 
 
154 INTRODUCTION. 
 
 tried, and the results were what was anticipated. 
 Saline solutions being placed in N and I, and puri- 
 fied water in P, the alkali of I first began to pass 
 into N; then the alkali of P, after passing through 
 I, reached N, and at the same time the acid of I 
 passed into P. Ultimately the two acids were col- 
 lected in P, and the two alkalies in N. As an ex- 
 ample of this, the cup N was filled with a solution 
 of the muriate of baryta, the cup I with sulphate of 
 potash, and the cup P with pure water. A battery 
 of 150 pairs brought sulphuric acid in five minutes, 
 and muriatic acid in two hours, into P. 
 
 When the cup P was filled with a solution of 
 sulphate of potash, I with muriate of baryta, and N 
 with distilled water, the baryta appeared in the 
 water in a few minutes; after an hour the potash 
 became sensible in it. 
 
 When the muriate of baryta was in P, the sul- 
 phate of potash in I, and water in N, the potash 
 soon appeared in the water; but the baryta was 
 arrested in the intermediate cup by the sulphuric 
 acid, and sulphate of baryta was abundantly pre- 
 cipitated. In like manner, when sulphate of silver 
 was placed in the cup I, muriate of baryta being in 
 N, and water in P, sulphuric acid alone passed into 
 P, and a precipitation took place in I. 
 
 (150.) The effects of the electric current on the 
 principles of vegetable and animal substances was 
 next tried. The fresh stalk of a polyanthus leaf was 
 used instead of the siphon of amianthus, to connect 
 the two cups P and N (fig. ?), the cup 1 being 
 omitted. The cup P was filled with a solution of 
 nitrate of strontia, and the cup N with purified 
 
DAVY S RESEARCHES. 
 
 155 
 
 water. The water soon became green, and showed 
 
 Fig. 7. 
 
 the presence of alkali ; and 
 the solution in the cup P 
 indicated the presence of 
 free nitric acid. After ten 
 minutes, the alkaline mat- 
 ter in N being examined 
 proved to be potash and 
 lime, but no strontia had 
 yet arrived in the cup. In 
 half an hour, however, 
 strontia appeared, and in 
 four hours was abundant. 
 
 A piece of the muscular flesh of beef was used 
 in like manner as a siphon connecting the two 
 cups, P containing a solution of muriate of baryta, 
 and N distilled water. Soda, ammonia, and lime 
 appeared first in the water, and after about an hour 
 and a quarter the baryta began to arrive. Mu- 
 riatic acid was abundantly liberated in the cup P. 
 
 It is nothing more than a general expression of 
 the phenomena which have been just detailed to 
 say, that hydrogen, alkaline matter, metals, and 
 certain metallic oxides, are attracted towards the 
 negative, and repelled from the positive pole of a 
 Voltaic apparatus ; and that oxygen and acid sub- 
 stances are affected with a similar attraction and 
 repulsion in the contrary direction. 
 
 (151.) As to the actual process by which the trans- 
 fer of the element decomposed takes place, either be- 
 tween the positive and negative wires in the solution 
 under decomposition, or through the intermediate 
 solution, no distinct opinion was expressed in the 
 
156 
 
 INTRODUCTION. 
 
 paper now noticed. Davy showed that it is natural 
 to suppose that the repellent and attractive energies 
 are conveyed from one particle to another of the 
 same kind, and that locomotion (of these particles) 
 takes place in consequence. He considered this to 
 be proved by many facts. Thus, when an acid was 
 drawn from the negative to the positive cup through 
 an alkaline solution contained in the intermediate 
 cup, if the Voltaic action was for a moment sus- 
 pended before the transfer of all the acid in the 
 negative cup had been effected, traces of acid were 
 always discoverable in the intermediate cup. It 
 appears from this that the series of acid molecules, 
 while moving between the ends of the amianthus 
 siphons in the intermediate cup, do not enter into 
 combination with the alkali ; but if the motion be 
 for a moment suspended, combination instantly 
 takes place. In this case, therefore, it would not 
 appear that any supposition of transmission by a 
 series of decompositions and recompositions is com- 
 patible with the phenomena. 
 
 In the cases, however, of the decomposition of 
 water (where the whole menstruum between the 
 decomposing wires is water), and of solution of 
 neutral salts (where also the menstruum is alto- 
 gether Qomposed of the same solution), he admits 
 that there may possibly be a succession of decom- 
 positions and recompositions throughout the fluid. 
 He admits also, that the impossibility of trans- 
 mitting through an acid or alkali any element 
 which forms with it an insoluble compound, al- 
 though the transmission is perfect when the com- 
 pound is soluble, supports the hypothesis of a sue- 
 
DAVY'S RESEARCHES. 157 
 
 cession of compositions and decompositions taking 
 place in every case. He maintains, that although 
 in some cases insoluble substances are transmitted, 
 the transmission is effected in a manner totally dif- 
 ferent from that which takes place in the more 
 general case. The insoluble matter was, in these 
 cases, carried over mechanically, either through 
 the interstices of the siphons, or by means of " a 
 thin stratum of pure water, where the solution had 
 been decomposed at the surface by carbonic acid." 
 It appears from the tenor of the observations in 
 this paper, " on the mode of decomposition and 
 transition," that the mind of the author had not yet 
 arrived at any opinion satisfactory to himself on 
 this subject. 
 
 By the experiments of Volta it had been shown 
 that different metals brought into contact were 
 oppositely electrified after separation. Davy found 
 that an acid and a metal being in contact, the 
 former became negative, and the latter positive ; 
 but that when an alkali and a metal were in con- 
 tact, the electrical effects were reversed. As a 
 general fact it appeared, therefore, that positive 
 electricity has a tendency to pass from acids to 
 metals, and from metals to alkalies, and negative 
 electricity to flow in the opposite direction. Dif- 
 ferent bodies were, therefore, regarded by Davy as 
 having with relation to each other specific electrical 
 energies. Acids have a negative and alkalies a 
 positive energy, with relation to metals ; while 
 metals have a positive energy with relation to acids, 
 and a negative energy with relation to alkalies. 
 Various experiments of a delicate kind were 
 
158 INTRODUCTION. 
 
 made to establish this general principle. To avoid 
 the disturbing effects which would be introduced 
 by chemical action, the substances of each kind 
 selected for experimental examination were in the 
 solid and dry form. When oxalic, succinic, benzoic, 
 or boracic acid, perfectly dry, either in powder or 
 crystals, was touched upon a large surface with a 
 disc of copper, zinc, or tin insulated, the metal 
 became positive, and the acid negative. Phos- 
 phoric acid and zinc gave a like result. 
 
 Metallic plates being brought in like manner in 
 contact with lime, strontia, magnesia, or soda, be- 
 came negative, the earths being positive. The 
 attraction of potash for water was too strong to 
 allow that alkali to be submitted to trial. Sulphur 
 became positive after contact with a metallic plate, 
 and the supposed exception to this in the case of lead 
 was removed by showing that the substance rubbed 
 against newly polished lead always became positive. 
 
 All these facts went to support the position, that 
 the electrical relation of different substances, as 
 shown by mere contact, was in harmony with the 
 law according to which electricity was developed in 
 the Voltaic apparatus, and with the phenomena of 
 decomposition. To complete the experimental proof 
 of this analogy, it would have been necessary to show 
 that oxygen has a negative and hydrogen a positive 
 electrical energy in relation to the metals. Not 
 being able to accomplish this, recourse was had to 
 the compounds of these substances. Sulphuretted 
 hydrogen in water, used in the Voltaic arrangement 
 of single metallic plates, plays the part of an alkali. 
 To support by a like analogy the negative character 
 
DAVY'S RESEARCHES. 159 
 
 of oxygen, he showed that oxymuriatic acid* 
 (chlorine) was more powerfully negative in rela- 
 tion to metal than muriatic acid, even in a higher 
 degree of concentration. 
 
 (152.) He assumed, as a principle suggested by 
 analogy and supported by experiment, that two bodies 
 which have contrary electrical energies in relation to 
 a third body have contrary electrical energies in re- 
 lation to each other ; that is to say, two bodies, A and 
 B, being successively brought into contact with a 
 third C ; if A is found to be positive after separa- 
 tion and B negative, then it follows that if A and 
 B be brought into mutual contact, A will be posi- 
 tive after separation and B negative. Lime and 
 oxalic acid in a dry and solid state, the former 
 being positive and the latter negative in relation to 
 metals, were brought into contact, and the electricity 
 collected after repeated contacts by a condensing 
 electrometer. The lime was found to be positive 
 and the acid negative. 
 
 Guided by the analogies suggested by such facts, 
 Davy maintained, as a general principle, that oxy- 
 gen and acid substances have a negative electrical 
 energy in relation to hydrogen and alkaline sub- 
 stances ; and that in the decompositions and changes 
 presented by the effects of electricity, the different 
 bodies naturally possessed of chemical affinities ap- 
 pear to be incapable of entering into combination 
 or of remaining in combination by virtue of these 
 affinities when they are placed in a state of elec- 
 tricity, contrary to the natural relation of their 
 
 * This substance was then supposed to be a compound. 
 
160 INTRODUCTION. 
 
 electrical energies. Thus the acids in the positive 
 part of the circuit separate themselves from the 
 alkalies, oxygen from hydrogen, and so on ; and 
 metals on the negative side do not unite with 
 oxgyen, and acids do not remain in union with their 
 oxides ; and in this way the attractive and repellant 
 agencies seem to be communicated from the me- 
 tallic surfaces (the poles of the pile) throughout 
 the whole of the menstruum. 
 
 (153.) In all cases in which bodies combine che- 
 mically, they are found to have contrary electrical 
 energies. Examples are numerous. The bodies in 
 the first of the following columns are all negative 
 with respect to those which are opposite to them in 
 the second : 
 
 Oxygen Zinc. 
 Oxygen Silver. 
 Copper Zinc. 
 
 Gold Mercury. 
 
 Metals Sulphur. 
 
 Acids Alkalies. 
 
 The constituent particles of each of these sub- 
 stances when brought into contact, being naturally 
 in opposite states of electricity, will, according to 
 the common laws of electricity, attract each other. 
 If they be solid bodies, the force of aggregation of 
 these particles, which constitutes the character of 
 their solidity, will resist their separation ; but if the 
 constituent particles be free to move and inter- 
 mingle among each other, then the attraction due 
 to their proper electricity will take effect, combin- 
 ation will ensue, the conditions of equilibrium of 
 the electrical forces will be satisfied, and all signs 
 of free electricity will cease. 
 
DAVY'S ELECTRO-CHEMICAL THEORY. l6l 
 
 In support of this hypothesis it is argued, 
 that when, by artificial means, the elements of any 
 compound are invested with electricity contrary to 
 that which naturally belongs to them, such electricity 
 exerting a force contrary to that which produces 
 or maintains, or tends to produce or maintain their 
 combination, that combination, if it exist, is dis- 
 solved, and if it tend to be effected, is prevented. 
 
 Thus zinc is one of the metals which have the 
 strongest natural tendency to combine with oxgyen. 
 Let it be charged with negative electricity, and its 
 oxydation becomes impossible, because, according 
 to Davy's hypothesis, the positive electricity natu- 
 rally belonging to its molecules is neutralized by 
 the negative electricity artificially imparted to it. 
 Again, silver is one of the metals which have the 
 least tendency to unite with oxygen ; but let silver 
 be charged with positive electricity, and it oxydates 
 easily. The positive electricity supplied artificially 
 gives increased power to that which the particles 
 possess, so as to augment their attraction for the 
 negative particles of the oxygen. 
 
 (155.) The cases of bodies which have contrary 
 electrical energies, either in relation to a third body 
 or in relation to each other, are therefore simple, and 
 easily apprehended. But two bodies may have 
 electrical energies with respect to a third, the same 
 in kind, but unequal in degree. Thus all acids are 
 negative in relation to metals, but any two of them 
 will be unequally so ; and in like manner all alkalies 
 are positive, but unequally positive in relation to 
 metals. Sulphuric acid is more negative than mu- 
 riatic acid in relation to lead, and potash is more 
 
 VOL. i. M 9 
 
12 INTRODUCTION. 
 
 positive than soda in relation to tin. Such bodies 
 compared with each other may have the same or 
 contrary electrical energies, or they may be neutral. 
 Sulphur and the alkalies are positive in relation to 
 the metals, but their electrical energies with respect 
 to each other are contrary. 
 
 (156.) The evolution of heat and light, which com- 
 monly attends the restoration of electrical equilibrium 
 between two bodies strongly charged with electricity 
 by artificial means, is brought by Davy in further 
 support of his theory. It is well known that heat 
 and light also result from intense chemical action. 
 When the electric current passes through bodies, 
 the electricity being then incomparably more feeble 
 in intensity than that which proceeds from the 
 common machine, heat is evolved without light, and 
 the degree of this heat is, ceteris paribus, augmented 
 as the intensity of the electricity is increased. In 
 the same manner in slow chemical combinations 
 there is an increase of temperature without lumi- 
 nous appearance. 
 
 (157.) Heat by producing fusion, and liberating 
 the constituent particles of bodies from their natural 
 aggregation, has been regarded as being conducive 
 to their chemical combination. In the theory pro- 
 posed by Davy it is, moreover, viewed as being 
 otherwise instrumental in giving play to the affi- 
 nities. That heat is one of the means of exalting 
 the electrical energy of bodies, is apparent from its 
 known effects on glass and tourmaline. But in the 
 experiments now noticed, more distinct and specific 
 evidence is adduced of its direct electric agency. 
 A plate of sulphur was placed on an insulated plate 
 
DAVY'S ELECTRO-CHEMICAL THEORY. 163 
 
 of copper, and the temperature of the bodies being 
 gradually elevated, their electrical state was ex- 
 amined at different stages of the experiment. At 
 56 the electricity was scarcely sensible to a con- 
 densing electrometer; at 100 it affected the gold 
 leaves without the condenser, and increased in a 
 still higher degree as the sulphur approached its 
 point of fusion. 
 
 Since heat, therefore, increases the natural elec- 
 trical energy of the component particles of bodies, 
 it gives them, according to the theory of Davy, an 
 increased tendency to combine chemically, if those 
 energies be contrary. Hence, when a spark, or 
 other sufficient source of heat, is introduced into a 
 mixture of oxygen and hydrogen, it renders the 
 contiguous molecules of oxygen more strongly ne- 
 gative, and those of hydrogen more strongly po- 
 sitive. In virtue of their increased mutual attraction 
 they combine, and in combining heat is evolved, 
 which affecting other contiguous molecules causes 
 further combination, and so on until the combination 
 is complete. 
 
 (158.) According to this hypothesis, combination 
 should be rapid, heat and light intense, and the com- 
 pound neutral in its properties, whenever the elec- 
 trical energies of the two constituents are strong and 
 perfectly equal. But when they are very unequal, the 
 effects would be less vivid, and the compound would 
 have the acid or alkaline character, according as the 
 energy of the negative or positive constituent is in 
 excess. 
 
 The production of water from the combination of 
 oxygen and hydrogen, and the formation of the 
 M 2 
 
164 
 
 INTRODUCTION. 
 
 metallic salts, are adduced as examples of strong 
 and equal energies. Like examples are afforded by 
 the nitrate, sulphate, and chlorate of potash and 
 muriate of lime, which severally, when touched upon 
 a large surface by plates of copper and zinc, gave no 
 electrical signs. Subcarbonate of soda and borax, 
 on the contrary, gave a slight negative charge, and 
 alum and superphosphate of lime a feeble positive 
 charge. 
 
 (159.) The next section of this remarkable paper 
 professes to explain the author's views of the " mode 
 of action " of the Voltaic pile. The absence of that 
 perspicuous style of expression which so generally 
 characterizes his writings, in this case justifies the 
 supposition that his own perceptions on the subject 
 of the theory he proposes were not at the time very 
 clear or well defined. It must be recollected that 
 Volta maintained that the source of electricity in the 
 pile was the contact of the dissimilar metals, and 
 that the intervening fluid merely acted the part of a 
 conductor to carry away, in a continued stream, the 
 positive electricity from each zinc surface, and the 
 negative electricity from each copper surface. Fa- 
 broni and Creve, and afterwards Wollaston and 
 others, maintained that the source of the electricity 
 was the chemical action between the zinc and the 
 fluid, and that the intervening copper acted as a 
 conductor to carry away, in a continued stream, the 
 positive electricity from one side of the fluid, and 
 the negative electricity from the other. Davy pro- 
 fessed to reconcile these conflicting hypotheses by 
 admitting, with Volta, that the opposite currents 
 were propagated from the surface of contact of the 
 
DAVY'S ELECTRO-CHEMICAL THEORY. 165 
 
 zinc and copper ; but that the liquid separating the 
 pairs of plates did not, and could not, carry forwards 
 the currents, as Volta maintained, by their conduct- 
 ing power, but that they effected that object by the 
 chemical action which took place between them and 
 the zinc. This is our view of the theory proposed 
 by Davy in the paper now referred to ; but, as has 
 been already stated, the expressions are not so clear 
 as to remove all doubt of his exact meaning. 
 
 Davy uses the term " electrical energy " appa- 
 rently to express the same phenomenon which Volta 
 called " electro-motive action," and which had been 
 also called " Voltaic action." This term denotes 
 the quantity of electricity evolved upon the two 
 metals on either side of their common surface, ac- 
 cording to Volta's theory of contact. The act of 
 conveying forward through the series in each di- 
 rection the electricity, positive and negative, thus 
 propagated at the common surface, is called by 
 Davy the " restoration of the electrical equilibrium 
 which was disturbed by the electrical energy of the 
 metals." Strictly speaking, there is no restoration 
 whatever of electrical equilibrium during the action 
 of the pile. The electric fluids are never in a state of 
 repose. Two currents run in uninterrupted streams 
 in opposite directions. When therefore Davy says 
 that " the chemical changes" produced by the 
 liquid interposed between the metallic elements of 
 the pile are " the causes that tend to restore the 
 equilibrium/' he must, as we conceive, be under- 
 stood to mean that these changes are " the causes by 
 which the electric currents are propagated towards 
 the poles of the pile." 
 
 M 3 
 
166 INTRODUCTION. 
 
 (160.) Having premised these explanations, let us 
 now consider the reasoning and the facts on which 
 this theory of Davy has been based. He denies that 
 the liquid elements of the pile can act as an or- 
 dinary conductor of electricity, the term conductor 
 being used in the same sense as when applied to 
 the metals and other solid conductors, because, with 
 regard to electricities of such very low intensity, 
 water (as well as liquids in general) is an insulating 
 body. Besides, there is every reason to believe 
 that, " if the fluid medium were a substance in- 
 capable of decomposition (by the metallic ele- 
 ments), the motion of the electricity would cease.'* 
 When the liquid in a Voltaic arrangement of zinc 
 and copper is a solution of muriate of soda, decom- 
 position ensues. The oxygen and muriatic acid 
 pass through the fluid from the copper towards the 
 zinc, transporting or transported by the negative 
 current ; and the hydrogen and soda pass from the 
 zinc towards the copper, transporting or transported 
 by the positive current. Whether the author con- 
 sidered that the transfer of the electricity is effected 
 by the locomotion of the decomposed elements 
 through the fluid, or by a series of decompositions 
 and recompositions, in which there is no motion of 
 translation imparted to any of the elements re- 
 sulting from the decomposition, and in which the 
 electricities themselves are not transferred through 
 the fluid, but rendered alternately free and latent as 
 the successive decompositions and recompositions 
 are effected, does not appear from the developments 
 contained in this paper. 
 
 A pile of twenty-four pairs, in which the con* 
 
DAVY'S ELECTRO-CHEMICAL THEORY. 167 
 
 necting fluid was water free from air, had no Voltaic 
 power. To determine whether another liquid with 
 superior conducting power, but still incapa'ble of 
 chemical action, would be affected, concentrated sul- 
 phuric acid was tried. No permanent current was 
 produced. Solutions of neutral salts render the 
 pile active at first ; but .when, by continued decom- 
 position, the solution in contact with the zinc be- 
 comes acid, and that in contact with the copper 
 alkali, the action ceases. Dilute acids being them- 
 selves easily decomposed, and promoting the de- 
 composition of the water, dissolving the oxide of zinc 
 as fast as it is formed, and evolving gases only on 
 the copper side, are the most powerful and durable- 
 fluid elements for a pile. All these facts supply 
 converging evidence upon the position that che- 
 mical action is essential to the vitality of the Voltaic 
 apparatus. 
 
 Against the hypothesis that chemical change is 
 the primary source of the action of the pile, it is 
 contended that in a combination of zinc and copper 
 plates with dilute nitrous acid, the side of the zinc 
 exposed to the acid is positive; but in a Voltaic 
 combination of zinc water and dilute nitric acid, 
 the side of the zinc exposed to the acid is negative. 
 The chemical action of the acid on the zinc being 
 in both cases the same, it is argued that if the 
 electric currents originated at the common surface 
 of the zinc and acid, which they would do if che- 
 mical change were their primary source, the di- 
 rection of the currents would be the same, instead 
 of being contrary in the two cases. 
 
 As a further argument against the chemical theory 
 M 4 
 
1 58 INTRODUCTION. 
 
 of the pile, Davy maintained that in mere cases of 
 chemical change, electricity is never exhibited ; and 
 endeavoured to support this position by the ex- 
 amples of iron burned in oxygen, the deflagration 
 of nitre and charcoal, the combination of solid 
 potash and sulphuric acid, and other chemical 
 actions. Subsequent investigation, however, has 
 shown that this principle is not tenable, and that 
 chemical change is attended with the evolution of 
 electricity. 
 
 (161.) With Davy, as with Franklin, application 
 ever trod closely on the heels of discovery. The 
 same memoir which disclosed the brilliant series of 
 discoveries of which we have here attempted to give 
 a brief analysis, also indicated the vast applications 
 of which they were susceptible, in the further in- 
 vestigation of the laws of nature, and in arts con- 
 ducive to the economy of life. The detection of 
 acid and alkaline matter in mineral, animal, and 
 vegetable substances, and their separation from 
 them, was sufficiently obvious. A piece of muscular 
 fibre, through which the electric current was trans- 
 mitted for five days, was rendered dry and hard. 
 Potash, soda, ammonia, lime, and oxide of iron 
 were carried from it by the negative current; and 
 the three mineral acids, with phosphoric acid, passed 
 off with the positive current. From a laurel leaf 
 the negative current carried green colouring matter, 
 resin, alkali, and lime, and the positive current took 
 vegetable prussic acid. Mint gave potash and lime 
 with the negative, and an acid matter with the po- 
 sitive current. The flesh of the living hand, care- 
 fully washed in pure water, gave a mixture of 
 
DAVYS RESEARCHES. 
 
 muriatic, sulphuric, and phosphoric acids with the 
 positive current, and fixed alkaline matter with the 
 negative current. This fact accounts for the acid 
 and alkaline tastes first observed by Sulzer given by 
 metals in contact. 
 
 By converting the processes, the Voltaic currents 
 may be made the means of introducing acids and 
 alkaline, or metallic principles, into the animal and 
 vegetable system. This idea has since been realized 
 in medical practice by some physicians. 
 
 (162.) In the experiments hitherto made, the acids 
 and alkalies themselves were not decomposed. The 
 history of scientific discovery affords no more 
 remarkable example of that instinctive foresight 
 which enables the philosopher to suspect the direc- 
 tion in which truth lies, and prompts him in the 
 selection of subjects of inquiry, than is apparent in 
 comparing Davy's present guesses with the result 
 of his subsequent researches. " These facts," says 
 he, ' ' induce us to hope that this new mode of ana- 
 lysis may lead to the discovery of the true elements 
 of bodies, if the materials acted on be employed 
 in a certain state of concentration, and the elec- 
 tricity be sufficiently exalted. For if chemical 
 union be of the nature which I have ventured 
 to suppose, however strong the natural electrical 
 energies of the elements of bodies may be, there is 
 yet every probability of a limit to their strength : 
 whereas the powers of our artificial instruments 
 seem capable of indefinite increase." 
 
 How soon he led the way towards the realiz- 
 ation of this magnificent conjecture will presently 
 appear. 
 
170 INTRODUCTION. 
 
 (163.)Suddenandviolentderangements of the elec- 
 trical equilibrium of the elements of our system are 
 manifested in other cases besides the glaring instances 
 offered by atmospheric phenomena. The electrical 
 appearances which precede and attend earthquakes 
 and volcanic eruptions admit of easy explanation 
 on the electro-chemical theory. The slow and gra- 
 dual changes observed by the geologist are indi- 
 cations of the more tranquil and incessant opera- 
 tions of electrical agency. Where strata of pyrites 
 and coalblende occur; where the pure metals or the 
 sulphurets are found in contact with each other, or 
 with any conducting substances ; and where dif- 
 ferent strata contain different saline menstrua, elec- 
 tricity must be evolved, and by its agency mineral 
 formations would probably be influenced or pro- 
 duced. 
 
 (164.) These views, which have been recently con- 
 firmed by the observations of Mr. Fox on the elec- 
 trical condition of the metallic veins in Cornwall, 
 were illustrated by experiment. A mixed solution of 
 muriates of iron, copper, tin, and cobalt, was placed 
 in the positive cup P, and distilled water in the 
 negative cup N, the cups being connected by as- 
 bestos. The four oxides passed through the asbestos 
 to the cup N ; a yellow metallic crust was formed 
 on the negative, wire, round the base of which the 
 oxides collected in a mixed state. In another ex- 
 periment the carbonate of copper was diffused in 
 minute subdivision through water, and a negative 
 wire placed in a small perforated cube of zeolite in 
 the liquid. Green crystals collected upon the cube 
 and adhered to it, the particles being incapable of 
 
DAVY'S RESEARCHES. 171 
 
 penetrating it. By the multiplication of such in- 
 stances, Davy conceived that the electrical power 
 of decomposition and transference would afford a 
 satisfactory explanation of some of the principal 
 facts in geology, and his anticipations have since 
 been to a considerable extent realized by the re- 
 searches of Becquerel and others. " Natural elec- 
 tricity/' says Davy in the conclusion of this me- 
 morable paper, " has hitherto been little investi- 
 gated, except in the case of its evident and power- 
 ful concentration in the atmosphere. Its slow and 
 silent operations in every part of the surface will 
 probably be found more immediately and import- 
 antly connected with the order and economy of 
 nature ; and investigation on this subject can hardly 
 fail to enlighten our philosophical systems of the 
 earth, and may possibly place new powers within 
 our reach."* 
 
 (165.) His theoretical ideas on the application 
 of electrical decomposition to the solution of the 
 phenomena of geology were seized with ardour 
 by GUYTON MORVEAU. That eminent chemist, 
 like Davy, endeavoured to exhibit on a small scale, 
 by direct experiments, the processes which are con- 
 tinually going on in the crust of the earth. The 
 native oxide of antimony he regarded as an ex- 
 ample of slow transition from the state of a sul- 
 phuret to that of a pure oxide, by means of the 
 decomposition of water by subterranean electricity. 
 By careful examination of a specimen of this mi- 
 neral, he observed that it still retained the structure 
 of the crystallized sulphuret of antimony, and even 
 * Philosophical Transactions, 1807. 
 
172 INTRODUCTION. 
 
 preserved partially its metallic lustre, and inferred 
 that a slow Voltaic action had changed its compo- 
 sition without disturbing the arrangement of its 
 constituent parts. To support those ideas suggested 
 to him in Davy's celebrated paper by direct expe- 
 riment, he submitted a piece of sulphuret of anti- 
 mony to the action of a powerful Voltaic apparatus. 
 An odour of sulphuretted hydrogen was soon per- 
 ceivable ; the liquid assumed a yellow colour, and 
 the sulphuret appeared of a darker tint and iri- 
 descent, indicating incipient decomposition. The 
 negative plate became black, and the positive one 
 was coated with a light yellow incrustation, which 
 proved to be the oxide of antimony. Thus it ap- 
 peared that the sulphuret of antimony was capable 
 of being transferred immediately into the oxide by 
 the mere operation of the Voltaic forces. Other 
 native sulphurets were tried in like manner, and gave 
 similar results.* 
 
 (166.) During the twelve months next succeeding 
 the date of the memoir above noticed, Davy devoted 
 his labours, and directed all the powers of his ge- 
 nius, to the development of the consequences of 
 the theoretical principles which he had propounded, 
 and to the realization of the ideas he had ventured 
 to throw out respecting the resolution of natural 
 substances, before regarded as simple, into their con- 
 stituents. Never before did Theory more surely 
 lead to Discovery ; never was the prophetic instinct 
 of a philosopher more speedily or more magnificently 
 satisfied. His foreknowledge of the facts to be dis- 
 
 * Annales de Chimie, torn. liii. p. 113. 
 
DAVYS RESEARCHES. 
 
 closed and the instruments for their disclosure, of 
 the end to be attained and the means of attaining 
 it, of the route to be followed and the goal to be 
 reached, was distinctly expressed; and with the con- 
 fidence inspired by clear perceptions and conscious 
 power, he immediately advanced in the course he 
 described, and attained the end he foresaw. The 
 resolution of the alkalies and earths into their ele- 
 ments was the splendid result of his labours during 
 the year 1807, and was consigned to the Bakerian 
 Lecture read before the Royal Society on the 19th 
 of November in that year. 
 
 (167.) His first efforts were directed to potash, 
 which was submitted in a state of solution to the elec- 
 tric current. The water only was decomposed, the 
 alkali refusing to yield. In its dry state it would not 
 transmit the current. In order to give it a con- 
 ducting power, and at the same time exclude water, 
 on which by preference the current appeared to act, 
 the alkali was now placed in a platinum spoon, and 
 exposed to the flame of a lamp directed upon it by 
 a blast of oxygen. When reduced to the fluid state 
 by such means, the potash transmitted the Voltaic 
 current. When the metal of the spoon was posi- 
 tive, and the point of a platinum wire inserted in 
 the fluid alkali negative, combustion attended by 
 intense splendour was exhibited at the wire, and a 
 column of flame arose from the point of contact of 
 the wire with the alkali. When the spoon was ne- 
 gative, and the wire positive, a vivid light appeared 
 on the former ; aeriform globules rose through the 
 liquid potash, which inflamed as soon as they escaped 
 into the air. 
 
2 74 INTRODUCTION. 
 
 It was conjectured that the constituent of the 
 potash, attracted by the negative pole, was the mat- 
 ter which in these cases escaped in bubbles ; and 
 that its affinity for oxygen was so strong, that the 
 moment it came in contact with the atmosphere it 
 recombined with oxygen and produced combustion. 
 The question, therefore, now was, how to arrest that 
 element, and submit it to examination ? 
 
 As the liquefaction of the alkali by heat appeared 
 to entail, as a consequence, the immediate recom- 
 bination of its separated constituent, it was now 
 attempted to give the necessary conducting power 
 to the potash, by allowing it to imbibe from the 
 atmosphere as much moisture as would give a con- 
 ducting power to its surface. The alkali in this 
 state was placed on a platinum disc, which was con- 
 nected with the negative pole, while a wire connected 
 with the positive pole was applied to its upper sur- 
 face. At the upper surface there was a disengage- 
 ment of gas ; at the lower surface small metallic 
 globules appeared, like mercury, in their visible 
 character. Some of these burnt by contact with 
 the air. Others had their metallic lustre tarnished, 
 and finally covered with a white film, which de- 
 fended them from the atmosphere, and preserved 
 them in their metallic state. 
 
 The gas disengaged at the positive wire was oxy- 
 gen, and the metal deposited was the base of the 
 alkali, afterwards called POTASSIUM. 
 
 (168.) Soda, when submitted to a like process, 
 gave a similar result, and the metal educed from 
 it was that which is now called SODIUM. 
 
 This capital discovery was made in October, 1807. 
 
DAVYS RESEARCHES. 1?5 
 
 Potassium was discovered on the 6th of that month, 
 and sodium a few days after. 
 
 Sensitive friends of the great British chemist 
 have been moved to vindicate the glory of this dis- 
 covery from those who would tarnish it, by ascribing 
 to the accidental possession of the laboratory and 
 apparatus of the Royal Institution of Great Britain 
 a share in producing it. These generous survivors 
 may tranquillize their fears. Possibly such vindica- 
 tion may be called for by a portion of the present 
 generation having pretensions sufficient to raise 
 them to the level of envy, but wanting those better 
 qualities which would elevate them above it. Cer- 
 tainly no such apology will be needful with posterity. 
 
 (169.) The strong affinities of these new metals for 
 one or other of the constituents of almost every body 
 with which they were brought in contact, and of 
 every menstruum or atmosphere with which they 
 could be surrounded, was very embarrassing, and 
 rendered the examination of their physical pro- 
 perties extremely difficult. It was found most con- 
 venient either to preserve them in a tube protected 
 from the contact of the air above recently distilled 
 naphtha, or to allow them to combine with mercury 
 so as to form an amalgam, and in that state to pre- 
 serve them, separating them by heat when the pure 
 metal was required. 
 
 (170.) The analogy suggested by the decomposition 
 of the fixed alkalies naturally led to a like inquiry 
 with respect to the earths which enjoy with the 
 former common properties, and those which seemed 
 most analogous to the alkalies. Baryta, strontia, 
 lime, and magnesia, were tried by like methods, but 
 
INTRODUCTION. 
 
 without any satisfactory result. Being slightly 
 moistened at their surfaces, they were exposed to 
 the electric current transmitted by iron wire un- 
 der naphtha. At the negative pole they assumed 
 a darker colour, and small particles appeared 
 there showing metallic lustre, and which gradually 
 whitened by exposure to air. In the experiments 
 on potassium it was found that when a mixture of 
 potash and the oxide of mercury, tin, or lead, was 
 exposed to the Voltaic current, decomposition en- 
 sued, and an amalgam of potassium was produced. 
 The same method was accordingly tried with the 
 alkaline earths. Mixtures of these substances with 
 oxides of tin, lead, silver, and mercury, were ex- 
 posed to the current. In these cases, a small 
 quantity of a substance having the whiteness of 
 silver was deposited at the negative pole, which 
 was found to be an amalgam. Still the results were 
 not conclusive or satisfactory. 
 
 (171.) The labours of Davy had attained thi 
 point, when, in June, 1808, he received a letter from 
 M. Berzelius, informing him that, assisted by Dr. 
 Pontin, that chemist had succeeded in decomposing 
 baryta and lime, by exposing them in contact with 
 mercury to the current. Davy immediately re- 
 peated the experiment, and obtained the amalgam 
 of the metallic base of baryta at the negative 
 pole. This was accomplished by a battery of 500 
 pairs, weakly charged, acting on a surface of slightly 
 moistened baryta through the medium of a globule 
 of mercury. The mercury gradually became less 
 fluid, and after a few minutes was found covered 
 with a white film of baryta ; and when the amalgam 
 
DAVY'S RESEARCHES. 177 
 
 was thrown into water, the latter was decomposed, 
 hydrogen was dismissed, mercury precipitated, and 
 a solution of baryta formed. A like process gave a 
 similar result with lime. 
 
 (172.) Having thus verified the results obtained 
 by Berzelius, Davy extended the same method to 
 strontia and magnesia. The former readily yielded; 
 the latter was more intractable. By continuing the 
 process, however, for a longer time, and keeping 
 the earth continually moist, at last a combination of 
 the basis with mercury was obtained, which slowly 
 produced magnesia by absorption of oxygen from 
 the air, or by decomposing water. 
 
 Thus were discovered BARIUM, STRONTIUM. 
 CALCIUM, and MAGNESIUM, as an immediate 
 consequence of the first great step made in this 
 course of investigation by the discovery of potas- 
 sium and sodium. 
 
 (173.) The next group of earths brought to trial 
 consisted of alumina, silica, zirconia, and glucinia, 
 which proved more refractory than any of the 
 former. Driven in search of other methods of 
 experimenting, he considered minutely their quali- 
 ties in relation to other bodies, with a view to the 
 discovery of analogies by which his researches 
 might be conducted. From the absence of any 
 tendency in alumina and silica to yield to the at- 
 traction of the electric current in the direction of 
 either pole, he inferred the probability of their par- 
 taking of the nature of neutro-saline substances, 
 and attempted their decomposition by processes 
 suggested by that supposition. Failing in these, 
 
178 INTRODUCTION. 
 
 and observing that alumina and silica have both a 
 strong affinity for potash and soda, and considering 
 that such affinity could not proceed from the oxy- 
 gen which might be one of their constituents, he 
 inferred that it must be a quality of their metallic 
 bases, and that it would, in that case, be probable 
 that, if mixed with soda or potash, and exposed to 
 the electric current, the base might be made to 
 separate, and to attach itself to the base of the 
 alkali. A mixture of silica and potash, in the pro- 
 portion of one to six, was accordingly put in a 
 platinum crucible, and reduced to a fluid state over 
 a charcoal fire. The crucible was put in connec- 
 tion with the positive pole of a battery of five 
 hundred pairs, and a rod of platinum connected 
 with the negative pole was brought in contact with 
 the alkaline menstruum. The moment the end of 
 the negative rod touched the liquid, globules rose 
 through it to the surface, on which they swam 
 about in a state of brilliant combustion. When the 
 mixture cooled, the platinum bar was removed, and 
 the alkali and silex which adhered to it detached ; 
 there remained upon it brilliant metallic scales, 
 which, immediately on exposure, became covered 
 with a white crust, and some of which burnt spon- 
 taneously. Being plunged in water, the end of the 
 platinum produced effervescence, and an alkaline 
 solution was formed, which, upon examination, 
 was proved to contain silica. The same process 
 applied to alumina gave a like result. 
 
 It was now determined to try the effect of the 
 Voltaic current upon the earths, in contact with 
 potassium itself. An amalgam of potassium, in 
 
DAVY S RESEARCHES. I 7.0 
 
 contact with silica, was negatively electrified under 
 naphtha. After being acted on for an hour, the 
 amalgam was made to decompose water, and the 
 alkali thus obtained was neutralized by acetous 
 acid. A white precipitate was obtained having all 
 the characters of silica. 
 
 The same process was applied, with the same 
 results, to alumina, glucinia, and zirconia. It was 
 inferred, therefore, that these earths were oxides of 
 metals, to which respectively the names of SILI- 
 CIUM, ALUMINIUM, GLUCINIUM, and ZIRCONIUM 
 were given. 
 
 (174.) Having established, by direct experiments, 
 the fact that so many of the alkaline and earthy sub- 
 stances were oxides with metallic bases^ it was con- 
 sistent with sound physical logic to assume, as a 
 general law, that " the alkalies and earths are oxides 
 of metals ." 
 
 (175.) The question, how far the volatile alkali, 
 Ammonia, was to be regarded in relation to such a 
 law, naturally presented itself. Without reference to 
 this analogy, or offering any hypothesis to explain the 
 fact, SEEBECK had already shown that an amalgam 
 could be obtained by the action of ammonia on 
 mercury. This fact was reproduced by Berzelius 
 and Pontin, and communicated by them, with vari- 
 ous circumstances attending it, to Davy. Berzelius 
 maintained that ammonia came within the scope 
 of the general law, and that an idea which had been 
 previously thrown out by Davy was justified by the 
 phenomena which showed that ammonia was a 
 binary metallic base. This question was then taken 
 N 2 
 
180 INTRODUCTION. 
 
 up by Davy, and the experiments of Berzelius re- 
 peated, but without arriving at any certain or clear 
 result. Gay-Lussac and Thenard opposed the 
 views of Davy and Berzelius ; and a contest arose, 
 for which, as it has little connexion with the pro- 
 gress of electrical science, we shall merely refer to 
 the scientific periodical works in which it was car- 
 ried on.* 
 
 (176.) It has been already observed, that the cha- 
 racter of Davy's mind was to pass directly from dis- 
 covery to application. In the same memoir which 
 contained the announcement of the subjugation of 
 the alkalies and earths by the powers of the pile, is 
 found his brilliant hypothesis to explain the pheno- 
 mena of volcanoes and aerolites. The metallic bases 
 of the alkalies and earths cannot exist at the surface 
 of the earth in their simple or uncombined form, 
 nor even alloyed with the more perfect metals., be- 
 cause of the intensity of their affinity for oxygen. 
 But the same cause does not prevent their existence 
 in the interior parts of the globe. Let the possibi- 
 lity of the existence of potassium, sodium, calcium,, 
 or any other metals of the same class in the inferior 
 strata of the earth, either in a separate state or in 
 combination with other metallic substances, be ad- 
 mitted ; and it is only necessary to imagine their 
 occasional exposure to the action of air or water, to 
 obtain a satisfactory solution for volcanic eruptions. 
 These highly combustible metallic principles, com- 
 bining with oxygen, attended by violent com- 
 
 * Annales de Chimie, torn. Ixxii. p. 193., Ixxv. 256 291 
 Biblioth. Brit, June, 1809, p. 122. 
 
DAVY*S RESEARCHES. 181 
 
 bustion, are ejected from the bowels of the earth, 
 and form the craters of volcanoes, the combination 
 being an earthy matter exhibited after its ejection as 
 lava. The formation of aerolites might proceed from 
 the same causes, their luminous appearance and 
 detonation being produced by the combustion at- 
 tending the combination of the metals with oxygen 
 as they enter the atmosphere. 
 
 With a view to test Ihe validity of these inge- 
 nious hypotheses, Davy investigated carefully the 
 phenomena of active volcanoes; and, not finding them 
 to be in sufficient accordance with these, he relin- 
 quished his theory, without any of that regret which 
 attends the failure of a favourite hypothesis, when 
 the discovery of truth is an object secondary to the 
 attainment of personal distinction. 
 
 (177.) The powers of decomposition and transfer 
 by Voltaic electricity, so strikingly exhibited in the 
 researches of Davy, directed the attention of physio- 
 logists and others once more to the investigation 
 of the agency of electricity in the vegetable and 
 animal economy. The experiments which had been 
 made to show that the alkaline and earthy ele- 
 ments found in organized vegetable substances were 
 evolved, by the process of vegetation, from air and 
 water, had always been inconclusive and unsatisfac- 
 tory; and Davy's experiments, in which it was shown 
 that even in water carefully distilled there is still 
 held in solution a portion of saline or metallic matter, 
 together with the known fact, that air almost always 
 holds in mechanical suspension solid matter of va- 
 rious kinds, finally overturned such hypotheses. All 
 the substances developed in organized nature may 
 N 3 
 
182 1NTROD U CTION. 
 
 be produced, by ordinary processes, from combin- 
 ation of known constituents. The compounds of 
 iron, alkalies, and earthy bodies with mineral acids, 
 abound in vegetable soil. The decomposition of 
 basaltic, granitic, and other rocks affords a constant 
 supply of earthy, alkaline, and ferruginous matter 
 to the superficial part of the earth. In the seeds of 
 all plants which have been examined, neutro-saline 
 compounds, containing potash, soda, or iron, have 
 been found. It is easy to imagine that these princi- 
 ples pass from vegetables to animals. 
 
 (178.) The same analogies suggested to Dr. Wol- 
 laston the idea, that something like the decomposing 
 and transmitting powers of the pile is the agent to 
 which the animal secretions are due, especially as the 
 existence of such agency in a considerable degree of 
 intensity, in certain animals, was proved by the effects 
 of the torpedo and Gymnotus electricus; and he con- 
 sidered that the universal prevalence of the same 
 power, lower only in degree in other animals, was 
 rendered highly probable by the extreme sudden- 
 ness with which the nervous influence is propagated 
 from one part of the living system to another. 
 Although the electric power of decomposition and 
 transfer has been experimentally demonstrated only 
 in cases of comparatively high intensity of action, 
 yet analogy countenanced the idea that very feeble 
 electric energies would produce like effects more 
 slowly, in proportion to their weakness. To illus- 
 trate this by immediate experiment, he tied a piece 
 of clean bladder over one end of a glass tube three 
 quarters of an inch in diameter, and two inches 
 long, and filled it with water holding ^^ of its 
 
EXPERIMENT OF WOLLASTON. 183 
 
 weight of salt in solution. Placing it on a shilling, 
 he connected the silver with the surface of the water 
 by a wire of zinc, and found that alkali was trans- 
 mitted through the bladder to the silver by the at- 
 traction of the negative electricity. Decisive indi- 
 cations of this were obtained in five minutes. The 
 efficacy of a power so feeble confirms the conjecture 
 that similar agents may be instrumental in various 
 animal secretions. The blood, which is alkaline, 
 supplies the bladder with matter in which acid is 
 strongly manifested ; while an excess of alkali, above 
 that contained in the blood, is manifested in bile. 
 These effects would be explained by admitting a 
 permanent state of positive electricity in the kidneys, 
 and negative electricity in the liver. The coinci- 
 dence of this view with the guesses of Napoleon, 
 already mentioned (117.), is curious and interest- 
 ing.* 
 
 (179.) The last great discovery of Davy directed 
 the attention of the philosophers of the Continent to 
 the same field of inquiry ; and, much as had been 
 expected from the powers of the pile when its illus- 
 trious inventor expounded its nature and properties 
 to the assembled members of the Institute in 1801, 
 it was now, from day to day, rendered more evident 
 that these powers were inadequately estimated, and 
 imperfectly understood, and that it was still destined 
 to enrich every branch of physical science by the 
 development of new and unlooked-for phenomena. 
 Napoleon, in the magnificent spirit with which his 
 encouragement of the sciences was always manifested, 
 
 * See Philosophical Magazine, vol. xxxiii. p. 1088. 
 
 N 4 
 
184 INTRODUCTION. 
 
 had presented to the laboratory of the Polytechnic 
 School a Voltaic apparatus of immense magnitude 
 and power. With this instrument MM. Gay-Lussac 
 and Thenard undertook an experimental investiga- 
 tion of the powers of the pile, with the view of de- 
 termining more especially the influence which the 
 number of the metallic elements, and the nature of 
 the liquid used to charge the pile, have on its chemical 
 action. Assuming, as a modulus of the chemical 
 energy of the pile, the quantity of gas evolved in 
 the process of decomposition in a given time, they 
 arrived at the following conclusions : 1. The de- 
 composing energy depends conjointly on the con- 
 ducting power of the liquid under decomposition, 
 and on the nature of that which is used to charge 
 the pile. 2. It is greater when the pile is charged 
 with a mixture of acid and salt, than with salt alone. 
 3. The chemical effects are proportional to the force 
 of the acids by which it is put in action : and, 4. 
 They do not augment in the same ratio as the 
 number of pairs of plates, but very nearly in the 
 ratio of the cube root of that number. 
 
 (1 80.) That part of the electro-chemical theory of 
 Davy in which the negative character natural to 
 certain physical elements, and the positive to others, 
 is assumed, was implicitly, if not expressly, included 
 in the hypothesis of Grotthus. Without such a sup- 
 position, the series of decompositions and recompo- 
 sitions imagined by that philosopher could scarcely 
 be admitted. The probable connexion of chemical 
 attractions with electric forces had been also con- 
 jectured by Hube in his Traite de Physique, and 
 Hitter obscurely expressed some ideas of the same 
 
DAV\ S RESEARCHES. 185 
 
 kind. Immediately before the commencement of 
 Davy's researches, OERSTED, since so celebrated for 
 his discoveries in electro-magnetism, promulgated a 
 theory *, in which he maintained that all the phe- 
 nomena of chemistry might be regarded as the result 
 of two general forces common to all matter, and that 
 the same forces produced those effects which were 
 rendered sensible in electric attractions and repul- 
 sions. This work, however, was exclusively of a 
 speculative kind, unsupported by any experiments 
 which could give force or validity to the theory it 
 proposed. 
 
 The electro- chemical theory of Davy was the first 
 which had ever professed to be based on clear and 
 well-ascertained facts. It was laid down as a fun- 
 damental principle in this theory, that when two 
 bodies, the particles of which are in opposite elec- 
 trical states, and sufficiently exalted to enable their 
 electric attraction to overcome the force of aggrega- 
 tion of their particles, are brought into contact, they 
 will unite, and heat and light will be developed by 
 the combination of the two electric fluids. When 
 the combination is effected, all signs of electricity 
 cease, as would necessarily ensue from the union of 
 the two fluids, but by what power the aggregation 
 of the new compound was maintained was not ex- 
 plained. 
 
 (181.) Berzelius and Amp ere, who, of all the philo- 
 sophers of the Continent, evinced most justice and 
 candour in their appreciation of Davy's merit, took up 
 
 * Recherches sur 1'Identite des Forces Chimiques et Elec* 
 triques. Traduit de I'Allemand. 1813, 
 
186 INTRODUCTION. 
 
 the electro-chemical theory, which was not pursued 
 through its consequences by its author, owing pro- 
 bably to the natural disposition of his mind to in- 
 vestigate new facts rather than discuss the merits of 
 hypotheses. Berzelius assumed that the constituent 
 atoms of bodies were not only naturally electrical, 
 as Davy had maintained, but that they possessed 
 electric polarity, and that the intensities of their 
 poles are unequal. He investigated, in the first 
 place, the two questions, How electricity exists in 
 bodies ? and, How it is that some bodies are natu- 
 rally negative, and others sometimes positive and 
 sometimes negative ? 
 
 (182.) A body never becomes electric, without ma- 
 nifesting the two opposite electric principles, either 
 in different parts of it, or in the sphere of its action; 
 when the two electricities appear separately in a 
 continuous body, they are always found on opposite 
 sides. The tourmaline and some other crystals 
 offer an example of this. But, since the parts of a 
 body possess the same properties as the body itself, 
 it is necessary to admit that bodies are composed of 
 atoms, each of which has an electric polarity, and its 
 poles have unequal intensities. On this polarity 
 depend the chemical phenomena, and its unequal 
 intensity is the cause of the different force exercised 
 by their affinities. Bodies are accordingly electro- 
 positive or electro-negative in combining, accord- 
 ing as the influence of the one or other of their 
 atomic poles predominates. 
 
 The degree of polarity in this theory is influenced 
 6y the temperature. Thus many substances at 
 common temperatures manifest but feeble electric 
 
ELECTRO-CHEMICAL THEORY OP BERZELIUS. 187 
 
 polarity, which, at a red heat, show a very strong 
 one. 
 
 No combination can be effected unless the polar- 
 ized molecules of one or both of the combining 
 bodies have free mobility amongst each other, each 
 being at liberty to turn on its own centre in any 
 direction, so that the particles may present towards 
 each other their contrary poles in obedience to their 
 electric attraction. This condition renders it neces- 
 sary that one or both of the combining bodies be 
 in the fluid state. 
 
 The vulnerable point of this theory was found in 
 the phenomena of aggregation. In what manner 
 can the electric forces which it assumes produce 
 the hardness, brittleness, ductility, and tenacity of 
 different species of solids ; the viscidity of liquids ; 
 or the elasticity of gases ? 
 
 Berzelius admits that these effects are not expli- 
 cable by this hypothesis. M. Ampere attempted to 
 solve this question *, by assuming that the atoms 
 of bodies possessing each its proper electricity, in 
 virtue of which they are united in combinations in 
 the same manner as two leaves of paper oppositely 
 electrified adhere to each other, also act by their 
 electricity on the electricity of the medium in which 
 they exist, attracting the fluid of the contrary name, 
 and repelling the fluid of the same name. The 
 atoms are therefore considered as strictly analogous 
 to the Leyden jar; the internal charge represent- 
 ing the natural electricity of the atom, and the ex- 
 ternal that which is drawn from the surrounding 
 
 * Journal de Physique, 1821. 
 
188 INTRODUCTION. 
 
 medium. If a combination is formed between an 
 electro-positive and an electro-negative body, a 
 discharge takes place ; the atoms dismiss their ex- 
 ternal charge, and rush into union in virtue of the 
 reciprocal attraction of their opposite natural elec- 
 tricities. The atmospheres of the atoms, as well as 
 the atoms themselves, are combined ; but, as the atoms 
 cannot emerge from them, their electricities act on 
 those of their atmospheres, exerting attractions and 
 repulsions, so as to produce electrical phenomena 
 the reverse of those which attended their combin- 
 ation. 
 
 (183.) The zinc plates of a Voltaic apparatus, 
 being subject to continual oxydation, are at length 
 so reduced in thickness, as to render it necessary to 
 replace them by new ones. This gradual wear of 
 the pile by use rendered it desirable to seek for 
 means of constructing a pile composed of solid 
 elements only; a project, however, which could only 
 be entertained by those who conceived that chemical 
 action was merely incidental, and not essential, to 
 the development of Voltaic electricity. Although 
 the high probability, if not the certainty, that che- 
 mical action is indispensable, must render abortive 
 all attempts at the discovery of a dry pile, such 
 researches have nevertheless been attended with 
 some advantage. In 1803, MM. Hachette and 
 Desormes substituted starch for the liquid in the 
 common pile ; and, in 1809, De Luc invented a pile 
 apparently free from any liquid element. This ap- 
 paratus consisted of a column formed of alternate 
 discs of zinc and paper gilt on one side, the gilt 
 sides of the paper discs being all turned in one 
 
DRY PILES. 189 
 
 direction. This was in reality not a dry pile ; the 
 paper imbibed and retained moisture enough to give 
 a feeble activity to the apparatus. 
 
 (1 84.) De Luc's pile was improved by Zamboni in 
 1812. He rejected the discs of zinc, and composed 
 the pile of discs of paper only, one surface being tin- 
 ned, and the other coated thinly with the peroxide 
 of manganese, brushed with a mixture of flour and 
 milk ; or gilt or silver paper may be used, the me- 
 tallic surface being wetted with a saturated solution 
 of the sulphate of zinc, on which, when dry, the 
 peroxide of maganese in powder may be spread. 
 Several leaves of paper thus prepared are placed 
 one upon the other, and cut into the required form 
 by a circular cutter. As many discs are thus formed 
 by one operation as there are leaves of paper super- 
 posed ; and these being afterwards laid one upon 
 the other, the pile is formed. This pile is usually 
 placed in a hollow cylinder of the same internal 
 diameter. The paper discs are forced into close 
 contact by pressure produced by screws. 
 
 In these, as in the pile of De Luc, the humidity 
 of the paper is the source of Voltaic power. Al- 
 though, by the aid of a condenser, the electricity 
 evolved in these piles may be rendered sensible, 
 and sparks may even be obtained, the power is in- 
 comparably more feeble than that of the common 
 pile, even in its most inefficient state. It is found 
 that by increasing beyond a certain limit the number 
 of discs composing these, their power is diminished. 
 Their effects have been generally limited to those 
 produced on the condenser ; but, by diminishing con* 
 siderably the number of discs, M. Pelletier has sue- 
 
190 INTRODUCTION. 
 
 ceeded in decomposing water by these instruments. 
 Their action, however, ceases after the lapse of a cer- 
 tain period, when the paper has lost all its humidity. 
 
 (185.) The only uses to which dry piles have been 
 hitherto applied are 1. To produce a continued 
 motion, by an electrical pendulum suspended between 
 the contrary poles of two such piles placed side by 
 side, so that the positive pole of one and the ne- 
 gative pole of the other shall be at the summit. 
 This motion will be continued as long as sufficient 
 moisture is retained by the elements of the piles 
 to sustain their activity ; but it will not be regular, 
 since the development of electricity will be affected 
 by variable atmospheric causes. 2. In condensing 
 electrometers, to detect the presence of very small 
 quantities of electricity on the inferior plate of the 
 condenser.* 
 
 (186.) We shall here conclude this brief sketch of 
 the progress of Voltaic electricity, regarding the phe- 
 nomena and natural laws which have been more re- 
 cently developed as belonging to the topics of present 
 investigation rather than to the history of the past, 
 and reserving them, therefore, for their suitable place 
 in the work we have undertaken. 
 
 III. MAGNETISM. 
 
 (187.) The substances endowed with magnetism 
 exhibit that property by three distinct effects : - 
 1. They attract iron and all ferruginous matter. 
 
 * Becquerel, Traite de 1'Electricite, torn. i. p. 166. 
 
MAGNETISM. 191 
 
 2. Two bodies endowed with the property of 
 magnetism will attract each other at one part of 
 their dimensions, and repel each other at another 
 part. These contrary effects, belonging to opposite 
 sides or ends, are called MAGNETIC POLARITY. 
 
 (188.) 3. When a magnet is placed on a vertical 
 axis through its centre of gravity, on which it is free 
 to revolve, the axis being between its poles, it will 
 oscillate on each side of a certain determinate po- 
 sition, in which it will at length come to rest. 
 When in this position, a vertical plane passing 
 through the axis and the poles will be nearly, but 
 not exactly, coincident with the plane of the meri- 
 dian of the place in which the magnet is situate. 
 For all magnets similarly supported, in the same 
 situation, these planes will be parallel. This plane 
 is called the MAGNETIC MERIDIAN. The angle 
 which the magnetic meridian makes with the ter- 
 restrial meridian is called the VARIATION of the 
 magnet. 
 
 (189.) 4. If a magnet be placed on a horizontal 
 axis passing through its centre of gravity at right 
 angles to the magnetic meridian and between its poles, 
 it will oscillate on each side of a certain determinate 
 position, in which it will at length come to rest. 
 When in this position, a plane passing through the 
 axis and the poles of the magnet will not be hori- 
 zontal, but will make a certain angle with a hori- 
 zontal plane through the axis. This angle is called 
 the DIP of the magnet. 
 
 The power of the magnet, when placed on a ver- 
 tical axis, to fix itself in the magnetic meridian of 
 any place to which it may be transported, is called 
 
192 INTRODUCTION. 
 
 its DIRECTIVE POWER, and is the principle on which 
 its application to navigation depends. 
 
 (190.) The attractive power of the magnet for iron 
 was the property which was first observed. This 
 property was known to the ancients, who gave to the 
 natural magnet (an oxide of iron) the name Mag- 
 nes (//ayrije); derived, as is supposed, from Mag- 
 nesia, a district of Lydia, in which the natural 
 magnet was found in greatest abundance. It was 
 also called Lapis Heracleus, from Heraclea, a city 
 of Lydia. From some passages in ancient authors, 
 it would seem that the force of magnetic attraction 
 in very high degrees of intensity was then generally 
 known. Pliny relates that Dinochares proposed to 
 Ptolemy Philadelphus to erect a temple at Alex- 
 andria, the dome of which should be built of load- 
 stone, so as to sustain in the air an iron statue of 
 Arsinoe. Saint Augustine also alludes to a statue 
 thus suspended in the air in the middle of the temple 
 of Serapis, at Alexandria. 
 
 (191.) The polarity and directive powers of the 
 magnet were discoveries of a much more recent date. 
 The application of the magnetic needle to navigation 
 must have immediately succeeded the first know- 
 ledge of its directive power, but the discoverer is 
 unknown ; and even the century which was ho- 
 noured by the invention of this most beautiful 
 application of science to the uses of man is un- 
 certain. It is stated, in the account of the Chinese 
 empire by Du Halde, that the directive power of the 
 magnet was used in that part of the globe, for the 
 purpose of land journeys, more than a thousand 
 years before the birth of Christ. If such were the 
 
MARINER'S COMPASS. 193 
 
 case, it is difficult to imagine that its use for sea 
 voyages should have failed to spread itself west- 
 ward until two thousand years later. But, besides 
 this, there are other reasons why little credit is to 
 be given to the accounts which ascribe this inven- 
 tion to the Chinese.* 
 
 (192.) The earliest work in which the use of the 
 mariner's compass is distinctly mentioned is a ma- 
 nuscript poem of the twelfth century, preserved in 
 the Royal Library at Paris, the authorship of which 
 is attributed to Guiot de Provins. Guiot was at 
 the court of the emperor Frederic Barbarossa, held 
 at Mentz, in the year 1181. 
 
 Hansteen, in his work on the " Magnetism of the 
 Earth," quotes an Icelandic historian, to show that 
 the directive power of the loadstone was known a 
 century antecedent to the date of this poem. That 
 annalist, relating a voyage made in those seas, says 
 incidentally, that " In those times, seamen had no 
 loadstone in the northern countries." It appears 
 that this writer, Arc Frode, was born about the 
 year 1068, and therefore probably published his 
 account early in the twelfth century. 
 
 Cardinal Jacques de Vitri, who lived about the 
 year 1200, speaks of the magnetic needle, in his His- 
 tory of Jerusalem, as indispensable to those who 
 make sea voyages. It has also been said that it 
 was first brought to Europe, from the East, by 
 Marco Polo. It is, however, certain that Vasco de 
 Gama, the Portuguese navigator, used the compass 
 in his voyage to India in 1497. 
 
 * See Kircher, De Maynete. 
 VOL, I. O 
 
1 94> INTRODUCTION. 
 
 (193.) Before it became the subject of accurate 
 investigation, it was supposed that the direction of 
 the compass was identical with that of the terrestrial 
 meridian, and that it pointed due north and south. 
 The discovery of its variation, and that the amount 
 and direction of the variation are different in dif- 
 ferent places, is generally ascribed to Columbus in 
 1492. There appears, however, in a volume of MS. 
 tracts in the University of Leyden, a letter dated 
 1269, by Peter Alsiger, in which the principal pro- 
 perties of the magnet are mentioned; and, among 
 others, the variation. The honour of this discovery . 
 has also been ascribed to Grignon, a pilot of Dieppe, 
 Sebastian Cabot, Gonzales, and others. 
 
 Accurate observations of the variation of the 
 needle began to be made at Paris about the year 
 1550. At this time the variation was towards the 
 east. It diminished in quantity, and became nothing 
 in 1663; after which it passed to the west, increas- 
 ing gradually till it attained a certain limit, after 
 which it diminished. 
 
 (194-.) The Dutch navigators, in 1599, also con- 
 structed accurate tables of variation. 
 
 (195.) In the year 1576, Robert Norman, a mathe- 
 matical instrument maker in London, discovered the 
 DIP. He found that the card of the compass near 
 the north point was always depressed or inclined 
 downwards, so that he was obliged to put a counter- 
 poise on the southern pole of the needle, to keep it 
 level. 
 
 (196.) Mentioning this circumstance to some sci- 
 entific friends, he was advised to construct a needle 
 on a horizontal axis, and to observe the position 
 
VARIATION AND DIP. 3 5 
 
 to which this downward inclination would bring 
 the northern pole. He accordingly constructed 
 the first DIPPING NEEDLE, and found the DIP to be 
 about 71 J. 
 
 (197.) The variation of the needle was accurately 
 observed at London by Burrough, the friend of 
 Norman, who found that in the year 1581 it was 
 11 15' east. In the treatises extant by Norman 
 and Burrough, no reference is made to any change, 
 periodical or otherwise, either in the variation or 
 the dip. 
 
 In the following century, the change to which 
 the variation is subject was observed by Mair, 
 Gunter, Gellibrand, and Bond. In the year 1599, 
 Edward Wright wrote a work on the compass, which 
 was published by Prince Maurice, lord high ad- 
 miral of the United Provinces, in which the advan- 
 tage of keeping registers of the variations observed 
 on all voyages is urged. Thus the variation of the 
 variation, not only as to time, but as to place, had 
 at this period begun to receive the attention of 
 those engaged in navigation. 
 
 (198.) When the influence of magnets on ferru- 
 ginous matter came to be examined, it was soon appa- 
 rent that they not only enjoyed the property of attrac- 
 tion, but that soft iron, so long as it remained within 
 the sphere of their influence, actually acquired their 
 own nature, and became magnetic also. When 
 withdrawn from the influence of the magnet, the 
 iron was found to return to its natural state. If, 
 however, the iron, while influenced by the magnet, 
 were twisted, filed, hammered, or submitted to other 
 violence affecting its structure, it was then found co 
 
9b INTRODUCTION. 
 
 preserve the magnetism it had acquired, even when 
 withdrawn from the magnet. 
 
 (199.) When iron filings were scattered over a 
 sheet of paper under which a magnetic bar was placed, 
 it was found that the metallic powder arranged itself 
 in a particular manner, indicating different intensi- 
 ties of attraction in different parts of the bar. At a 
 point near the centre the attraction seemed to cease, 
 and to be augmented in each direction towards the 
 extremities. The POLARITY of the magnet was 
 consequently apparent. The points where the at- 
 traction seemed to be most intense were called the 
 POLES. 
 
 When a magnetic bar was broken in the middle, 
 or at the neutral point, each part was found to ac- 
 quire separate polarity, and, like the original magnet, 
 to have two poles with neutral points intermediate. 
 When magnetism was imparted by a magnet to a 
 bar of iron, the former lost none of its own magnetic 
 force. Hence it was inferred that, in giving mag- 
 netism, the magnet lost none of the magnetic fluid. 
 (200.) When a magnet was brought in contact with 
 a piece of steel, the effect was first discovered to be 
 feebly but gradually increased, until the steel itself 
 became a permanent magnet, but that this might be 
 effected suddenly by friction. Bars of steel, thus 
 magnetized, were called artificial magnets. 
 
 (201.) Gilbert, in his work already referred to 
 published in the sixteenth century, mentions that the 
 fact of magnetism being imparted to a bar of iron 
 by the earth itself, was first discovered by examin- 
 ing the rod of the weathercock of the church of 
 the Augustines at Mantua. 
 
COULOMB'S RESEARCHES. 197 
 
 The possibility of conferring magnetism on sub- 
 stances which are not ferruginous was shown in 
 1733 by Brandt, who imparted magnetism to the 
 metal cobalt. Croustedt, in 1750, showed that nickel 
 is also susceptible of this property. 
 
 (202.) After philosophers had become familiar 
 with the attractions and repulsions, the polarity and 
 directive power of magnets, their attention was di- 
 rected to the establishment of a numerical measure 
 of the actual amount of attractive or repulsive force 
 which they exerted under given circumstances. For 
 a long period, no estimate of this was formed more 
 accurate than the weight which, by attraction, the 
 magnet was capable of supporting attached to a 
 piece of soft iron adhering to it. In 1789, COULOMB 
 applied to magnetism those beautiful and accurate 
 instruments of investigation which were so suc- 
 cessfully employed in electricity and other depart- 
 ments of experimental physics, and determined by 
 their means the intensities and laws of magnetic 
 forces. Two methods of measuring the force ex- 
 erted were practised by him, similar to those by 
 which electric attractions and repulsions had been 
 measured. These were, the balance of torsion, by 
 which the amount of the force was estimated by the 
 action of a twisted wire or fibre of silk; and the ob- 
 servation of the number of oscillations which the 
 attracted or repelled body made in a given time, on 
 each side of the line of attraction or repulsion. By 
 these means it was demonstrated that the force of a 
 magnet was, cceteris paribus, in the direct ratio of 
 the absolute intensity of the magnetism, and inversely 
 as the square of the distance of the attracted or 
 o 3 
 
198 INTRODUCTION. 
 
 repelled body from it ; a law identical in all respects 
 with that by which electrical attractions and re- 
 pulsions are governed. He also estimated, as he 
 had done with electrified conductors, the distri- 
 bution of magnetism on the surface of magnetized 
 bars ; and found that in bars of equal transverse sec- 
 tion, of which the length was considerable compared 
 with the magnitude of the section, the poles or 
 points of maximum intensity were always at a dis- 
 tance of about an inch and a half from the extre- 
 mities ; and that, in very short bars, the poles are at 
 one third of their length from the extremities, and 
 that this latter position is the limit to which the 
 poles approach as the bars are diminished in length. 
 
 (203.) In making artificial magnets, either by 
 means of natural magnets or by other artificial mag- 
 nets already made, the process first adopted was to 
 rub the bar to be magnetized, from end to end, with 
 one of the poles of the magnet by which it was to be 
 magnetized. This method succeeded sufficiently 
 well in magnetizing short needles ; but, when applied 
 to bars of any considerable length, it was attended 
 with the liability of producing consequent points^ 
 that is, in fact, making the bar into a succession of 
 magnets instead of a single magnet. Thus, a certain 
 portion of the entire length, measured from the ex- 
 tremity, would possess two poles and an interme- 
 diate neutral point ; then another succeeding portion 
 of the length would possess other two poles with 
 another intermediate neutral point, and so on. 
 
 (204.) In 1745, Dr. Gowan Knight of London 
 practised an improved method. He placed two strong 
 bar magnets end to end in the same line, the north 
 
ARTIFICIAL MAGNETS. 1 99 
 
 pole of the one being in contact with the south pole 
 of the other. Over them he laid the bar to be mag- 
 netized, its centre coinciding with the united ends 
 of the two magnets, and its length laid along them. 
 In this position the two magnets were drawn asunder 
 lengthwise, their poles passing under each half of 
 the length of the bar to be magnetized. By this 
 method the bar acquired much stronger magnetism 
 than by that which had previously been practised. 
 
 (205.) Du HAMEL further improved this process. 
 The bar to be magnetized being placed between the 
 pieces of soft iron, he took two bar magnets, and 
 placing the north end of one and the south end of the 
 other upon the centre of the bar, and inclining them 
 at an angle of about 30 to it, he drew them upon it 
 from the centre to the extremities, and repeated this 
 process until the bar was strongly magnetized* This 
 method was modified by MITCHELL, who placed a 
 series of bars to be magnetized in the same straight 
 line,, with their extremities successively in contact. 
 He then placed two bundles of strong magnets per- 
 pendicular to them, with their ends resting upon 
 them, the northern end of one bundle and the 
 southern end of the other being downwards. These 
 two bundles of magnets, being attached to each 
 other, were moved over the series of bars to be 
 magnetized. 
 
 (206.) In 1789, COULOMB directed his investiga- 
 tions to the processes of producing artificial magnets. 
 He showed that the susceptibility of bars of steel for 
 magnetism depended conjointly on the temper of 
 the steel and the force of the magnets, and that 
 there was a certain limit to the magnetic force 
 o 4 
 
200 INTRODUCTION. 
 
 which a bar could receive. When a bar attained this 
 limit, it was said to be magnetized to saturation. 
 
 (207.) The magnetic needles of ships' compasses 
 being liable to great vicissitudes of temperature, it 
 was a question of considerable importance to navi- 
 gation whether heat affected the magnetic virtue. 
 Gilbert was the first who observed that a magnet 
 lost all its power when raised to a white heat, and 
 on being cooled did not recover its magnetism. It 
 was not, however, till a much later period, that the 
 influence of neat on magnetism was submitted to 
 accurate inquiry. 
 
 (208.) It was natural that the directive power of 
 the magnet, and its application to navigation, should 
 engross a large share of attention ; and that the go- 
 vernments of maritime countries, more especially, 
 should cause to be carefully and accurately observed 
 all those phenomena by which that property was 
 affected. The variation of the . needle, and the 
 changes periodical and local to which it is subject, 
 were questions of the highest importance to national 
 and commercial interests in every part of the world. 
 So early as 1722, Graham had observed that in a 
 given place the needle was subject to a diurnal va- 
 riation, which was afterwards ascertained with great 
 precision in different parts of Europe. It was ob- 
 served by WARGENTIN, secretary to the Swedish 
 Academy in 1750, and by CANTON in London in 
 1756, and subsequently by VAN SWIETEN, with 
 nearly the same results. From all these observations 
 it appeared that the north pole of the needle be- 
 gins to turn westward at seven or eight o'clock in 
 the morning, and continues to deviate in that clirec- 
 
DIURNAL VARIATION. 201 
 
 tion till about two o'clock, when it becomes station- 
 ary, and soon begins to return eastward, arriving at 
 the position it had in the morning at the same hour 
 in the evening. Canton's observations showed that 
 the amount of this deviation varied from seven to 
 thirteen or fourteen minutes, being greatest at mid- 
 summer and least at midwinter, and increasing and 
 decreasing gradually between these seasons. 
 
 More recently the same phenomenon has been 
 observed by Colonel Beaufoy, Professor Hansteen, 
 and others. 
 
 ( 209.) Cassini, who observed the diurnal variation 
 of the needle at Paris, found that neither the solar 
 heat nor light influenced it ; for it was the same in 
 the deep caves constructed under the Observatory in 
 Paris, where a sensibly constant temperature is pre- 
 served, and from which light is excluded, as at the 
 surface. In northern regions these diurnal changes 
 are greater and more irregular; while, towards the 
 line, their amplitudes are gradually diminished until 
 at length they disappear. 
 
 (210.) The investigation of the changes produced 
 in the direction of the needle, and in the intensity of 
 the earth's attraction upon it, by change of place upon 
 the surface, being a matter vitally important to com- 
 merce and navigation, has engaged the attention of 
 all maritime and commercial countries, from an 
 early period in the history of the mariner's compass. 
 In fact, what may be not improperly called magnetic 
 geography has been, and still is, a subject of pro- 
 found interest, as well to the merchant as to the 
 philosopher. 
 
 It has been already stated that the discoverer of 
 
202 INTRODUCTION. 
 
 the DIP found that at London a magnetic needle, 
 free to move on an axis perpendicular to the mag- 
 netic meridian, presented its north pole downwards 
 forming an angle of above 71. If the instrument 
 be carried northward, it is found that the dip 
 gradually increases ; and, on reaching a certain 
 region near the pole, the needle would become ver- 
 tical, the dip being then 90, and its north pole 
 pointing downwards. At such a place, the common 
 compass needle, moving on a vertical support, would 
 lose its directive power, and rest indifferently in any 
 position. A place where these effects would be pro- 
 duced is called a NORTHERN MAGNETIC POLE. 
 
 (211.) If, on the other hand, the dipping needle, 
 were carried towards the equator, the magnitude of 
 the DIP would be gradually diminished, until, on 
 arriving at a certain region near the equator, the 
 needle would become horizontal, and the dip would 
 become nothing ; and if the dipping needle could be 
 carried round the globe, always following such a 
 course as would allow it to retain its horizontal po- 
 sition, its course traced on the globe would be the 
 
 MAGNETIC EQUATOR. 
 
 The magnetic equator does not coincide with the 
 equator of the globe, nor is it a great circle of the 
 earth. It never departs from the equator, however, 
 more than twelve or thirteen degrees. 
 
 (212.) If, after passing the magnetic equator, the 
 dipping needle be carried southwards, its south pole 
 will dip or be directed downwards ; and this dip will 
 increase in magnitude as the needle approaches the 
 south pole. A place near that pole, where the needle 
 becomes vertical, is a SOUTHERN MAGNETIC POLE. 
 
ELECTRO-MAGNETISM. 203 
 
 The first national project to determine the ele- 
 ments of magnetic geography was undertaken by 
 the British government about the year 1700, when 
 the celebrated Halley was commissioned to make a 
 voyage with the view to collect the necessary ob- 
 servations. The results obtained by him were, 
 however, deprived of the chief part of the advan- 
 tages which ought to have attended them, because 
 of the absence of uniformity in his instruments, and 
 the neglect of making proper comparisons of them 
 with others. 
 
 Since that period, observations have been made 
 and recorded in all extensive voyages, and the data 
 for the determination of the elements of this part 
 of physical geography have been greatly aug- 
 mented. The question being still, however, under 
 investigation, the statement of the circumstances 
 attending it will find a more proper place in the 
 following treatise, than in the present historical 
 summary. 
 
 IV. ELECTRO-MAGNETISM. 
 
 (213.) Those capital experiments by which the 
 science of magnetism has been reduced to the rank of 
 a branch of electricity, by showing that all magnetical 
 phenomena are merely effects of electrical currents 
 modified by physical influences peculiar to certain 
 substances, are of so very recent a date that they 
 can scarcely be considered as yet falling within the 
 scope of Scientific History. Nevertheless, the im- 
 portant relations they bear to other parts of physics, 
 
204 INTRODUCTION. 
 
 the high generality of the phenomena themselves, 
 and especially their susceptibility of being re- 
 duced to mathematical analysis, require that they 
 should not be passed without some notice, even in 
 a sketch so brief and rapid as the present. Since, 
 however, it is proposed in these volumes to enter 
 very fully into the details of the chief experiments 
 which form the foundation of this part of electrical 
 science, it will be sufficient here to notice concisely 
 the chief results, in the order of their discovery, of 
 those experimental investigations which may be 
 regarded as forming the basis of the division of the 
 science now denominated ELECTRO-MAGNETISM. 
 
 At a very early period in the progress of electri- - 
 cal inquiries, indications were observed of a relation 
 existing between electricity and magnetism. Ships' 
 compasses had their directive powers impaired by 
 lightning, and sewing needles were rendered mag- 
 netic by electric discharges passed through them. 
 The influence of electricity over the magnetic pro- 
 perties of iron had been sufficiently noticed to 
 suggest to the clear and far-sighted mind of Bec- 
 caria a notion, which can scarcely be called a vague 
 one, of that theory of terrestrial magnetism which 
 may now be regarded as established on the basis of 
 electro-magnetical phenomena. 
 
 No facts sufficiently clear and decisive to afford 
 general conclusions were produced until the year 
 1280, which was signalised by the greatest discovery 
 in physical science since the memorable invention 
 of the pile. 
 
 (214.) Professor OERSTED, of Copenhagen, had 
 promulgated certain theoretical views on the subject 
 
OERSTED'S EXPERIMENTS. 205 
 
 of the relations of electricity and magnetism in the 
 year 1807, which obtained little attention, being un- 
 accompanied by any new facts, and the community 
 of science being then engrossed by the various and 
 interesting experimental applications of the pile, 
 and the magnificent series of discoveries which 
 Davy was beginning to unfold. In 1820, however, 
 Oersted supplied all that was wanting in 1807 to 
 fix the attention of scientific inquirers a capital 
 experiment. In that year he announced the fact, 
 that a magnetized needle placed near a metallic 
 wire connecting the poles of a pile was compelled 
 to change its direction ; that the new direction 
 .which it assumed was determined by its position in 
 relation to the wire, and to the direction of the 
 current transmitted along the wire ; that when the 
 current was sufficiently strong, and the needle suffi- 
 ciently sensitive, the latter always assumed a position 
 at right angles to the wire ; and that, whenever the 
 direction of the current along the wire is reversed, 
 the needle, making half a revolution, reverses the 
 direction of its poles, keeping still perpendicular to 
 the wire. This discovery being made known caused 
 unqualified astonishment throughout Europe ; the 
 more especially, as all the attempts made before to 
 trace the relation between the electric current and 
 the magnet had been unavailing. The enthusiasm 
 which had been lighted up by the great discovery 
 of Volta twenty years before, and which time had 
 moderated, was relumined, and the experimental 
 resources of every cabinet and laboratory were 
 brought to bear on the pursuit of the consequ'ences 
 of this new relation between sciences so long sus- 
 
206 
 
 INTRODUCTION. 
 
 pected of closer ties. The inquiry was taken up by 
 AMPERE, ARAGO, BIOT, SAVART^ and SAVARY, in 
 France; by DAVY, GUMMING, and FARADAY, in 
 England ; and by DE LA RIVE, BERZELIUS, SEE- 
 BECK, SCHWEIGER, NOBILI, and others, in various 
 parts of Europe. 
 
 (215.) Among these, in the inquiry now before 
 us, AMPERE has assumed the first and highest place. 
 No sooner was the fact discovered by OERSTED made 
 known, than that philosopher commenced the beau- 
 tiful series of researches which has since surrounded 
 his name with so much lustre, and brought electro- 
 dynamics within the pale of mathematical physics. 
 On the 1 8th of September, 1 820, within less than three 
 months of the publication of Oersted's experiments 
 in France, Ampere communicated his first memoir 
 on electro-magnetism to the Academy of Sciences. 
 
 (216.) In this paper was explained the law 
 which determined the position of the magnetic 
 needle in relation to the electric current. In order 
 to illustrate this, he proposes that a man should 
 imagine the current to be transmitted through his 
 body, the positive wire being applied to his feet 
 and the negative wire to his head, so that the cur- 
 rent of positive fluid shall pass upwards from the 
 feet to the head, and that of the negative fluid 
 downwards from the head to the feet. This being 
 premised, a magnetic needle freely supported on its 
 centre of gravity, so as to be capable of assuming 
 any direction, and placed before him, will throw 
 itself at right angles to him ; the north pole of the 
 needle pointing towards his left, and the south pole 
 towards his right. 
 
AMPERE'S RESEARCHES. 207 
 
 If the person through whose body the current 
 thus passes turn round, so as to present his face in 
 different directions, a magnetic needle, still placed 
 before him, will have its direction determined by 
 the same condition ; the north pole pointing always 
 to the left, and the south to the right. 
 
 In the same memoir were described several in- 
 struments intended to be constructed; especially 
 spiral, or helical wires, through which it was pro- 
 posed to transmit the electric currents, and which, it 
 was expected, would thereby acquire the properties 
 of magnets, and retain these properties so long as 
 the current might be transmitted through them. 
 The author also explained his theory of magnets, 
 ascribing their attractive and directive powers to 
 currents of electricity circulating constantly round 
 their molecules, in planes at right angles to the line 
 joining their poles ; the position of the poles, on the 
 one side or the other of these planes of revolution, 
 depending on the direction of the revolving current. 
 
 (217.) On the 25th of the same month, Ampere 
 communicated to the Academy another paper.* In 
 this he delivered the results of his experiments 
 on the reciprocal attractions and repulsions of 
 electric currents acting on each other. He showed 
 that two straight wires along which currents are 
 transmitted will attract or repel each other, ac- 
 cording to the direction of the currents. Let a line 
 be imagined intersecting both wires at right an- 
 gles. If both currents move towards this per- 
 pendicular or both from it, the wires will attract 
 each other ; but, if while one of the currents moves 
 * Annales de Chimie et Physique, torn. xv. 59 170. 
 
208 INTRODUCTION. 
 
 
 towards this perpendicular the other moves from it, 
 then they will repel each other. If the wires be 
 parallel to each other, they will attract or repel each 
 other, according as the currents move in the same 
 or opposite directions. If the wires be in the same 
 plane, but not parallel, their directions will meet if 
 produced : in this case they will attract each other, 
 if the currents be both directed towards, or from, the 
 point where their directions meet ; and they will 
 repel each other, if one current be directed towards, 
 and the other from, that point. 
 
 (218.) In the same paper he proposes the hypo- 
 thesis of currents of electricity circulating round the 
 terrestrial globe, from east .to west, in planes at right 
 angles to the direction of the dipping needle, to ac- 
 count for the phenomena of terrestrial magnetism. 
 
 These researches proceeded with unusual celerity. 
 On the 9th of the following month (October), three 
 weeks after the reading of the last-mentioned paper, 
 he presented another memoir to the Academy, in 
 which he investigated the properties of currents 
 transmitted through wires forming closed curves 
 (courbes fermees), or complete geometrical figures. 
 
 (219.) While Ampere was proceeding with these 
 researches, ARAGO directed his inquiries to the state 
 of the wire through which the current was trans- 
 mitted, more especially with a view to determine 
 whether every part of its surface was endowed with 
 the same magnetic properties. With this view he 
 placed iron filings within the sphere of attraction of 
 the wire, and found that they adhered to it, so as 
 to form concentric rings upon it. The moment the 
 connexion of the wire with the pile was broken, 
 
ARAGO S RESEARCHES. 209 
 
 and the current was no longer transmitted along it, 
 the filings fell off and all attraction disappeared. 
 
 (220.) By a process inferred from the theory of 
 Ampere, M.Arago succeeded in imparting perma- 
 nent magnetism to needles and bars of steel by 
 means of the electric current. This was accomplished 
 by making a spiral of wire, through which the cur- 
 rent was transmitted, while the needle or bar to be 
 magnetized was placed within its coils. The posi- 
 tion of the poles of the magnets thus made depended 
 on the direction of the screw, or helix, formed by the 
 conducting wire. If the wire formed a right-handed 
 screw, the poles were placed in one direction ; and 
 if it made a left-handed screw, they were reversed. 
 When the wire was made to form a succession of 
 screws alternately right-handed and left-handed, the 
 bar was magnetized with a corresponding series of 
 consequent points. The same results were obtained 
 whether the electricity transmitted through the wire 
 proceeded from a Voltaic apparatus or from the 
 common electrical machine. * 
 
 (221.) At the same time, or very little later, and be- 
 fore the information of Arago's experiments reached 
 England, Davy succeeded also in imparting mag- 
 netism to needles by the Voltaic current, and by 
 common electricity; and also showed the effect of 
 the conducting wire on iron filings.f 
 
 M. Ampere, with the view of more completely 
 developing the action of electric currents and mag- 
 nets separately and on each other, contrived various 
 
 * Annales de Chimie et Physique, torn. xv. p. 93. 
 
 f Letter to Wollaston, 12th Nov. 1820, Phil. Trans. 1821. 
 
 VOL. I. P 
 
210 INTRODUCTION. 
 
 methods by which wires, formed into parallelo- 
 grams, circles, and other geometrical figures, could 
 have a current transmitted round them, and be at 
 the same time so supported or suspended as to be 
 capable of assuming any position or direction to 
 which their mutual attraction, or the attraction be- 
 tween them and magnets placed near them, or the 
 influence of the magnetism of the earth upon them, 
 might dispose them. These contrivances afterwards 
 became instruments by which many important ex- 
 periments were made ; the first of which was com- 
 municated to the Academy on the 30th of October, 
 1820. This was the fact, that a wire forming a 
 plane geometrical figure through which the electric 
 current is transmitted will, if free to move, dispose 
 itself so that its plane shall be at right angles to the 
 dipping needle. 
 
 (222.) On the same day, MM. BIOT and SA VART 
 communicated to the Academy the results of experi- 
 ments made with the view to determine the law of 
 the mutual attraction and repulsion of electric 
 currents. The results of these experiments were 
 reduced to analytical investigation by LA PLACE, 
 who showed that the law resulting from them was, 
 that the attraction or repulsion of each elementary 
 .part of the current diminishes in the same ratio as 
 the square of the distance of the object on which 
 it acts increases; a law identical with that of all 
 other modes of electrical attraction and repulsion. 
 ;The effect of the obliquity of the current to the 
 direction in which the force acted was also deter- 
 mined. 
 
 (223.) On the 4th of December following, M. AM- 
 
BIOT. SAVART. LA PLACE. FARADAY. 211 
 
 PERE read to the Academy the memoir which con- 
 tains the reduction of the phenomena of electro- 
 magnetism to mathematical analysis. He showed that 
 all the various phenomena attending the action of 
 magnets on each other, of electric currents on mag- 
 nets, and of magnets on electric currents, and, in 
 fine, of electric currents on each other, could be 
 derived, by mathematical calculation, from formulae 
 expressing the action of two infinitely small ele- 
 ments of electric currents, acting on each other in 
 the direction of the line joining their middle points. 
 The discussion of this subject was concluded in 
 another memoir, read to the Academy on the 8th 
 and 15th of January, 1821. 
 
 (224-.) This year, 1821, was signalized by the com- 
 mencement of the labours of FARADAY in electro- 
 magnetism. This philosopher, who has since attained 
 such well-merited celebrity, realized a suggestion 
 which originated with Dr.Wollaston. A magnet 
 being placed in a vertical position, a wire was so 
 suspended that, while the electric current was pass- 
 ing through it, it was capable of moving round the 
 axis of the magnet so as to describe a conical or 
 cylindrical surface. While the current was main- 
 tained, the wire took spontaneously this motion ; and 
 when the direction of the current along it was re- 
 versed, it reversed its motion, and turned round the 
 magnet the contrary way. Reversing these con- 
 ditions, and instead of fixing the magnet and leaving 
 the wire free, he fixed the wire, and so adjusted the 
 magnet that it was at liberty to revolve round the 
 wire as an axis. When the current was transmitted 
 through the wire, the magnet spontaneously revolved 
 p 2 
 
212 INTRODUCTION. 
 
 round it; and when the direction of the current 
 through the wire was changed, the motion of the 
 magnet was reversed. 
 
 FARADAY attempted, without success, to cause a 
 magnet to revolve on its own axis ; but, the memoir 
 containing the account of his experiments being pub- 
 lished in France, AMPERE succeeded in producing 
 rapid rotatory motion of magnets on their own axes, 
 and showed that this and the two former results of 
 Faraday's experiments followed as necessary con- 
 sequences of his own mathematical principles. He 
 also showed that the same effects could be produced 
 with helical currents, thus affording a further cor- 
 roboration of his theory of magnetic action. 
 
 (225.) Immediately after the publication of these 
 experiments of Faraday, DAVY thought that the effect 
 of the magnet on the current might be obtained in a 
 more simple state by transmitting the current through 
 a fluid conductor, and exposing the conductor to the 
 action of a strong magnet. With this view, two 
 copper wires, about a sixth of an inch in diameter, 
 coated with sealing-wax, and flattened and polished 
 at the ends, were cemented into two holes three 
 inches apart in the bottom of a glass dish, so that 
 the direction of the wires was perpendicular to the 
 dish. The coating of sealing-wax rendered the wires 
 non-conductors, except at their flattened and po- 
 lished ends, which were not coated. Mercury was 
 poured into the dish so as to cover the ends of the 
 wires to the depth of the tenth or twelfth of an inch. 
 The parts of the wires proceeding from the bottom 
 of the dish were now put in connexion with a 
 powerful Voltaic battery, the positive current flow- 
 
DAVY'S EXPERIMENTS. 213 
 
 ing into the mercury at one wire, and passing from 
 it at the other. The moment the current commenced, 
 the mercury over each wire was thrown into a state 
 of violent agitation. Its surface was raised into the 
 form of two small cones one over each wire ; waves 
 flowed off in all directions from these cones. On 
 holding the pole of a powerful bar magnet some 
 inches above one of the cones, its vertex was lowered; 
 and according as the magnet descended towards the 
 mercury the subsidence of the cone continued, and 
 the propagation of waves around it ceased, until at 
 length the surface of the mercury became perfectly 
 level, and a slow revolving motion of the mercury 
 round the pole of the magnet began to be manifested. 
 As the magnet was brought still closer to the mer- 
 cury this gyration of the fluid became more rapid, 
 and the centre round which the gyration took place 
 (which was directly over the end of the wire) be- 
 came depressed. The rapidity of the rotation of the 
 . mercury, and the depression of the centre of the 
 vortex, continued to increase as the magnet was 
 brought nearer to the mercury, until no more mer- 
 cury remained over the end of the wire than was 
 barely sufficient to cover it. This rotation took 
 place with either pole of the magnet, and over either 
 wire, changing its direction when either the pole of 
 ' the magnet or the direction of the current was 
 changed. It is evident that these phenomena are in 
 accordance with, and referable to, the same general 
 law as those previously discovered by Faraday. The 
 same effects were observed when fused tin was sub- 
 stituted for mercury, and when steel wires were used 
 p 3 
 
214 INTRODUCTION. 
 
 to conduct the current. The current was also con- 
 ducted to the dish by tubes filled with mercury, with 
 like results.* 
 
 (226.) In order to determine whether the matter 
 forming the conductor along which the electric cur- 
 rent passed had any influence on the electro-magnetic 
 phenomena which at this time engaged the attention 
 of philosophers, Davy placed two pieces of charcoal 
 in connexion with the wires of a powerful Voltaic 
 battery, and, by presenting their points towards 
 each other, at a distance varying from one to four 
 inches, according to the density of the air in which 
 the experiment was made, he obtained a column of 
 electric fluid formed by the current passing through 
 the space between the charcoal points. This cur- 
 rent was not transmitted, as usual, along any con- 
 ductor, but merely passed through the air between 
 the points ; and its presence was rendered manifest 
 by the light evolved. When a powerful magnet 
 was presented to this column, with its pole at a 
 very acute angle to it, the column was attracted or 
 repelled with a rotatory motion, or made to revolve 
 by placing the poles in different positions, in the 
 same manner as metallic wire conducting the cur- 
 rent would have been. The electric column was 
 more easily affected by the magnet, and its motion 
 was more rapid, when it passed through dense than 
 through rarefied air; and, in this case, the con- 
 ducting medium, or chain of aeriform particles, was 
 much shorter, f 
 
 * Phil. Trans. 1823; also Davy's Vorks, vol. vi. p. 258. 
 | Phil. Trans, 1821 ; Davy's Works, vol. vi. p. 232. 
 
SCHWEIGER. 215 
 
 While these investigations were proceeding in 
 France and England, the discoveries to which they 
 led conducted a German philosopher to the inven- 
 tion of an instrument of physical inquiry of sur- 
 passing beauty and utility, and equalled in import- 
 ance by none which had appeared since the balance 
 of torsion. 
 
 (227.) The multiplier, or, as it has been called in 
 England, the galvanometer, invented by SCHWEIGER, 
 is an instrument by which the presence of an electric 
 current is detected, and its intensity measured. It 
 is based upon the fact, that a wire through which a 
 current passes will have a tendency to turn a mag- 
 netic needle at right angles to it. By this beautiful 
 instrument the most feeble currents may be made 
 manifest, and their intensities compared. It is 
 various in its construction, according to the nature 
 and power of the electric currents intended to be 
 observed, but generally consists of a rectangular 
 frame of wood, round two parallel sides of which a 
 copper wire, lapped with silk, is rolled, so that the 
 coils of wire shall be close beside each other, and 
 parallel in their general direction to the other two 
 sides of the frame. Within the frame, and between 
 the two surfaces formed by the coils of wire, is 
 suspended a magnetic needle. If the frame be so 
 placed that the needle, when at rest, shall be parallel 
 to the coils of wire, these coils will be parallel to 
 the magnetic meridian. Matters being thus dis- 
 posed, let the extremities of the wire be put in 
 connexion with the poles of a Voltaic pile. The 
 current passing through the wire will act upon the 
 needle, and each coil of the wire will affect it as a 
 p 4 
 
216 INTRODUCTION. 
 
 separate current, so that the total effect will be in 
 proportion to the number of coils. If the current 
 in the lower coils moved in the same direction as 
 the upper, it would have a contrary effect on the 
 needle ; but, by the manner in which the wire is 
 carried round the frame, the systems of inferior 
 currents are contrary in their direction to the supe- 
 rior currents, and they have, consequently, the 
 same effect on the needle. If the effect of the 
 current thus multiplied be sufficient, the effect of 
 the earth's magnetism will be overcome, and the 
 needle will be turned at right angles to the wires, 
 and, consequently, will take the direction of mag- 
 netic east and west ; but if the force of the current 
 be insufficient for this, the needle will be deflected 
 at some definite angle with the magnetic meridian, 
 the magnitude of which angle will supply the means 
 of estimating the force of the current. 
 
 It is evident that the sensibility of this instrument 
 will be augmented in proportion as the magnetism 
 of the needle is enfeebled, and the number of coils 
 of wire augmented. 
 
 The direction of the current is indicated by the 
 direction in which the deflection of the needle takes 
 place. If the north pole of the needle be deflected 
 towards the east when the current passes in one 
 direction through the wire of the multiplier, it will 
 be equally deflected towards the west when the same 
 current is reversed. 
 
 (228.) When Ampere had demonstrated the reci- 
 procal action of electric currents on each other and 
 on magnets, he showed that the terrestrial globe ex- 
 erted an influence on magnets freely suspended, and 
 
THEORY OF TERRESTRIAL MAGNETISM. 217 
 
 on electric currents transmitted through wires so 
 supported as to be capable of obeying any forces 
 exerted upon them, identical in all respects with 
 the influence which a sphere would exert round 
 which a wire coiled so that its coils shall nearly 
 coincide with the parallels of latitude, through 
 which wire an electric current is transmitted, run- 
 ning continually from east to west, or contrary to 
 the diurnal motion of the earth ; or, since the wire 
 in such case is merely necessary to conduct the 
 electricity, the phenomena of terrestrial magnetism 
 only require the admission that a series of electric 
 currents continually circulate round the globe, 
 according to lines which intersect the magnetic 
 meridians perpendicularly. 
 
 (229.) To present an experimental verification of 
 this theory, M. Ampere constructed a plane geome- 
 trical figure, a circle, for example, of wire, and sus- 
 pended it in such a manner that, while the current 
 circulated upon it, the figure was capable of moving 
 on a vertical axis through its centre of gravity. It 
 was observed to throw its plane into a position at 
 right angles to the magnetic meridian. When the 
 current was reversed, it turned round through 1 80, 
 and reversed its plane. When a helix was suspended 
 on its centre of gravity, and a current was transmitted 
 through the wire, it exhibited all the properties of a 
 magnet ; when suspended on a vertical axis, it as- 
 sumed the direction of the magnetic meridian ; and 
 when suspended on a horizontal axis at right angles 
 to the magnetic meridian, it threw itself parallel to 
 the dipping needle. 
 
 The hypothesis of Davy, that the nucleus of the 
 
INTRODUCTION. 
 
 globe consisted of the metallic bases of the alkalies 
 and earths, and that its surface was oxydated, sup- 
 plied Ampere with strong grounds of probability 
 in support of these theoretical ideas of terrestrial 
 magnetism. It was easy to imagine that, at the 
 surface of contact of the metallic nucleus and the 
 surrounding shell of oxydated matter, there were 
 constant chemical actions in progress, which might 
 produce a series of electric currents at some dis- 
 tance below the surface of the earth, and that these 
 currents, acting through the shell of oxides, would 
 produce the phenomena of terrestrial magnetism. 
 
 (230.) In the same year, M. de la Rive, of Geneva, 
 published a memoir, in which he showed that when 
 a current is transmitted through a closed circuit of 
 a rectangular form, for example, it affected only the 
 sides which have a vertical position. He esta- 
 blished, as a general law, that a vertical current, 
 capable of revolving round a fixed vertical line as 
 an axis, will place itself so that the plane passing 
 through its own direction, and the axis round which 
 it revolves, shall be at right angles to the magnetic 
 meridian, the side on which the current descends 
 being on the east of the axis, and the side on which 
 it ascends being on the west. 
 
 He also showed that a horizontal current, though 
 not susceptible of being influenced by the magnet- 
 ism of the earth, is not therefore free from all ac- 
 tion; on the contrary, he proved that when it is 
 free to move parallel to itself, it will move in this 
 manner in the one direction or the other, according 
 to its own direction ; and that this motion will equally 
 ensue in all positions in which it may be placed, 
 
1)E LA HIVE. SAVARY. 219 
 
 whether it be directed north and south, east and 
 west, or in any intermediate azimuth. 
 
 These laws, proved experimentally by M. de la 
 Rive, were immediately shown by M. Ampere to be 
 direct consequences of his theoretical principles. 
 
 (231.) In the year 1827, M. SAVARY directed his 
 labours to follow out the researches on the power of 
 theVoltaic current to impart magnetism to iron, which 
 had been demonstrated by the experiments of Davy 
 and Arago. M. Savary discharged a Ley den jar 
 through a metallic wire, needles placed near which 
 were found to be magnetized, and the strength oi 
 the magnetism imparted to them was observed to 
 vary with their distance from the wire. Being 
 placed at various distances from it, the magnetizing 
 power of the current was not found either conti- 
 nually augmented, or continually decreased ; but, as 
 the needle receded, it first increased and then di- 
 minished, attaining a maximum at a certain posi- 
 tion. He also showed that as the distance varied, 
 not only the intensity of the magnetic force passed 
 thus successively through maxima and minima, but 
 the polarity was reversed, taking alternately one 
 direction or the other. These alternations of in- 
 tensity and polarity appeared to be determined in a 
 great measure by the weight, diameter, and con- 
 ducting power of the wire, and the strength of the 
 electric discharge. 
 
 (232.) One of the most novel and unexpected cir- 
 cumstances attending the experiments of M. Savary, 
 was the manner in which he showed that the mag- 
 netizing influence of the current was modified by 
 transmitting it through other metals. When the 
 
220 INTRODUCTION. 
 
 needle to be magnetized was enveloped in metallic 
 leaf, the magnetism it received was augmented. 
 By gradually increasing the thickness of its metallic 
 coating, the force of the magnetism it received in- 
 creased by degrees till it attained a maximum, after 
 which it diminished; and, by further augmenting 
 the thickness of its coating, it was diminished so as 
 to be equal to the magnetism it would receive with- 
 out any coating. Copper, tin, gold, silver, and 
 mercury, used as coating, were found to possess this 
 property in different degrees. The force of the 
 electric charge transmitted through the wire was 
 found to have a singular influence on the pheno- 
 menon; for, according as this force was increased or 
 diminished, different thicknesses of the same coating 
 were necessary to produce equal effects. These consi- 
 derations also affected the direction of the polarity. 
 (233.) These facts appeared to M. Savary to 
 be scarcely compatible with any hypothesis which 
 requires the admission or the translation of electric 
 matter by the current ; and he considered that they 
 indicated rather that the current proceeds from a 
 system of undulations propagated along the wire, 
 and transmitted by it laterally to adjacent media. 
 
 V. THERMO-ELECTRICITY. 
 
 (234.) The fact that a derangement of the equili- 
 brium of temperature was attended with the evolution 
 of electric effects was observed by VOLT A, and subse- 
 quently by DESSAIGNES. VOLTA found that a plate 
 of silver, one end of which was more heated than the 
 other, produced Galvanic effects ; and DESSAIGNES 
 
THERMO-ELECTRICITY. 221 
 
 observed that convulsions were produced in the frog, 
 when the muscles and nerves were connected by a 
 silver spoon in which lighted charcoal was placed. 
 These isolated observations, however, led to no con- 
 clusions affecting the progress of discovery. 
 
 Immediately after the discovery of OERSTED be- 
 came known throughout Europe, Professor SEEB EC K, 
 of Berlin, engaged in a series of researches on the 
 Voltaic effects produced by derangement of tem- 
 perature; and communicated to the Academy of 
 Sciences of Berlin, during the years 1821 and 1822, 
 the results of his experiments, which were published 
 in the Transactions of that body, and form the basis 
 of whatever has since been collected under the title 
 
 of THERMO-ELECTRICITY. 
 
 (235.) A rod of copper being bent into a semi- 
 circle, a bar of antimony was soldered to it, so that the 
 two metals had the form of a stirrup. The tempera- 
 ture of one of the points of junction of the metals was 
 raised, while that of the other was unchanged. An 
 electric current was immediately excited, passing 
 from the copper at the heated point through the 
 antimony. The intensity of the current was aug- 
 mented by augmenting the difference of temperature 
 of the two points of connexion of the metals, and 
 the direction of the current was reversed when the 
 source of heat was removed from one point of junc- 
 tion to the other. The current was rendered mani- 
 fest by its power to deflect a magnetic needle. 
 
 Seebeck observed similar effects by combining- 
 various other metals in pairs ; and found that the 
 relative electric state of the metals did not corre- 
 spond with that assigned to them in Volta's theory 
 
222 INTRODUCTION. 
 
 of contact. He also observed that currents were 
 produced by inequality of temperature in bars of a 
 single metal, when they have a crystalline structure ; 
 and suggested that the changes of temperature of 
 the metallic nucleus supposed by Davy to be within 
 the external crust of the earth, might produce those 
 currents circulating round the globe to the influ- 
 ence of which Ampere ascribed the magnetism of 
 the globe. 
 
 (236.) In the year 1823, these inquiries were pro- 
 secuted by the Chevalier YELIN, and MM. MARSH 
 and GUMMING.* The first investigated the influ- 
 ence of the nature and form of homogeneous 
 metals on the direction and intensity of the elec- 
 tric current. The two latter philosophers pro- 
 duced the revolution of thermo-electric currents 
 round magnets. Soon afterwards MM. OERSTED 
 and FOURIER communicated to the Academy of Sci- 
 ences a series of experiments on currents obtained 
 by thermo-electric piles, made by combining bars 
 of antimony and bismuth in a series. The alter- 
 nate points of junction were heated, and the current 
 was manifested by the deflection of a magnetic 
 needle. This deflection, though considerable, was 
 not observed to increase in proportion to the num- 
 ber of thermo-electric elements constituting the 
 pile. They attempted, without success, to effect 
 chemical decompositions by the current thus ob- 
 tained. This has, however, been since effected by 
 Becquerel, by exposing to the action of the thermo- 
 electric current solutions easily decomposable. M. 
 Bottot, of Turin, has also succeeded in decom- 
 * Bibl. Univ. torn. xxiv. xxv. xxvii. and xxix. 
 
THERMO-ELECTRICITY. 223 
 
 posing water, and various solutions, by the thermo- 
 electric current obtained from a pile formed of a 
 series of wires of platinum and iron. 
 
 (237.) The result of these and subsequent investi- 
 gations of SEEBECK, BECQUEREL, and others, has led 
 to the establishment of the following series of metals 
 possessing thermo-electric excitability, in the order 
 in which they stand. 
 
 1. Bismuth. 7. Silver. 
 
 2. Platinum. 8. Copper. 
 
 3. Mercury. 9. Zinc. 
 
 4. Lead. 10. Iron. 
 
 5. Tin. 11. Antimony. 
 
 6. Gold. 
 
 If a thermo-electric couple be formed by any 
 two metals in this series, the positive electricity at 
 the heated point will pass from that metal which 
 holds the higher to that which holds the lower 
 place in the series ; consequently, each of the metals 
 in the series is thermo-electrically positive to all 
 above it, and negative to all below it. 
 
 Becquerel showed that when one end of a homo- 
 geneous metallic wire was heated, and brought into 
 contact with the other end, an electric current was 
 formed, passing from the heated to the cold part of 
 the wire across the point of contact. These in- 
 quiries were attended with several curious and im- 
 portant results, when the temperatures were carried 
 to extreme points in the case of certain metals ; but 
 as they will necessarily form a part of the subject 
 matter of these volumes, it is needless to enter here 
 into any details respecting them. 
 
224 INTRODUCTION. 
 
 (238.) In concluding this brief outline of the his- 
 tory of the progress of electrical science, it may b' 
 proper to remind the reader, that the necessary 
 limits of such a compendium as the present have 
 precluded many details which would be regarded 
 with interest by those who prosecute scientific in- 
 quiries. The chief discoveries only, including those 
 which have had most influence in establishing gene- 
 ral theories, were entitled to any detailed notice ; 
 and the labours of contemporary philosophers have 
 been for the most part excluded, as they will find a 
 more suitable place in the manual wiiicii we now 
 offer to the public. 
 
BOOK THE FIRST. 
 
 ELECTRO-STATICS. 
 
 CHAPTER I. 
 
 DEFINITIONS AND PRIMARY FACTS. 
 
 (1 .) As the observation of the phenomena of nature pro- 
 gressively unfolded those general laws which constitute 
 the body of physical science in its present advanced state, 
 effects which were tangible, and admitted of direct nu- 
 merical estimation by weight and measure, first pressed 
 themselves on the attention of philosophers. Weight, 
 extension, magnitude, and form, are properties insepa- 
 rable from matter, in whatever state it may be, and di- 
 vested of which, indeed, it is impossible to imagine its 
 existence. Such qualities, and their consequences, were 
 therefore early subjects of investigation. Some of them 
 are not only inseparable from the matter to which they 
 appertain, but are unchangeable in quantity or degree so 
 long as the identity of that matter continues. The 
 weight, or inertia, of a body must be the same so long 
 as no particle is added to it or subtracted from it; and 
 although the effects of the mutual attraction of its con- 
 stituent molecules, which determine its hardness, brittle- 
 ness, ductility, and elasticity, may be modified by the 
 action of other antagonist forces depending on the agency 
 of heat and the play of the chemical affinities, yet these 
 molecules can by no means be deprived of any portion 
 of that reciprocally attractive force. It is otherwise witi 
 the class of physical agents called the imponderables 
 one of the most remarkable of which is about to engagd 
 VOL. j. Q 
 
226 ELECTRO-STATICS. PART I. 
 
 our attention. Material substances are composed of 
 particles or atoms,, maintained in their position with a 
 greater or less degree of mutual proximity by the opera- 
 tion of forces, the sphere of whose action is extremely 
 minute. But these constituent particles, though in close 
 juxtaposition, are not in absolute contact : on the con- 
 trary, it is demonstrable that they are separated by in- 
 terstitial spaces, which, though small in magnitude 
 compared with the total dimensions of the bodies they 
 pervade, are great, perhaps infinite, compared with 
 the dimensions of the molecules which they separate. 
 Is evidence required of this proposition ? Take the 
 most solid and adamantine body, and accurately measure 
 its volume. Let it then be exposed to cold, so as to 
 reduce its temperature any required number of de- 
 grees. If its magnitude be now measured, it will be 
 found to be less than before. But this diminution of 
 volume has not arisen from any loss of ponderable matter, 
 for the body before and after the change of temperature 
 would be found to have the same weight. It has, 
 therefore, contracted its dimensions by the nearer ap- 
 proach of its constituent particles to each other. Their 
 mutual attraction has been rendered more energetic by 
 the diminished force of that agent which keeps them se- 
 parated ; but had they been in actual contact before the 
 change of temperature, they would have had no room to 
 approach each other, and therefore no diminution of vo- 
 lume would have ensued. To this diminution of volume, 
 of which the hardest and most solid bodies are suscep- 
 tible, there is no known limit, save that which is imposed 
 on the reduction of their temperature ; and the spaces 
 which separate their ^articles must, therefore, bear to 
 the dimensions of the particles themselves a proportion 
 indefinitely great. 
 
 These interstitial spaces or pores, though void of all 
 ponderable matter, are nevertheless the region of physical 
 agents of vast importance in the great economy of nature. 
 The researches of modern philosophers respecting the 
 phenomena of light and heat have rendered it probable, 
 
ELECTRICAL EXCITATION. 227 
 
228 ELECTRO- STATICS. PART I. 
 
 briskly rubbed with a dry woollen cloth,, and then pre- 
 sented to the feather : when the side of the tube where 
 it has been rubbed is brought near the feather, the latter 
 will manifest a tendency to approach the tube ; and when 
 the distance is further diminished, the feather will sud- 
 denly fly to the tube, and adhere to it. 
 
 It is apparent, then, that friction with a woollen cloth 
 has imparted to the glass a property which previously it 
 did not possess, in virtue of which it exerts an attrac- 
 tion on the matter of the feather, and produces a distinct 
 and measurable mechanical effect. The mechanical pro- 
 perties of the tube, and cloth with which it was rubbed, 
 remain, however, unchanged, neither having gained or 
 lost a single particle of ponderable matter. 
 
 This property is not peculiar to glass. If a stick of 
 sealing-wax, or a piece of resin or amber, be rubbed, in a 
 similar manne'r, these substances will manifest the same 
 attraction for the feather. 
 
 Neither is a feather the only substance which will be 
 affected by this attraction. If bits of paper, straw, or 
 other similar light substances, be suspended like the fea- 
 ther, they will be similarly attracted by the glass. 
 
 (3.) The physical agent, whatever be its nature, which 
 is thus called into operation by the friction of the glass, 
 wax, resin, or amber, and to which the attraction is due, 
 is called Electricity. 
 
 The first substance which was known to be capable of 
 acquiring this property was amber, in which it was ob- 
 served by Thales, about six centuries before the birth of 
 Christ. The name electricity has therefore been given 
 to this part of physical science, from the Greek word 
 yXsKTpov (electron), signifying amber, the first substance 
 in which the property had been observed. 
 
 (4.) To examine all the circumstances attending the 
 phenomena to which we have above referred, let us 
 suppose that pieces of glass, sulphur, resin, and sealing- 
 wax are provided, formed into cylinders of such a mag- 
 nitude as to produce the desired phenomena with suf- 
 ficient intensity. For this purpose, cylindrical pieces 
 
CHAP. I. ATTRACTION AND REPULSION. 22Q 
 
 of these substances should be about an inch in diameter, 
 and twelve or fourteen inches in length. Let a small 
 ball, with a diameter of about the eighth of an inch, be 
 turned from the pith of the elder tree ; and let it be 
 suspended from a hook by a fine silken thread, sup- 
 ported by a convenient stand, as represented in fig. 1. 
 This ball will supply the place of the feather in the ex- 
 periments already explained. 
 
 Let a glass rod, S (fig. 1.), be now briskly rubbed with a 
 .,._ , woollen cloth previously well dried, and 
 
 let it be presented to the pith ball B, 
 The ball will approach the glass, and 
 adhere to it. If they be separated by 
 / ' drawing the glass away, and after. 
 / wards the glass be again presented 
 
 / to the pith ball as before, the ball, 
 
 / instead of being attracted, will recoil 
 
 / 5 from it, taking the position B', and 
 
 I rt (J) will remain in that position so long 
 
 jIL B ' as the glass is held near it ; but when 
 
 i-^^ the glass is removed, the ball will again 
 
 descend to its natural position of rest, the silken string 
 by which it is suspended recovering its vertical position. 
 If a stick of resin, sulphur, or sealing-wax be simi- 
 larly rubbed, and presented to the pith ball, the same 
 effect precisely will ensue. 
 
 From these experiments the following consequences 
 may be inferred : 
 
 1st. By the friction of the dry woollen cloth, a quality 
 is imparted to glass, sulphur, resin, or sealing-wax, in 
 virtue of which the pith ball is attracted to it. 
 
 2d. After being brought in contact with the surface 
 of the glass, sulphur, resin, or sealing-wax, and sepa- 
 rated from it, the state of the ball is changed ; and it 
 acquires a property, relative to the glass, sulphur, resin, 
 or wax, in virtue of which, instead of being attracted, it 
 is repelled. 
 
 (5.) Let two pith balls be now suspended from the 
 game hook by two separate silken threads of the same 
 Q 3 
 
230 ELECTRO-STATICS. PART I. 
 
 length, so that the balls shall rest in contact. Let the 
 glass rod, after being rubbed as before, be presented to 
 these balls. It will attract both of them, and they will 
 adhere to it. Let the glass rod be then withdrawn 
 from them. They will no longer 
 fall into the vertical position, and rest \ 
 in contact with each other, as they 
 did before they were touched by the 
 glass : on the contrary, they will 
 repel each other, so that the threads by 
 which they are suspended will diverge, 
 as represented in fig. 2. In fact, the 
 baUs will acquire a property, relative 
 to each other, similar to that which 
 the glass and the single ball had after 
 contact in the preceding experiments. 
 The same effects would ensue if the sulphur, resin, 
 or sealing-wax were used in the experiment. 
 
 (6'.) Whatever theory may be adopted for the ex- 
 planation of electrical phenomena, or to whatever phy- 
 sical agency the immediate effects of electricity may be 
 ascribed, it will be convenient to express the phenomena 
 which are to be described, arid whose laws are to be 
 developed, by terms which must unavoidably have occa- 
 sional reference to one or other of the various hypotheses 
 which have been invented to connect together, system- 
 atize, and generalize the facts which form this branch 
 of physical science. We shall thus frequently refer to 
 electricity as & fluid. In so doing, however, we desire for 
 the present to be understood as using the phrase electric 
 fluid, and similar terms, merely to express ascertained and 
 observed facts, without asssuming anything respecting 
 the physical agent from which electrical phenomena pro- 
 ceed. Whatever the nature of electricity may be, 
 whether it be a fluid sui generis which pervades all bodies, 
 and possesses distinct properties ; or a property of the 
 fluid or fluids from which the effects of heat, light, and 
 other imponderable agents arise; or whether, finally, it be 
 an affection of the constituent molecules of matter^ 
 
CHAP. I. ELECTRICITY A FLUID. 231 
 
 the effects we shall have to explain being such as might 
 and would belong to an elastic or self-expansive fluid, 
 we shall apply that term to express the physical agent, 
 whatever it may be, to which such effects belong ; and 
 this, and this only, is what we would be understood to 
 convey whenever the phrase " electric fluid " is used. 
 
 (7.) The preceding experiments indicate the trans- 
 mission of a physical influence from the glass to the 
 pith balls, which influence the glass itself acquired by 
 friction. The electric fluid, produced on the surface of 
 the glass by friction, being self-expansive, and the 
 surface of the pith balls being of a nature to allow an 
 unobstructed passage to it, a portion of the fluid dis- 
 tributed upon the glass at the point of contact passes, 
 by virtue of its expansive power, to the surface of the 
 pith balls ; and this fluid being self-repulsive, the pith 
 ball, when covered with it, is repelled by the glass. 
 
 That the pith ball, after contact with the glass, pos- 
 sesses the property which by friction was imparted to 
 the glass, may be proved by causing the ball to approach 
 another ball in its natural state. The latter will then be 
 attracted by the former in the same manner as it would 
 be attracted by the glass ; and, after contact, the two 
 balls will repel each other in the same manner as that in 
 which the glass repelled the pith ball after contact in 
 the first experiments, and in the same manner as the 
 two pith balls repelled each other after they had both 
 touched the glass. 
 
 (8.) When glass, sulphur, resin, sealing-wax, or 
 any other substance susceptible of like effects, is sub- 
 mitted to friction, so as to acquire the property here 
 described, such surface is said to be excited; and when, 
 by contact with it, a pith ball or any other body ac- 
 quires alike property, that body is said to be electrified. 
 
 (9.) Besides the effects of attraction and repulsion 
 here described, an electrified or excited body is attended 
 with other effects, not less remarkable, which will be 
 more fully developed as we proceed. If a glass tube, 
 excited by friction, be brought near the surface of the 
 Q 4 
 
232 ELECTRO-STATICS- PART I. 
 
 skin, a sensation will be produced, when the distance 
 is diminished to a certain limit,, similar to that which is 
 felt when we touch a cobweb. Also, if a strongly elec- 
 trified or excited surface be brought nearly into contact 
 with the knuckle, or with a small metallic ball, a little 
 luminous spark, accompanied with a cracking noise, 
 will be observed to pass between the electrified body and 
 the knuckle or to the metallic ball. If the experiment 
 be made in the dark, this spark will be more distinctly 
 visible. 
 
 If any of the bodies above mentioned be excited by 
 friction in the dark, a blueish light will be observed 
 constantly to follow the motion of the cloth with which 
 the friction is performed. 
 
 (10.) Besides the substances above mentioned, there 
 are a vast number of others capable of evolving elec- 
 tricity by friction in the same manner. All vitreous and 
 resinous bodies whatever produce like phenomena in 
 different degrees. They are also obtained with stuffs of 
 silk of every kind. 
 
 (11.) The metals form a class of bodies which at 
 first appear incapable of producing these effects. If a 
 tube or rod of metal be taken in one hand, and any 
 stuff by which other substances are excited be briskly 
 rubbed upon it with the other hand, none of the usual 
 indications of electricity will follow. The metal will 
 not attract light substances ; it will have no effect upon 
 the pith balls; no luminous appearance will follow the 
 rubbing, nor will any effect be produced upon the 
 organs by approaching or touching it. A more close 
 and attentive examination will show, however, that 
 this absence of electrical effects does not arise from the 
 circumstance of the metal being incapable of evolving 
 the electric fluid by friction, but that the proper effects 
 of such an evolution of electricity are masked by others 
 depending on a property of the metals in which vitreous 
 and resinous substances do not participate. To make this 
 apparent, let us suppose that, instead of holding the 
 metallic tube in the hand, we attach to the end of it a 
 
CHAP. I. EXCITATION OF ELECTRICITY. 233 
 
 glass handle, which enables us to hold it in d convenient 
 manner for rubbing it, but at the same time without 
 touching it. If we now rub it as before, taking care 
 not to touch it except with the cloth with which it 
 is rubbed, it will be found to acquire the same elec- 
 trical properties as the glass, sulphur, and resinous sub- 
 stances acquired in the former experiments. 
 
 (12.) The possibility of exciting electricity on the 
 surface of metals may also be shown in the following 
 manner. Let a tube of metal be suspended by a silken 
 cord, and let it be struck two or three times with the 
 skin of a cat ; it will be then found to be electrified, or 
 excited, in the same manner as the glass or resin. 
 
 So long as the metal thus excited is kept suspended 
 by silk, or supported by glass, the electrical state of its 
 surface will continue ; but if it be touched, even for an 
 instant, by the hand, or by a rod of metal held in the 
 hand, it will suddenly lose its electricity altogether. 
 
 There are a variety of other substances, besides silk 
 and glass, by which metal may be suspended or supported 
 so as to enable it to be excited by friction : if it be 
 supported, for example, by any resinous substance, it 
 will be equally capable of being excited. 
 
 (13.) These effects are easily explained. Metallic 
 bodies are susceptible of excitement as easily as the 
 vitreous or resinous bodies ; but they likewise possess 
 a property which the latter do not enjoy, in virtue 
 of which the electric fluid moves freely upon them. 
 In the experiment, therefore, in which a metallic rod 
 held in one hand is rubbed by a woollen cloth held 
 in the other, the electricity which is excited upon the 
 rod of metal passes away through the hand to the 
 body of the operator. That portion of the electricity 
 which is nearest the hand first passes away, and the 
 facility with which the electricity moves upon the sur- 
 face of the metal causes the remaining portion of it 
 immediately to follow. As fast, therefore, as the electric 
 fluid is evolved by the friction upon the metal, it passes 
 
234 ELECTRO-STATICS. PART I. 
 
 away to the- hand and thence to the body of the operator, 
 and no portion remains on the surface of the metal. 
 
 The case, however, is quite otherwise with a rod of 
 glass similarly treated. Unlike metallic surfaces, the 
 surface of glass does not give free motion to electricity. 
 On the contrary, the electricity appears, as it were, 
 to be so obstructed in its movement over a vitreous sur- 
 face, that it may be accumulated in one part, while 
 another part is altogether free from it. If, therefore, a 
 rod of glass be held in one hand, while with the other 
 it is excited by friction, that part of the rod which has 
 been thus submitted to friction will be covered with 
 electricity, whilst that part which is held by the hand 
 will be free from it. The electricity which is accu- 
 mulated on the part of the surface which has been 
 rubbed, does not, as was the case with the metallic sur- 
 face, pass over the surface to the hand and thence into 
 the body of the operator, but remains on that part of the 
 surface where it was excited. The glass, however, may 
 be deprived of its electricity by the operator passing his 
 hand in contact with that part of the surface which has 
 been excited. All the electricity produced upon the 
 glass will then pass into the body of the operator. 
 
 (14.) These and similar effects lead to the distribu- 
 tion of natural substances, in reference to their electrical 
 properties, into two great classes ; those which, like 
 the metals, allow the free motion of the electric fluid 
 over their surfaces; and those which, like glass and 
 resinous substances, do not allow its free motion. The 
 first are called conductors of electricity, and the second 
 non-conductors. 
 
 (15.) The impracticability of producing electrical 
 effects by friction upon the metals and similar substances, 
 when the experiment is conducted in the same manner 
 as with glass or resin, for a long time led to the suppo- 
 sition that such substances were incapable of evolving 
 electricity, and hence they were called non-electrics or 
 analectrics; while, on the other hand, vitreous and resin- 
 ous substances, and the like, were called electrics. When, 
 
CHAP. I. CONDUCTORS AND NON-CONDUCTORS. 235 
 
 however,, it was discovered that the absence of the elec- 
 trical effects on conducting bodies, when excited by 
 friction or otherwise, was owing not to their inability to 
 evolve electricity, but to the escape of this electricity as 
 fast as it was evolved, these terms were abandoned as ex- 
 pressing properties and effects which have no actual 
 existence ; and the more appropriate terms conductors 
 and non-conductors were retained. 
 
 (16.) When it is desired to preserve on a conducting 
 body the electricity which has been imparted to it or 
 excited upon it, it is usual either to suspend or support 
 it by a non-conducting substance: such a substance, not 
 giving free passage to the electricity, prevents its escape 
 from the electrified body. An electrified body thus 
 placed is said to be insulated ; and non-conductors are 
 accordingly called also insulators. 
 
 (17.) Atmospheric air must manifestly belong to the 
 class of non-conducting bodies; for if it gave a free passage 
 to electricity, the electrical effects excited on the surface 
 of any body surrounded with it would quickly pass 
 away, and no electrical phenomena of a permanent or 
 durable nature could be produced, unless the bodies ex- 
 perimented on were removed from contact with the air. 
 On the contrary, it is found that resin or glass, when 
 submitted to friction, preserves its electrical properties 
 for a considerable time, even though surrounded by at- 
 mospheric air. 
 
 (18.) In the experiments with the pith balls already 
 explained, we have stated that the balls should be sus- 
 pended by threads of silk. If, instead of silk threads, fine 
 metallic wires had been used, the same effects would not 
 have ensued. The balls could not then have been perma- 
 nently electrified by contact with the excited tube, and 
 consequently the effects of electricity upon them would 
 not be manifested. 
 
 The cause of this admits of easy explanation. When 
 the pith ball touches the glass, being itself a conductor 
 of electricity, its surface becomes covered with the 
 electric fluid taken from that part of the glass which 
 
236 ELECTRO-STATICS. PART I. 
 
 it touches. When detached from the glass, this fluid 
 flows along the wire hy which the ball is suspended, this 
 wire heing also a conductor of electricity ; and from this 
 wire it passes away through the material of the stand 
 by which the wire is supported. But a silk thread 
 being a non-conductor of electricity, the electric fluid 
 taken by the ball from the glass cannot escape when the 
 ball is suspended by such a thread, and is therefore re- 
 tained upon it. 
 
 (19.) If the stand to which the ball is suspended 
 consist of a pillar of glass, the same effects will ensue, 
 whether the ball be suspended by a conducting or a non- 
 conducting thread ; for if the ball be suspended by a 
 metallic wire, the electric fluid received by the ball when 
 it touches the glass will be spread over the ball and the 
 wire; but its escape will be prevented by the glass pillar 
 from which the wire is suspended, glass being a non- 
 conductor. 
 
 (20.) Water, whether existing in the liquid or va- 
 porous form, is a conductor of electricity * ; and this 
 property affects, in a very important manner, all electri- 
 cal experiments. The atmosphere contains suspended 
 in it at all times more or less aqueous vapour ; and the 
 presence of this conducting substance, mingled with the 
 pure atmospheric air, which is a non-conductor, impairs 
 the non-conducting quality of the latter, and facilitates 
 the escape of electricity from all excited or electrified 
 bodies. This is one of the reasons why electrical ex- 
 periments are made with more facility, and the desired 
 effects produced with more certainty and success, in cold 
 and dry weather, the atmosphere then holding but little 
 aqueous vapour suspended in it. 
 
 (21.) It also happens that, when the atmosphere is 
 highly impregnated with vapour, more or less of this 
 vapour is deposited in a film of moisture on the surfaces 
 of bodies exposed to the air, more especially on surfaces 
 which have an attraction for such vapour. When such 
 
 * It will appear hereafter that its conducting power is of an order very 
 inferior to that of the metals. 
 
CHAP. I. CONDUCTORS AND NON-CONDUCTORS. 237 
 
 a coating of moisture is deposited on the surface of a 
 non-conducting body., it impairs or destroys its non- 
 conducting power. If, under such circumstances., glass 
 or resin, or any other non-conductor, be excited by fric- 
 tion, the electricity evolved will escape by means of the 
 conducting power of the moisture which rests upon the 
 surface. In like manner, if pillars of glass or resin be 
 used as supports to insulate electrified bodies, or if 
 threads of silk be used for the same purpose to suspend 
 electrified bodies, those substances will lose their insu- 
 lating power, since the moisture deposited upon them 
 will enable the electricity to pass away. In warm 
 ^eather, therefore, and, in general, at any time when 
 the atmosphere is strongly impregnated with vapour, 
 the success of electrical experiments can only be insured 
 by keeping the insulating supports dry, by constantly 
 rubbing them with a dry warm cloth, which is itself a 
 non-conductor. A silk handkerchief is very fit for this 
 purpose. 
 
 (22.) The distribution of all bodies into two great 
 classes of conductors and non-conductors, like most other 
 systematic classifications in physical science, is not in 
 strict conformity with the natural properties of material 
 substances ; and though such a distinction is useful and 
 necessary, it must be adopted subject to a clear know- 
 ledge of the restrictions under which only it can be ap- 
 plied. Few bodies can be found which, in a strict sense, 
 belong to either of the specified classes ; and many exist 
 which present nearly equal claims to be placed under 
 either of them. There is, in fact, no substance whose 
 surface is strictly impassable by the electric fluid, though 
 there are many which offer such obstruction to its pro- 
 pagation over them, that, in a practical sense, they may 
 be regarded as non-conductors. It is equally impossible 
 to find any body whose surface offers so free a passage 
 to the electric fluid, that under no conceivable circum- 
 stance is any the least obstruction discoverable; but 
 there are many which offer an obstruction so extremely 
 small in amount, that th^y may be, and are, regarded as 
 
238 ELECTRO-STATICS. PART I. 
 
 practically perfect conductors. Finally, there are many 
 substances which possess the conducting power so im- 
 perfectly, that it seems doubtful to which class they 
 should most properly be assigned. There is, in a word, 
 a progression of degrees in which the conducting power 
 is found in bodies ; and the various substances in nature 
 might be tabulated or arranged in a series, beginning 
 with that substance over which electricity passes most 
 freely, proceeding through gradations to those substances 
 which offer such obstruction to its passage as scarcely to 
 be considered as conductors, and from these through the 
 catalogue of bodies offering more and more obstruction 
 to its transmission, until we arrive at that substance which 
 approaches nearest to an absolute non-conductor. In 
 the formation of such a series, however, much difficulty 
 is found, owing partly to the absence of any precise 
 measure of the conducting power of bodies, and partly 
 to the fact that the conducting power of the same body 
 at different times is subject to variations, proceeding from 
 causes external to it; such as its hygrometric state, or 
 its temperature. 
 
 (23.) Of all known substances, those which offer least 
 obstruction to the passage of electricity are the metals. 
 These bodies all appear to transmit common electricity 
 without sensible obstruction : but, from experiments 
 made with galvanic electricity, there is reason to think 
 that even the rnetals are permeable in different degrees 
 by the electric fluid. 
 
 (24.) Mr. Singer, so well known for his investigations 
 in electricity, observes, that a tendency of the electric 
 fluid to pass through good conductors offers a measure of 
 their conducting power : for if various substances of the 
 same length and magnitude are used simultaneously to 
 connect an electrified conductor with one not electrified, 
 that through which the fluid passes in preference to the 
 others is the best conductor ; or if they are placed in 
 succession, that which conveys the charge most com- 
 pletely may be considered the most perfect. Metals, al- 
 though the most perfect of known conductors, offer some 
 
CHAP. I. CONDUCTORS AND NON-CONDUCTORS. 239 
 
 slight resistance to the transmission of electricity ; and a 
 charge will even prefer a short passage through air to a 
 current of twenty or thirty feet along thin wire. 
 
 (25.) The hodies composing the following series are 
 arranged in the order in which experiment has shown 
 them to possess the conducting power ; the most 
 perfect conductors standing at the head,, and the most 
 perfect non-conductor at the conclusion, of the series. 
 The black line divides the most imperfect conductors 
 from the most imperfect non-conductors : but, accord- 
 ing to wnat nas been already explained, the position of 
 this line is in a great measure arbitrary, and the ex- 
 act relative position of many of the substances composing 
 the series is, as yet, unascertained : 
 
 All the metals. 
 
 Well-burnt charcoal. 
 
 Plumbago. 
 
 Concentrated acids. 
 
 Powdered charcoal. 
 
 Dilute acids. 
 
 Saline solutions. 
 
 Metallic ores. 
 
 Animal fluids. 
 
 Sea water. 
 
 Spring water. 
 
 Rain water. 
 
 Ice above 13 Fahrenheit. 
 
 Snow. 
 
 Living vegetables. 
 
 Living animals. 
 
 Flame. 
 
 Smoke. 
 
 Steam. 
 
 Salts soluble in water. 
 
 Rarefied air. 
 
 Vapour of alcohol. 
 
 Vapour of ether. 
 
 Moist earths and stones. 
 
 Powdered glass. 
 
 Flowers of sulphur. 
 
 Dry metallic oxydes. 
 Oils, the heaviest the best. 
 
240 ELECTRO-STATICS. PART I. 
 
 Ashes of vegetable bodies. 
 
 Ashes of animal bodies. 
 
 Many transparent crystals, ory. 
 
 Ice below 13 Fahrenheit. 
 
 Phosphorus. 
 
 Lime. 
 
 Dry chalk. 
 
 Native carbonate of barytes. 
 
 Lycopodium. 
 
 Caoutchouc. 
 
 Camphor. 
 
 Some siliceous and argillaceous stones. 
 
 Dry marble. 
 
 Porcelain. 
 
 Dry vegetable bodies. 
 
 Baked wood. 
 
 Dry gases and air. 
 
 Leather. 
 
 Parchment. 
 
 Dry paper. 
 
 Feathers. 
 
 Hair. Wool. 
 
 Dyed silk. 
 
 Bleached silk. 
 
 Raw silk. 
 
 Transparent gems. 
 
 Diamond. 
 
 Mice. 
 
 All vitrifications. 
 
 Glass. 
 
 Jet. 
 
 Wax. 
 
 Sulphur. 
 
 Resins. 
 
 Amber. 
 
 Gum-lac. 
 
 (26.) According to the experiments of M. Achard, 
 of Berlin, ice whose temperature is below 13 Fahr. is 
 a non-conductor, though at all higher temperatures it 
 is a conductor. That philosopher experimented on a 
 rod of ice two feet long and two inches in diameter, 
 and found that at 18 Fahr. it became an imperfect 
 conductor, and that at 13 it ceased to have any dis- 
 coverable conducting power whatever. 
 
CHAP. I. CONDUCTING POWER. 241 
 
 Since the best test of a non-conductor is to ascertain 
 whether electricity can be excited on its - surface so as 
 to remain on it, M. Achard, having frozen some water 
 so as to exclude all air-bubbles from it, formed it into 
 a spheroid, and mounted it on an axis. When the tem- 
 perature of this was reduced below 13, he was able to 
 excite upon it a very high degree of electricity by the 
 ordinary process. 
 
 (27.) It is doubtful whether rarefied air should oc- 
 cupy a place among conducting bodies,, for the manner 
 in which it admits the motion of electricity is probably 
 very different from that in which other conducting sub- 
 stances exert that power. It will hereafter appear that 
 the electric fluid is retained on the surfaces of electrified 
 bodies by the atmospheric pressure, and that when its 
 tension exceeds this pressure it escapes spontaneously. 
 Whatever, therefore, be the intensity of the electric fluid 
 on any electrified body, if the atmosphere surrounding 
 it is so rarefied that its pressure shall be less than that 
 tension, the electricity must escape by its self-expansive 
 power ; and, in this sense, the surrounding air thus rare- 
 fied may be regarded as a conductor. Various experi- 
 ments have been made to ascertain whether a vacuum is 
 or is not a non-conductor ; and, although the question 
 cannot be considered as finally settled, there appears 
 every reason from analogy to consider it a perfect con- 
 ductor. 
 
 (28.) In conformity with the usage of all writers on 
 this branch of physics, we have adopted, and shall con- 
 tinue to use, the term conducting power to express that 
 quality of bodies in virtue of which they afford a free 
 passage to electricity. It were, however, to be wished 
 that this property had been designated by some term 
 which would more correctly express what appears, from 
 observation and experiment, to be its nature. Expe- 
 riment seems to prove that the particles of bodies have 
 no peculiar affinity for the free electric fluid, and they 
 neither attract nor repel it. So far, therefore, as the 
 phrase conducting power implies an active quality in 
 
 VOL. i. B 
 
ELECTRO-STATICS. PART I. 
 
 relation to that fluid, it does not correctly express the 
 property to which it is applied. A conductor exer- 
 cises no action on the electric fluid, and is merely 
 characterised by the negative or passive condition of 
 offering no obstruction to its motion. The electric fluid, 
 being self-expansive, has a natural tendency to diffuse 
 itself into the surrounding space; and when, in virtue of 
 this elasticity, it passes from the surface of one con- 
 ductor to the surface of another, the effect is analogous 
 to that which takes place when a vessel filled with com- 
 mon air is put into communication with another vessel 
 in which there is a vacuum. The air, by its elasticity, 
 expands and diffuses itself through the dimensions of the 
 two vessels, having before been confined to one of them. 
 What the vacuum is to a vessel filled with air, a con- 
 ductor in its natural state is to an electrified conductor. 
 We do not wish, however, to be understood to state 
 that, when an electrified conductor is brought into contact 
 with another conductor not electrified, the electric fluid 
 diffuses itself over both conductors according to the same 
 law as air would distribute itself between the two re- 
 ceivers just referred to. It does, however, diffuse itself 
 over both conductors according to its own peculiar laws. 
 
 (29.) The physical condition which confers on bodies 
 this conducting power has been a subject of fruitless 
 inquiry among electricians. All that is known respect- 
 ing it is, that the conducting quality depends partly, if 
 not altogether, on the peculiar arrangement of the par- 
 ticles of bodies, and is not dependent on the particles 
 themselves. It has been ascertained, that all bodies be- 
 come conductors in a state of solution. 
 
 It is natural to inquire whether any relation exists 
 between the power of conducting electricity and other 
 imponderable physical influences, such as light and heat 
 There is, however, one obvious distinction to be observed 
 between the manner in which light and heat are trans- 
 mitted through bodies, and that in which electricity is 
 transmitted by them. If a body be capable of conduct- 
 ing or transmitting light, that fluid will pass through its 
 
CHAP. I. CONDUCTING POWER. 2^3 
 
 solid dimensions ; thus glass, water, air, and other trans- 
 parent bodies, allow light to pass through them ; in other 
 words, they are conductors of light, while the metals 
 generally, and other opaque substances, refuse to admit 
 light through their dimensions, and either reflect it from 
 their surfaces or absorb it upon them. The metals in 
 general are free conductors of heat: if one end of a 
 metallic bar be heated, the heat soon passes through all 
 its dimensions, and the temperature of the other end 
 rises. But, on the other hand, glass and water, which 
 are such perfect conductors of light, scarcely possess the 
 power of conducting heat at all : one end of a rod of 
 glass may be rendered white hot, while no sensible ele- 
 vation of temperature takes place at the other end. 
 
 Electricity, however, is transmitted or excited not 
 through the interior dimensions, of bodies, but only on 
 their surfaces ; and the conducting power, therefore, 
 belongs solely to the surface. No relation exists between 
 the conductors of heat or light and those of electricity. 
 Glass, which is almost a perfect conductor of light, is a 
 non-conductor of heat, and also of electricity. Sealing- 
 wax, which is an opaque substance, and therefore a non- 
 conductor of light, is likewise a non-conductor of heat 
 and electricity. The metals, on the other hand, which 
 are non-conductors of light, are conductors of both heat 
 and electricity. Water is a conductor of electricity and 
 light, but a non-conductor of heat. 
 
 Neither does there appear to exist any general or con- 
 stant relation between the conducting power of bodies 
 and the state of cohesion of their particles. The metals,, 
 and vitreous and resinous bodies, when in the solid state, 
 manifest an active principle of cohesion among their par- 
 ticles ; but the metals are good conductors, while glass, 
 gum, and resin are non-conductors. Most liquids are 
 conductors, but possess this quality in very different 
 degrees. The oils are very imperfect conductors. Wax 
 and tallow, when cold, and therefore solid, are very bad 
 conductors, but when melted they are comparatively 
 good ones. The conducting power is found to exist in 
 B 2 
 
244 ELECTRO-STATICS. PART I. 
 
 bodies having the most opposite physical characters : 
 the flame of alcohol is a conductor, and so is ice. At 
 common temperatures, the bodies over which electricity 
 passes have no sensible effect upon it. The same elec- 
 tric spark will be derived from electricity transmitted by 
 an iron conductor, whether the iron is at the tempera- 
 ture of melting ice or red hot. The same spark is ob- 
 tained if the electric fluid be conducted by ice itself. 
 
 (30.) The non-conducting quality of atmospheric air 
 is shared by all gases when in a dry state ; but, besides 
 the insulating property which these bodies possess in 
 virtue of the absence of the conducting power, they all, 
 in common with atmospheric air, owe a great part of 
 their insulating power to the mechanical pressure which 
 they excite upon the surface of electrified bodies. It 
 appears that this pressure is the chief agent by which 
 the electricity is retained upon such bodies. This fact 
 may be established by experiment in the following man- 
 ner : Let a metallic ball,, suspended by a silken thread, 
 be electrified,, and placed under the receiver of an air- 
 pump. Let the air under the receiver be then rarefied 
 by the action of the pump : the ball will lose its elec- 
 tricity by degrees, according as it is relieved from the 
 pressure of the surrounding air by gradual rarefication. 
 
 If the same experiment be made with a ball of 
 glass, of equal magnitude, similarly suspended, and also 
 electrified by previous friction, it will be found that 
 this ball will also lose its electricity when the air in the 
 receiver is rarefied ; but it will not lose it so rapidly as 
 the metallic ball. 
 
 (31.) The conducting power of the metals is of great 
 use in the construction and adaptation of 'apparatus for 
 experimental investigations in electricity. A body 
 having an extensive metallic surface, placed on an in- 
 sulating support such as a pillar of glass or resin, or 
 suspended by insulating cords such as threads of silk, 
 may be regarded as a reservoir or vessel in which 
 electricity may be collected and retained for experi- 
 mental purposes. It is true that, after the lapse of a 
 
CHAP. I. ACCUMULATION OP ELECTRICITY. 245 
 
 certain time, it will be dissipated ; but it can always be 
 retained for a time sufficient for the purposes of experi- 
 mental inquiry. 
 
 It is very easy to comprehend how electricity may 
 be accumulated on such a body. Let a glass tube be 
 electrified by friction, and brought in contact with an 
 insulated metallic surface. The electricity upon the 
 tube at and immediately around the point of contact 
 will pass to the metallic surface, and, in virtue of its 
 conducting power, will be diffused uniformly over it. 
 If another part of the tube be then brought in contact 
 with the metal, a further communication of electricity 
 will take place ; and in the same manner, if, by any 
 contrivance, all the parts of the tube which have been 
 electrified by friction be brought successively in contact 
 with the metal, all the electricity excited on the tube will 
 be transmitted to the metallic surface, and will be uni- 
 formly diffused over it. The tube may then again be 
 excited by friction, and the electricity with which it is 
 charged may, in like manner, be communicated to the 
 metallic surface. By continuing this process, the insu- 
 lated metallic surface may be charged with any quantity 
 of accumulated electricity. 
 
 But it is evident that the mechanical labour of 
 exciting the tube in the manner already described, 
 by holding it in one hand, and the cloth to rub it in the 
 other, would be considerable, and the process would 
 be otherwise inconvenient. If the tube were mounted 
 so as to be capable of revolving on an axis, while the 
 cloth were held against it by a fixed support, the manual 
 labour would consist merely in giving a motion of ro- 
 tation to the^ tube, which might be accomplished by a 
 small handle or winch. The tube would be thus made 
 to revolve by a manual operation similar to that by which 
 a barrel-organ is worked. 
 
 Such is, in fact, with certain modifications and 
 additions, the ELECTRICAL MACHINE, a detailed de- 
 scription of which we shall give hereafter ; but it 
 will be convenient at present to render the general 
 B 3 
 
246 
 
 ELECTRO-STATICS. 
 
 PART I. 
 
 principle and purpose of such a machine understood. 
 Instead of a tube, a cylinder of glass, of considerable 
 magnitude, is mounted on bearings placed at the ex- 
 tremities of its geometrical axis, and is made to re- 
 volve on that axis by a winch or handle attached at 
 the extremity, so as to move outside the supports of 
 the cylinder. A transverse vertical section of such a 
 cylinder is represented at C (fig. 3.), supported at the 
 Fig. 3. 
 
 V~N i ^\ 
 
 Gjr-O 
 
 ends of its axis by uprights, D. The cylinder Is kept 
 moving, in the direction of the arrow, by the winch 
 already referred to. A cushion, A, is supported by a 
 pillar, B, and pressed by a spring against the glass 
 cylinder. A cloth of the proper material for exciting 
 electricity is attached to the cushion, and carried round 
 part of the cylinder terminating at E. As the cylinder 
 revolves, its surface becomes covered with electricity, 
 excited by friction with the cloth A E ; and if the appa- 
 ratus were limited to such a cylinder, it would soon be 
 covered with all the electricity it is capable of re- 
 taining. 
 
 This cessation of the production of electricity is pro- 
 vided against by a conductor, F. This is a body with 
 a metallic suface in the form of a cylinder of consider- 
 able length, having extremities which are segments of 
 spheres greater than a hemisphere ; and it is supported 
 on a pillar G of glass, or other non-conducting sub- 
 stance. From the end of the conductor, which is pre- 
 sented to the side of the cylinder, a horizontal row of 
 metallic points project, one. of which is represented at 
 P. The height of the pillar G is so adjusted, that 
 
CHAP. I. ELECTRICAL MACHINE. 247 
 
 these points shall be at the same level as the axis of the 
 cylinder, and they are so disposed as to extend through- 
 out the length of the cylinder. 
 
 The -conductor is so placed that these points shall be 
 very nearly in contact with the glass. While any por- 
 tion of the cylinder's surface is moved from the rubber 
 A to the point E, electricity is excited upon it by the 
 friction of the cloth. As it descends to P, it gives up 
 this electricity to the metallic points, which have an at- 
 traction for it, the nature of which will be explained 
 hereafter ; and from these points it passes to the con- 
 ductor F, over the surface of which it diffuses itself. 
 This process may be continued until the conductor has 
 received from the cylinder all the electricity which the 
 pressure of the air surrounding it is capable of retaining 
 on its surface. 
 
 If it be desired to collect a still greater quantity of 
 electricity, it is only necessary to provide two or more 
 insulated conductors, similar to F, which, being put 
 successively in communication with the cylinder C, 
 may be similarly charged ; or if two or more of these 
 insulated conductors be connected together by metallic 
 chains or wires, which will give free passage to the 
 electricity from one to the other, they may be all simul- 
 taneously charged by the cylinder C. In fact, they may 
 be then regarded as one continued conductor. 
 
 Such is the general principle of the ELECTRICAL 
 MACHINE, by means of which electricity is produced and 
 accumulated at pleasure, for the purpose of experi- 
 mental investigation. In the details of the construction 
 of these machines, there are, however, many other 
 principles brought into operation, depending on more 
 complicated phenomena, which will be explained here- 
 after. The description of the actual form and con- 
 struction, therefore, of electrical machines, as now used, 
 must be postponed until the phenomena on which they 
 depend have been explained. Meanwhile, it will be 
 apparent, from what has been above stated, that, by the 
 aid of apparatus such as we have here described, 
 
248 
 
 ELECTRO-STATICS. 
 
 electricity may be evolved and accumulated, and., when 
 so accumulated, may be transferred from place to place 
 by insulated conductors, or may be made to pass from 
 one conducting body to another by wires, chains, or 
 cords of any conducting substance. 
 
 (32.) When an electrified conductor is touched by 
 the finger, it instantly loses all its electricity. Where, 
 it may be asked, does this electricity escape to ? It is 
 easy to show that it passes through the body of the 
 person touching the conductor, and from his body to the 
 earth. 
 
 To demonstrate this, let us first suppose that a pith 
 ball, suspended by a non-conducting thread, is charged 
 with electricity. We have seen that if it be touched by 
 the finger it will immediately lose all its electricity. 
 But, instead of being touched by the finger, let it be 
 brought in contact with another pith ball, of ten times 
 its diameter, also suspended by a non- conducting thread. 
 It will be found that the smaller ball, after the contact, 
 will have retained but a small portion of its electricity. 
 In fact, the greater ball, having a diameter ten times 
 greater, has a surface a hundred times greater, than the 
 smaller one ; and the electricity which, before the con- 
 tact, was confined to the surface of the smaller ball, is 
 diffused, after the contact, by the conducting power of 
 the balls over the surface of both. The greater ball 
 takes a portion of it nearly a hundred times more than 
 is left on the smaller ball, and the electricity remaining 
 on the smaller ball is therefore nearly a hundred times 
 more feeble than it was before the contact. It is evi- 
 dent, that the degree in which the electricity on the 
 smaller ball is diminished by such means will depend 
 on the proportion of the surfaces of the two balls ; and 
 that, by augmenting the magnitude of the greater, the 
 quantity of electricity left upon the lesser will become 
 insensible. 
 
 If, then, an electrified conductor be taken to re- 
 present the smaller ball, and the whole globe of the 
 earth the greater,, it will be easily perceived that, whea 
 
CHAP. I. COMMON RESERVOIR. 249 
 
 such a conductor is made to communicate with the 
 earth by any conducting substance, such as a metallic 
 wire or chain, or the human body, the electricity which 
 was before accumulated on the insulated conductor will 
 be shared between it and the whole surface of the 
 earth ; and as its surface, however extensive it may be, 
 must be infinitely small compared with that of the 
 earth, the electricity remaining on the conductor must 
 be proportionately small compared with what it has lost. 
 In fact, the quantity remaining on it is inappreciable, 
 and the conductor may be considered as restored to its 
 natural state. 
 
 (32.) As all bodies temporarily electrified must, in 
 this manner, give up their electricity to the earth, the 
 moment that the means taken to insulate them are re- 
 moved, or whenever they are touched by any conduct- 
 ing substance, or even by the conducting power of the 
 aqueous vapour suspended in the air, the earth is called 
 the common reservoir of electricity. All free electricity, 
 wherever and however confined and accumulated, must 
 return to it. 
 
 The conducting power of the human body, as well 
 as the use of insulators in electrical investigations, may 
 be illustrated by the following interesting experiment. 
 
 If a metallic body or other conductor, insulated by 
 standing on supports of glass, be charged with electri- 
 city, this electricity may be drawn off and transmitted to 
 the earth by presenting to the body so insulated any 
 conducting body which communicates with the earth. 
 If the extremity of such conductor consists of a knob 
 or ball of metal, when brought near the insulated con- 
 ductor the electricity will pass from the latter to the 
 former, accompanied by a bright spark distinctly visible 
 and a cracking noise; the brightness of the one and 
 the loudness of the other being proportionate to the 
 quantity of electricity accumulated in the insulated con- 
 ductor. As the animal body is known to be a conduct- 
 ing substance there is no reason why a man might not 
 be used, instead of the insulated conductor, in this ex- 
 
250 ELECTRO-STATICS. PART I. 
 
 periment. Let a stool with glass legs be provided,, on 
 which a man shall stand : electricity may be communi- 
 cated to his body thus placed. The electricity thus im- 
 parted to him, being prevented from escaping by the 
 non-conducting power of the legs of the stool, will be 
 retained in his body. When a sufficient quantity is 
 thus accumulated in his person, a knob of metal, com- 
 municating by a chain or wire with the earth, being 
 brought near some part of his person, his head, for 
 example, a spark of electricity, accompanied by a noise 
 like that of a small explosion, will pass from his head 
 to the ball. (INTROD. (16.) ) 
 
OPPOSITE ELECTRICITIES. 251 
 
 CHAP. II. 
 
 OPPOSITE ELECTRICITIES. 
 
 (34.) IT has been shown that two pith balls, suspended 
 in contact by non-conducting threads, when electrified 
 repel each other. This is a general law. All bodies 
 whatever, when electrified by the contact of the same 
 excited surface, repel each other, and continue to ex- 
 ercise such reciprocal repulsion until they are deprived 
 of the electricity diffused upon them. This repulsion 
 is manifested not only between bodies like the two pith 
 balls, which have derived their electricity from contact 
 with the same excited surface, but also between a body 
 excited by friction and another which has been elec- 
 trified by contact with it. When a glass tube has been 
 electrified by friction with a woollen cloth, and has im- 
 parted a portion of its electricity to a pith ball sus- 
 pended by a non-conducting thread, it will repel that 
 ball, as has been already shown; and the reciprocity of 
 this effect is only prevented from being visible by the 
 comparative inertia of the glass. The repulsive force 
 which produces a visible displacement on so light a 
 mass of matter as the pith ball, when shared by the 
 component parts of so much heavier a mass as the glass 
 tube, can produce no sensible effect. If, however, a 
 hollow glass ball, equal in weight to the pith ball, were 
 electrified by friction, and suspended similarly to the 
 pith ball, the same reciprocal repulsion would be pro- 
 duced between it and the pith ball as was manifested 
 between the two pith balls electrified by contact with 
 the glass tube. 
 
 (35.) It will be observed that the pith balls, which 
 manifest a mutual repulsion, have in this case been 
 both electrified by the same body. As, however, it has 
 
252 ELECTRO-STATICS. PART I. 
 
 appeared that a great variety of substances are capable 
 of being excited by friction, it is natural to inquire 
 whether the electrical state to which they are brought 
 is the same in all, or, if not, in what respects it differs 
 in different bodies. For this purpose, let a stick of 
 sealing-wax and a piece of amber be excited by the 
 friction of the same woollen cloth ; and let two pith 
 balls, suspended on different stands, be electrified by 
 contact with them. If these pith balls be then brought 
 near each other, a mutual repulsion will ensue. The 
 same effect would have been produced if both pith balls 
 had been electrified by contact with the same excited 
 body, whatever that body might be. 
 
 If, after the pith balls are restored to their natural 
 state by being touched by the finger, one of them be 
 electrified by sealing-wax excited by the friction of a 
 woollen cloth, and the other by glass excited by friction 
 with the same cloth, the same effects will not ensue. 
 On bringing one ball near the other, instead of mani- 
 festing a mutual repulsion, they will now attract each 
 other. It is evident, therefore, that the electricity 
 produced by friction upon the glass has a different 
 quality from that produced by friction upon the wax. 
 These electricities impart a power of mutual attraction 
 to bodies to which they are communicated ; while, on 
 the other hand, bodies both electrified by either glass or 
 wax acquire a power of reciprocal repulsion. 
 
 To extend and vary these experiments still further, 
 let us suppose a pith ball to be suspended by a non- 
 conducting thread, and electrified by a glass tube excited 
 by friction. Let a variety of substances be provided, 
 susceptible of electrical excitation. These being elec- 
 trified by friction, let them be successively presented 
 to the pith ball electrified by the glass tube. By some 
 of them this ball will be attracted, and by others it will 
 be repelled. It is evident, therefore, that the elec- 
 tricity on the latter is contrary in its nature and effects 
 to the electricity excited on the former. 
 
 Since this electricity excited on glass is found to 
 
CHAP. II. POSITIVE AND NEGATIVE ELECTRICITY. 253 
 
 repel the pith ball, to which the same electricity has 
 been imparted, all those substances which, when ex- 
 cited, also repel the pith ball are said to be electrified 
 similarly to the glass ; while all those substances which 
 have the contrary effect of attracting the pith ball, are 
 said to possess a contrary kind of electricity. Vitreous 
 substances generally are among those which produce 
 the electricity by which the ball is repelled; while wax, 
 amber, and resinous substances generally, are among 
 those by which the ball is attracted. The name of 
 vitreous electricity has therefore been applied to the one, 
 and resinous electricity to the other. It will, how- 
 ever, presently appear that the actual phenomena are 
 not in accordance with these terms ; and the names po- 
 sitive and negative electricity have partially superseded 
 them, positive electricity being substituted for vi- 
 treous, and negative for resinous. These latter terms are 
 not free from objection, taking their origin, as they do, 
 from a theory of electricity which, being proved to be 
 incompatible with the phenomena, has been superseded 
 by one more adequately representing them. Both 
 systems, however, are used by writers of the present 
 day, and both, therefore, should be rendered familiar to 
 the student. 
 
 Since, then, the substances are so various which pro- 
 duce these two electricities, and since even the same sub- 
 stance, as will presently appear, may produce each of 
 them, what, it will be asked, is the test of similar or 
 different electricities ? and how is the positive or vitre- 
 ous to be known from the negative or resinous ? 
 
 This question is answered by the statement of the fol- 
 lowing law, to which there is no exception, and which, 
 indeed, must be regarded as constituting the definition 
 of similar and opposite electricities: 
 
 (3 6.) Bodies charged with similar electricities mu- 
 tually repel each other; and bodies charged with opposite 
 electricities mutually attract each other. 
 
 (37.) Positive) or vitreous., electricity is that which is 
 produced upon polished glass when rubbed with a woollen 
 
254 ELECTRO-STATICS. PART I. 
 
 cloth; and electricity of the contrary kind is negative, or 
 resinous. 
 
 (38.) It has been shown that these attractions and 
 repulsions are transmitted without obstruction through 
 the body of air by which the electrified bodies are sur- 
 rounded, and which intervenes between them. Air, how- 
 ever, is not the only medium through which these attrac- 
 tions and repulsions act. They are likewise transmitted 
 freely through all non-conductors, such as glass or resin. 
 
 If a pith ball, suspended by a silk thread, and elec- 
 trified by a glass tube excited by friction with a wool- 
 len cloth, be placed within a glass receiver, it will be 
 repelled when the excited glass tube is brought near the 
 side of the receiver, in the same manner as would happen 
 if the glass receiver were not interposed between the tube 
 and ball. If, on the other hand, a stick of sealing-wax, 
 excited by friction with the same woollen cloth, be 
 brought near the side of the receiver, the pith ball, elec- 
 trified by the glass, will be attracted. 
 
 (39.) The attractions and repulsions of electrified 
 bodies are likewise transmitted through conducting bo- 
 dies, such as the metals ; but their effects are so modi- 
 fied in that case by other phenomena, which will be 
 explained hereafter, that they are not observable. 
 
 (40.) The simple apparatus of a small pith ball, sus- 
 pended by a silken thread, and electrified by glass ex- 
 cited by the friction of a woollen cloth, is sufficient, in 
 all ordinary cases, to detect the presence and species of 
 electricity on any body. 
 
 If a body produce neither attraction nor repulsion on 
 such a ball, it is not electrified; or, at least, not suffi- 
 ciently so to produce a force strong enough to overcome 
 the rigidity of the silken string. If, however, it repels 
 the ball, it must possess positive electricity. 
 
 The electricity upon the body under examination may 
 be so feeble, that its attraction or repulsion shall be inca- 
 pable of moving the ball through any sensible space. The 
 sensibility of the apparatus may, under such circum- 
 stances, be augmented by using a fine thread and a lighter 
 
CHAP. II. OPPOSITE ELECTRICITIES. 255 
 
 ball ; but we shall hereafter explain the construction of 
 instruments by which the presence of electricity is more 
 accurately determined and measured. Such an appara- 
 tus as is described above is, in many cases,, sufficiently 
 sensible, and will serve to explain the phenomena to 
 which we shall at present confine our attention. 
 
 (41.) When two bodies are submitted to the process 
 of friction, there is nothing in the mechanical circum- 
 stances of the operation to lead to the supposition that 
 any peculiar effect could be produced upon one without 
 some corresponding effect being produced upon the other. 
 When, therefore, glass, sealing-wax, or any other non- 
 conductor, is electrified by friction with another sub- 
 stance, it is natural to infer that the substance with which 
 it is rubbed must undergo some physical change, or ac- 
 quire some quality having an analogy to that which is 
 acquired by the body rubbed. Such an inference is ren- 
 dered more probable by the circumstance that the elec- 
 tricity acquired by glass or resin is the same, whether 
 the glass or resin be kept at rest and the cloth rubbed 
 upon it, or the cloth be kept at rest and the glass or 
 resin rubbed against it ; or, finally, whether the friction 
 be produced by motion imparted to both. 
 
 To determine this, it is only necessary to bring the 
 cloth with which the conductor is rubbed to the test 
 of the electrified pith ball above explained. If glass be 
 excited by the friction of a woollen cloth, the glass will 
 be vitreously electrified, as has been already shown, and 
 will therefore repel the pith ball. If the woollen cloth 
 be presented to the same pith ball, it will attract it ; and 
 we therefore infer that the woollen cloth is resinously, 
 or negatively, electrified. 
 
 Again : let the same woollen cloth be rubbed upon a 
 stick of sealing-wax, and the pith ball being as before 
 positively electrified, the sealing-wax will attract it, since 
 the latter is negatively electrified ; but if the woollen 
 cloth with which the sealing-wax has been rubbed be 
 presented to the pith ball, repulsion will take place. 
 It appears, then, that the woollen cloth is capable of 
 
256 ELECTRO-STATICS. PART I. 
 
 being electrified either positively or negatively, accord- 
 ing as it is rubbed against the glass or the wax, and that 
 in each case it acquires an electricity of a kind contrary 
 to that which it imparts to the substance against which 
 it is rubbed. If it be rubbed against glass, the glass 
 acquires positive electricity, while the cloth acquires 
 negative electricity. If it be rubbed against sealing- 
 wax, the latter acquires negative electricity, and the 
 cloth will be positively electrified. 
 
 (42.) If these experiments be carried further, it 
 will soon become apparent that not only the substance 
 used as a rubber is capable of acquiring either kind of 
 electricity according to the substance against which it 
 is rubbed, but that the same substance, when submitted 
 to the friction of different rubbers, may be either posi- 
 tively or negatively electrified. 
 
 To prove this, let a tube of polished glass be rubbed 
 first with a woollen cloth, and then with the fur of a 
 cat. In the first case, as has already been explained, the 
 glass will be positively electrified, but in the second it 
 will be found to be negatively electrified. The species 
 of electricity excited upon glass is, therefore, not always 
 the same ; but is sometimes positive and sometimes 
 negative, according to the substance with which it has 
 been rubbed. 
 
 If the fur of a cat, after having been rubbed on glass, 
 be examined, it will be found to be positively elec- 
 trified, while the glass is negatively electrified. The 
 glass and cat's fur are, therefore, oppositely electrified. 
 By continuing and varying those experiments it will be 
 found that the electricity produced when two bodies are 
 submitted to friction is subject to the following law : 
 
 (43.) The electricities excited by the mutual friction 
 of two bodies are always of contrary kinds ; one being 
 positive } and the other negative. 
 
 The objection against the use of the terms vitreous 
 and resinous, to express the two opposite species of elec- 
 tricity, will now be apparent. These terms imply, that 
 Tvhen vitreous bodies are excited, they are always elec- 
 
CHAP. II. OPPOSITE ELECTRICITIES. 257 
 
 trifled with the one species of electricity; and that when 
 resinous bodies are excited, they are always electrified 
 with the other. Such an hypothesis is, however, at 
 variance with the facts which have been just demon- 
 strated; the same body being capable of being elec- 
 trified with either kind of electricity, according to the 
 substance with which it has been rubbed. 
 
 (44.) In order to verify more extensively the above 
 important law, that all bodies submitted to friction 
 acquire opposite kinds of electricity, the following ar- 
 rangement should be made. If the bodies under ex- 
 periment be solid, it will be advantageous to form them 
 into flat plates, the faces of which may be exposed to 
 friction. The magnitude of the surface operated upon 
 is thus increased, and the quantity of electricity evolved 
 proportionably augmented. If the bodies be conductors 
 of electricity, they should be furnished with non-con- 
 ducting handles or supports, to prevent the escape of 
 the electricity excited upon them. Whether the bodies 
 under examination be solids, or have the form of stuffs 
 or cloths, or are the skins of animals, they may be 
 insulated by these means, or by being suspended by 
 threads of silk kept well dried. When the two bodies 
 have been submitted to friction they must be separated, 
 and, while still insulated, presented successively to the 
 electrified pith ball previously described. One of them 
 will always be found to attract, and the other to repel 
 it. A vast number of experiments have been made 
 with a view to the discovery of the physical circum- 
 stances which determine the species of electricity which 
 different substances acquire, but hitherto this inquiry 
 has not been attended with satisfactory results. The 
 following is a series of non-conducting substances, ar- 
 ranged in such an order that, when any one of them is 
 rubbed against any other, that which stands first in the 
 list becomes positively electrified, and therefore the other 
 will be negatively electrified : 
 
258 ELECTRO-STATICS. PART I* 
 
 1. Fur of a cat. 6. Paper. 
 
 2. Polished glass. 7. Silk. 
 
 3. Woollen cloth. 8. Gum lac. 
 
 4. Feathers. 9. Rough glass. 
 
 5. Wood. 
 
 Thus, if the fur of a cat be rubbed against any one 
 of the eight substances which follow it upon the list, it 
 will be positively electrified, while the other substances 
 will be negatively electrified. If wood be rubbed 
 against any of the four substances which precede it, it 
 will be negatively electrified, while the substance against 
 which it is rubbed will be positively electrified ; but if 
 it be rubbed against any of the four substances which 
 follow it upon the list, it will be positively electrified, 
 while the substance against which it is rubbed will be 
 negatively electrified. 
 
 (45.) The species of electricity which two bodies 
 acquire by friction is not always the same, and is some- 
 times influenced by circumstances apparently of an in- 
 different nature. For it appears by the above series 
 (which has been deduced entirely from experiments, 
 independently of any theory) that if a piece of polished 
 glass be rubbed against a piece of rough glass, the 
 former becomes positively, and the latter negatively, 
 electrified. But there appears no reason why the polish 
 of the surface should produce this effect. If two lengths 
 of white silk ribbon, cut from the same piece, be rubbed 
 one across the other, that which is rubbed transversely 
 will become negatively electrified, while that which is 
 rubbed longitudinally will be positively electrified. 
 There is, however, no apparent reason why the di- 
 rection of the threads forming the ribbon should affect 
 the species of electricity it acquires. JEpinus found 
 that a plate of copper rubbed against sulphur, and that 
 two similar squares of glass rubbed against one another, 
 produced electricity that they were always oppositely 
 electrified, but that the electricity each acquired was 
 not always the same. The copper, when rubbed against 
 the sulphur, was sometimes positive, and sometimes ne- 
 
CHAP, II. OPPOSITE ELECTRICITIES. 259 
 
 gative, the sulphur always having an electricity of an 
 opposite kind ; and the plate of glass, which was at one 
 time positively electrified, was at another time electrified 
 negatively ; the other plate undergoing corresponding 
 variations. 
 
 (46.) The following experiment furnishes an inter- 
 esting illustration of the production of opposite elec- 
 tricities. Let two persons be placed upon insulating 
 stools (that is, stools having glass legs to prevent the 
 escape of electricity to the earth), and let one of them, 
 holding in his hand the fur of a cat, well dried, strike 
 the other with it two or three times. He that strikes 
 will have his body charged with positive electricity, and 
 he that is struck will be charged with negative electri- 
 city. This may be proved by the usual test of the 
 electrified pith ball. The ball being positively electri- 
 fied, let the first present his hand near it, and it will be 
 repelled ; but if the second present his hand towards it, 
 it will be attracted. If a person not insulated present 
 his finger to the face of either of them, a spark will 
 pass from their flesh to his finger. 
 
 If either of those persons descend for a moment, he 
 will lose all the electricity with which he is charged, 
 which will escape to the earth. If, in performing the 
 above experiment, one of the two persons only stand 
 on the insulating stool, he alone will be charged with 
 electricity, the electricity excited in the other escaping 
 immediately to the earth. If neither of these persons 
 be so insulated, neither will exhibit signs of electricity. 
 The electricity will be excited in each of them as before, 
 but, as they are not separated from the earth by any 
 non-conducting substance, it will immediately escape. 
 
 In experiments of this kind, the fur of the cat is the 
 most convenient substance which can be used, owing t^ 
 the facility with which electricity is produced by it. If 
 in dry weather, the hand be passed over the back of a 
 living cat, the hairs will bristle and be attracted by the 
 hand ; and sometimes a cracking noise will be heard, 
 and sparks observed. These latter effects, however, 
 s 2 
 
260 ELECTRO-STATICS. PART I. 
 
 which are also observable with human hair, only take 
 place in very cold weather, when the air, from its ex- 
 treme dryness, becomes a good insulator. The human 
 hair,, when clean and dry, and not greased, may be elec- 
 trified with great facility by friction. This is especially 
 the case with fair hair, which is generally fine and 
 pliable. 
 
 (47.) The opposite electricities produced by friction 
 may be demonstrated by the electrical machine, of 
 which the general principle has been explained. Be- 
 hind the rubber or cushion A (fig. 3.) let a conductor, 
 H, be placed, and let it be insulated by standing upon 
 a glass pillar, the cushion being also insulated. When 
 electricity has been excited by turning the glass cylinder, 
 the glass and the cushion, in accordance with what has 
 been explained in this chapter, acquire opposite kinds 
 of electricity. The positive electricity produced upon 
 the glass passes off to the conductor F, and the negative 
 electricity produced upon the rubber passes off to the 
 conductor H. These conductors are accordingly called, 
 respectively, positive and negative conductors. Elec- 
 tricity will continue to be accumulated on each of them 
 until as much has been collected as the atmospheric 
 pressure is capable of retaining on their surfaces. As 
 opposite electricities have an attraction for each other, 
 there is a continual tendency of the negative electricity 
 to flow towards the positive, and of the positive to flow 
 towards the negative, conductor ; and if the action of 
 the machine were suspended, nothing would prevent the 
 excess of electricity on each conductor from flowing t< 
 the other, and thus restoring the electrical equilibrium, 
 and reducing the conductors to their natural state, ex- 
 cept the non-conducting quality of the glass cylinde v 
 which stands between them. 
 
 (48. ) The property of exciting electricity by friction 
 is not confined to solids. The friction of liquids against 
 solids and against each other produces like effects. On 
 the top of a glass receiver let a wooden cup be attached, 
 so that no air can pass between it and the glass. The 
 
CHAP. II. EXCITEMENT OF ELECTRICITY. 26l 
 
 receiver thus arranged being placed upon the plate of an 
 air-pump, and the wooden cup being filled with mer- 
 cury, let the air within the receiver be rarefied by the 
 pump. The pressure of the external atmosphere will 
 force the mercury through the pores of the wood, and 
 it will fall like a shower of silver within the receiver, 
 striking against the sides of the glass as it descends. If 
 the electrified pith bah 1 be brought near the side of the 
 glass while the mercury is thus falling, it will be at- 
 tracted or repelled according to the species of electricity 
 with which the ball is charged, demonstrating that the 
 inner surface of the glass is electrified. 
 
 (49.) If the column of mercury supported in the 
 tube of a barometer be made to ascend and descend by 
 alternately inclining the barometer from side to side, the 
 friction of the mercury within the tube will be attended 
 with the evolution of electricity ; and if the experiment 
 be made in the dark, a faint luminous appearance will 
 be observed to follow the motion of the mercury. 
 
 (50.) Electricity may be also excited by the friction 
 of gases against solids. If a blast of air be directed by 
 bellows upon a plate of glass, the glass will be posi- 
 tively electrified, and the electricity may be detected by 
 the usual tests. 
 
 (51.) Friction, though the most convenient and usual, 
 is not the only, method of exciting electricity. Electricity 
 is evolved in almost every important change of form or 
 constitution which bodies undergo. It is evolved in 
 the fusion of solids. Let a metallic vase be placed on 
 . insulating supports, and let melted sulphur be poured 
 ~ into it. When the sulphur cools and hardens, and is 
 removed from the vessel, it will be found to be electri- 
 fied, and the metal will be found to have an electricity 
 of an opposite kind. Sometimes the sulphur in this 
 case is positively electrified, and the metal negatively; 
 and sometimes the contrary happens. 
 
 Various mineral substances of a crystallised structure 
 and vitreous nature are found to be electrified when their 
 temperature is raised to a certain point. In such cases, 
 s 3 
 
262 ELECTRO-STATICS. PART I. 
 
 the two extremities of the crystal are sometimes oppo- 
 sitely electrified, the one being positive and the other 
 negative. The effects, however, vary. 
 
 There are various other sources of electricity, such as 
 chemical action, and the contact of heterogeneous sub- 
 stances, especially metals. As these phenomena, how- 
 ever, will be fully explained at the proper places in these 
 volumes, it will not be necessary here to enter into de- 
 velopments on the subject. 
 
, /:; 
 
 263 
 
 CHAP. III. 
 
 LAW OF ELECTRICAL ATTRACTION AND REPULSION. 
 
 (52.) HAVING ascertained the existence of the attractive 
 and repulsive forces manifested by bodies on which elec- 
 tricity has been excited, or to which it has been im- 
 parted by contact with other bodies on which it has 
 been excited, it will next be necessary to investigate the 
 laws which govern the variation of the intensity or energy 
 of these attractions and repulsions under the various cir- 
 cumstances in which the bodies affected by them may 
 be placed. But, before commencing this inquiry, we 
 shall explain the principle and struc- 
 ture of the instrument by the aid of 
 which such forces have been mea- 
 sured. 
 
 The BALANCE OP TORSION, in- 
 vented and applied by COULOMB with 
 signal success in various physical in- 
 vestigations, and used by him in 
 his electrical experiments, is repre- 
 sented in fig. 4. A B C 1) is a 
 glass cylinder twelve inches in dia- 
 meter, and twelve inches in height ; 
 on the top is placed a glass plate A B 
 thirteen inches in diameter, which 
 completely covers the cylinder. In 
 the centre of this plate a hole E is 
 fo med, in which is inserted a glass 
 tube E F, about two feet high. In 
 the top of this tube is inserted the 
 balance of torsion, the parts of 
 which are separately exhibited in 
 The top of the tube E F is represented 
 s 4 
 
 figs. 5, 6, 7- 
 
264 ELECTRO-STATICS. PART I. 
 
 at F (fig. 6.). Into this passes like a telescope joint 
 a short tube H (fig. 5.), to the top of which is at- 
 
 Fig. 5. 
 
 Fig. 7. 
 
 tached a circular plate I, the 
 edge of which is divided into 
 360 degrees. The tube H is 
 permanently soldered into F. A 
 button K carries an index L at- 
 tached to a small cylinder M, to 
 the lower end of which is at- 
 tached a short rod N, at the bot- 
 tom of which is a small pincers 
 O, capable of being tightened 
 by pressing down a ring sliding 
 on N. These pincers hold the 
 upper extremity of an extremely 
 fine silver wire, kept in the ver- 
 tical position by a weight hang- 
 ing from it below. In the centre 
 of the circular plate I a circular 
 hole is formed, corresponding 
 in magnitude to the small cylin- 
 _A der M. When the wire and the 
 
 pincers which support it are let 
 into this hole, the cylinder M 
 is capable of revolving within 
 it with a little friction. The 
 length of the index L is such, that its extremity bent 
 downwards at right angles to its length, plays upon the 
 graduated edge of the circular plate I. The fine wire 
 thus held by the pincer O, descends through the centre 
 of the cylindrical tube E F (fig.l4>.) to the centre P. 
 of the great cylinder A B C I). The lower extremity 
 of this wire is held by another similar metaUic pincers 
 P, closed by a ring like the former, and held in the 
 vertical position by the weight of a small cylinder of 
 metal Q. This needle and its appendages are shown in 
 fig. 7. The pincers P are pierced with a small hole in the 
 horizontal direction, through which a needle R S passes. 
 
CHAP. in. COULOMB'S ELECTROMETER. 25 
 
 This needle is usually formed of gum lac, and is termi- 
 nated at one extremity by a ball R of the pith of elder of 
 about a quarter of an inch in diameter, and at the other 
 end by a circular plane of paper S, steeped in turpen- 
 tine, which serves as a counterpoise for the ball R, and 
 which, by means of the resistance of the air, retards the 
 oscillation of the needle. In the other hole T (fig. 4.) 
 made in the circular top A B of the great cylinder, is in- 
 serted a small rod T V formed of gum lac terminated in 
 a small ball V of the pith of elder. Round the great 
 cylinder at the level of the needle S R is attached a 
 graduated circle, by which the movements of the sus- 
 pended needle S R are measured. This graduated circle 
 is so numbered, that its degrees commence from the 
 point which is opposite the pith ball V. 
 
 Since the tube H turns with a little friction within 
 the tube F, the zero of the graduated cirjcle I, or the 
 point from which its divisions commence to be num- 
 bered, may be turned into any required position rela- 
 tively to the great cylinder A B C D. 
 
 (53.) Since the small tube M turns freely within the 
 hole formed in the centre of the circle I, the index L may 
 be turned round, causing the piece K, the tube M, the 
 pincers O, and the wire suspended from it, also to turn. 
 Let us suppose that the index L is turned so as to coin- 
 cide with the zero of the circle I, the wire by which the 
 needle R S is suspended being then free from any 
 torsion. Let the divided circle I be now turned until 
 the pith ball R is brought opposite to the zero of the 
 divided circle : it will be then immediately under the 
 opening T, by which the rod of gum lac bearing the 
 other pith ball V is let into the great cylinder. The 
 instrument is now reduced to a state in which it is 
 capable of measuring the force with which the balls R 
 and V would repel each other. If the centre of the 
 pith ball R be supposed to correspond precisely with 
 the zero of the circle before the introduction of the pith 
 ball V, and if the centre of the pith ball V when intro- 
 
266 ELECTRO-STATICS. PART I. 
 
 dueed be made to take the place of the centre of the 
 pith ball R, it will press the latter aside, and therefore 
 turn the needle R S through a space equal to the sum 
 of the radii of the two balls. This will produce a very 
 slight torsion of the wire suspending the needle R S, 
 the reaction of which will be just sufficient to keep the 
 two balls in contact. If either of the two balls thus in 
 contact be electrified, the electricity will be shared by 
 the other ball, and they will repel each other ; but as 
 the ball V is fixed, the repulsion will only take effect 
 upon the ball R, which will separate from V, and 
 make the needle R S move from V. R will continue 
 thus to depart from V, twisting the wire as it turns, 
 until the reaction of the torsion of the wire shall ba- 
 lance the repulsive force exercised by V upon R ; R 
 will then remain at rest at a distance from V, indicated 
 by the division of the circle opposite to which it stands. 
 The number of that division will also indicate the 
 angle of torsion through which the suspending wire has 
 been twisted, and this angle is always proportional to 
 the reaction of the wire, or the force with which it en- 
 deavours to recover its position of rest. In this manner 
 the force of repulsion at different distances may be mea- 
 sured and observed. 
 
 (54.) The extreme degree of sensibility of this in- 
 strument, and the minuteness of the quantities which it 
 is capable of measuring, may be collected from the con- 
 sideration of the dimension of the wires which were used. 
 In the experiments which will be hereafter explained, and 
 by which- the laws of electrical attraction and repulsion 
 were discovered, the suspending wire was 28 inches in 
 length, and was so fine that 1 foot of its length weighed 
 only the -$ part of a grain. The radius of a circle 
 described by the centre of the ball R was 4 inches ; the 
 force of torsion of this wire was found to be such, that 
 when turned through one entire revolution, or 360 de- 
 grees, its reaction amounted to no more than the 340th 
 part of a grain ; and since the reaction of torsion is pro- 
 
CHAP. in. COULOMB'S ELECTROMETER. 267 
 
 portional to the angle of torsion, the force corresponding 
 to the motion of the ball R through one degree of the 
 circle was only the 122,400th part of a grain. Thus 
 this balance so constructed was capable of dividing a 
 single grain into 122,400 parts, and rendering each 
 part distinctly observable. 
 
 (55.) Such a wire, on account of its extreme tenuity, 
 and, consequently, liability to be broken by the least dis- 
 turbance of the apparatus, was found to be inconvenient 
 in experiments where the forces to be measured were not 
 so extremely minute as to require so high a degree of sen- 
 sibility. Coulomb found it therefore more convenient 
 to employ in such cases a wire of double the diameter, 
 and of equal length. He showed that the reaction of 
 torsion, under like circumstances, of different wires was 
 diminished by augmenting the length of the wire, and 
 diminishing its diameter. This reaction, the smallness 
 of which is the measure of the sensibility of the instru- 
 ment, increases in the same proportion as the length of 
 the wire is diminished, when its thickness remains the 
 same ; and it increases in the same proportion as the 
 fourth power of its diameter when its length remains 
 the same. Thus the sensibility of the instrument is in 
 the direct proportion of the length of the suspending 
 wire, and in the inverse proportion of the fourth power 
 of its diameter. If, for example, the length of the wire 
 be doubled, the sensibility of the instrument will be 
 doubled, since the reaction produced by a given torsion 
 will be diminished in the proportion of two to one ; and 
 if the thickness of the wire be reduced in the proportion 
 of two to one, its length being preserved, the sensibility 
 of the instrument will be augmented in the proportion of 
 sixteen to one, the reaction produced by a given torsion 
 being sixteen times less than before. 
 
 (56.) In cases where the presence of much smaller 
 quantities of electricity was required to be detected, or 
 their quantity estimated, Coulomb used an electrometer 
 of a still higher degree of sensibility. From a micrometer 
 
268 
 
 ELECTRO-STATICS. 
 
 of torsion A {fig. 8.), placed at the top of a cylindrical 
 glass receiver, he suspended a single fibre 
 of natural silk, as produced by the silk- 
 worm, four inches in length. This thread 
 supported a small needle C D formed of 
 gum lac, twelve lines in length, and ter- 
 minated at one of its extremities by a 
 small circular disc of gilt paper. In the ap- 
 paratus employed by Coulomb, the weight 
 of the needle and disc, taken together, did 
 not exceed a quarter of a grain. These 
 delicate needles are easily formed by holding a thin 
 stick of gum lac in the flame of a lamp, and, when it is 
 softened, drawing the ends in opposite directions. The 
 melted part will thus be drawn into an extremely fine 
 thread, of which the needle of the electrometer may be 
 formed. The silk fibre, of four inches in length, by 
 which it is suspended, has such a flexibility, that when 
 twisted by a force acting with a leverage of one inch, 
 its reaction after one revolution is only the 60,000th 
 part of a grain ; and consequently its reaction, when 
 turned through one degree, would only amount to the 
 Fig. 9. 21,600,000th part of a grain. Thus, by this 
 * exquisite contrivance, a force is rendered ac- 
 jf tually observable amounting to less than the 
 20,000,000th part of a grain. 
 
 (57.) To communicate electricity to the disc, 
 a small copper wire is surrounded by a stick of 
 sealing-wax A B {fig. Q.\ extending at each 
 end beyond the extremities of the wax. At one 
 end the wire terminates in a small gilded pith 
 ball C, and at the other end in a hook D. The 
 stick is introduced into the glass case of the 
 electrometer through the opening T {fig. 4.), 
 and the ball C, is placed in contact with the 
 disc. Electricity is then communicated to the 
 hook D, and in virtue of the conducting power 
 of the wire it passes to the ball C, and is shared with the 
 
CHAP. III. ELECTRICAL REPULSION. 269 
 
 disc in contact with that ball. The disc is then repelled 
 with a force, the amount of which may be observed in 
 the manner already explained. 
 
 The sensibility of these instruments is so great, that 
 if, after having electrified a stick of sealing-wax by 
 friction, it be held at a distance of three feet from the 
 hook D, the disc will be repelled to the distance of 90 
 from the ball C. The manner in which this repulsion 
 is produced by the action of an electrified body at a 
 distance and without contact will be explained hereafter ; 
 it is only referred to at present as a proof of the extreme 
 sensibility of this apparatus. 
 
 (58.) In the forces which are manifested between 
 electrified bodies, the first circumstance which will at- 
 tract attention is the fact that the energy of these 
 forces is augmented as the distance between the elec- 
 trified bodies is lessened. The analogies suggested by 
 various other physical forces, whose intensities likewise 
 increase with the diminution of the distance, and more 
 especially the law of gravitation, by which the energy 
 of that force increases in the same proportion as the 
 square of the number expressing the distance between 
 the gravitating bodies is diminished, naturally leads to 
 the question, According to what law does the force of 
 electrical attraction or repulsion increase as the distance 
 between the electrified bodies is lessened ? If the nature 
 of electricity were perfectly known, this law could be 
 deduced by general reasoning, so that the manner in 
 which electrified bodies would comport themselves, in 
 any position in which they might be respectively placed, 
 could be certainly foretold. But the physical principles 
 from which electricity arises not being known, investi- 
 gation must proceed from the discovery of its laws by 
 the direct observation and comparison of its phenomena, 
 to the establishment of a just theory respecting its nature. 
 To determine, therefore, the law of its variation, it is 
 necessary to submit electrified bodies to their mutual 
 attraction or repulsion at different distances, to measure 
 
270 ELECTRO-STATICS. PART 
 
 the actual amount of that attraction or repulsion at those 
 distances, and by comparing the results of such measure- 
 ment with the distances themselves, to discover the de- 
 pendence of one upon the other. This was effected by 
 Coulomb by the aid of electrometers of torsion, such as 
 have been described. 
 
 (59.) Let us suppose the suspended needle of the 
 electrometer to carry at its extremity a small disc of 
 gilt paper S (fig. 7.)* tne plane of which shall be ver- 
 tical, and so placed that its edge shall be presented to 
 the wire of suspension. Let the index of the micro- 
 meter at the top of the tube E F (fig. 4.) be then turned 
 to the zero of the divided circle I (fig. 5.), on which 
 it moves ; also let the piece I itself, carrying with it the 
 micrometer, be turned until the edge of the disc of gilt 
 paper shall be presented to the zero of the divisions sur- 
 rounding the case ABCD (fig. 4.) of the instrument. 
 Let another similar disc of gilt paper be now introduced 
 into the case through the opening T (fig. 4.), and be 
 let down to the level of the needle, and so placed that the 
 gilt surfaces of the two discs shall be parallel to each 
 other and in contact. If a feeble electricity be imparted 
 to these discs by means of the head of a pin held by a 
 stick of sealing-wax or gum lac, the two discs, being si- 
 milarly electrified, will repel each other ; and that which 
 is attached to the needle, being free, will recoil from the 
 fixed disc under the opening T, and the needle will move 
 through part of a revolution, producing a corresponding 
 torsion in the suspending wire. When the reaction of 
 this torsion becomes equal to the repulsion, the needle, 
 after a few oscillations, will come to rest. This will 
 necessarily take place in some position of the needle 
 if the discs be not too strongly electrified, because 
 while the repulsive force diminishes by the increasing 
 distance of the moveable from the fixed disc, the re- 
 action of torsion which resists this repulsion will be 
 augmented. 
 
 If the index L (fig. 5.) be now turned in such a 
 
CHAP. III. LAW OF REPULSION. 271 
 
 direction as to force the moveable disc nearer to the fixed 
 disc, and be adjusted so as to maintain the moveable 
 disc at rest at any required distance from the fixed disc, 
 the force of repulsion, which must always be equal to 
 the reaction of torsion, will be proportional to the angle 
 of torsion, and that angle will be measured by the dis- 
 tance of the index L from the zero of the divisions I 
 added to the distance of the moveable disc from the 
 zero of the divisions surrounding the case A B C D 
 (fig. 4.). In this manner the repulsive force corre- 
 sponding to any distance of the moveable from the fixed 
 disc less than that at which it first settles itself may be 
 observed. 
 
 If the repulsive force at greater distance be required, 
 then let the index L (fig. 5.) be turned in the opposite 
 direction, so as to turn the moveable disc from the fixed 
 disc, and when the former is brought to rest in any re- 
 quired position, the angle of torsion will be found by 
 subtracting the distance of the index L from the zero of 
 the divisions I (fig. 5.) from the distance of the move- 
 able disc from the zero of the divisions surrounding the 
 case A B C D (fig. 4.). Thus in every case the angle 
 of torsion corresponding to any given position of the 
 moveable disc may be found, and thereby the repulsive 
 force at that distance may be determined. 
 
 Instead of discs of metal or gilt paper, the experiment 
 may be made with pith balls gilt, or balls of metal. 
 
 (60.) To show the method which led Coulomb to the 
 discovery of the law of electrical repulsion, we shall give 
 the details of one of his experiments. 
 
 Having electrified the two discs, he found that the 
 moveable disc, when repelled, remained at rest after it 
 had turned through an angle of 36 from the fixed disc. 
 The torsion therefore produced by turning the suspend- 
 ing wire through this angle balanced the repulsive force 
 of the discs in this position. He now applied his hand 
 to the index L (fig. 5.), and turned it round in such a 
 direction as to move the disc upon the needle towards 
 the fixed disc. He found that, to bring the moveable 
 
272 ELECTRO-STATICS. PART III. 
 
 disc to a distance of 18 from the fixed disc, the index 
 L had moved oyer the divided circumference I from 
 zero to 126. The angle of torsion, therefore, corre- 
 sponding to this position of the moveable disc, was found 
 by adding 18 to 126, and was therefore 144. 
 
 He now continued the motion of the index L in the 
 same direction, so as to force the moveable disc still 
 nearer to the fixed disc ; and he found that when the 
 distance between the discs was 8^, the distance between 
 the Index L and the moveable disc or the angle of 
 torsion was 575 J. The results of these three experi- 
 ments are exhibited in the following table : 
 
 Distance between the Discs. 
 
 Torsion measuring the Repulsive 
 Force. 
 
 36 
 18 
 SJ 
 
 36 
 
 144 
 5751 
 
 (6l). On comparing these two series of numbers, it 
 will be observed, that those in the first column form a 
 decreasing series, in which each number is nearly double 
 that which follows it. This would be exactly the case, 
 if the last distance were 9 On comparing the numbers 
 in the second column, they will be observed to be an 
 increasing series, in which each number is very nearly 
 four times that which precedes it ; this would be the 
 case exactly, if the last number were 576; hence, it 
 appears, that if the distances between the electrified 
 bodies be constantly diminished in the proportion of 
 2 to 1, their repulsive force will be increased in the 
 proportion of 1 to 4, or as the squares of the distances. 
 If the law, therefore, developed in these particular ex- 
 periments prevail generally, we shall arrive at the re- 
 markable conclusion, that the same law which reigns 
 among the great bodies of the universe, and regulates 
 their mutual attractions, also governs the attractions 
 and repulsions of electrified bodies. 
 
 (62.) In deducing such a conclusion from these ex- 
 
CHAP. III. LAW OF REPULSION. 2? 
 
 periments, it may be objected, that the distances be- 
 tween the electrified bodies have been measured,, not by 
 straight lines drawn from one to the other, but by the 
 arcs of a circle, of which the suspending wire is the 
 centre, and the distance from that wire to the bodies, 
 the radius. It is, however, easy to show by geometrical 
 calculation, that, in cases like the present, where the 
 arcs do not exceed 36, the distances between the 
 bodies measured upon the arcs differ little from the 
 distances measured in a straight line, and that the law 
 deduced from the comparison of the arcs will there- 
 fore be applicable to the rectilinear distance. 
 
 (63.) It may also be objected that the force of electrical 
 repulsion is exerted in a straight line joining the two 
 electrified bodies, while the force of torsion by which 
 this repulsion is balanced acts in the direction of a 
 tangent to the circle described by the moveable disc ; 
 that, therefore, the two forces not acting in imme- 
 diately opposite directions will not be equal ; and that 
 since the obliquity of the tangent representing the 
 direction of the force of torsion to the straight line 
 joining the electrified bodies is variable, the force of 
 torsion will not even be proportional to the force of 
 repulsion. 
 
 This objection, like the former, would be valid, if 
 the experiment were extended to such a distance be- 
 tween the electrified bodies as would cause a consider- 
 able obliquity between the tangent and the line joining 
 them ; but, the present experiments being limited to 
 angles under 36, this obliquity is so small, that the 
 force of torsion may, without sensible error, be taken as 
 equal to the repulsion of the electrified bodies. 
 
 (64.) By varying the experiments, and extending them 
 to other distances, Coulomb succeeded in establishing the 
 universality of the law, that bodies electrified by similar 
 electricities repel each other with a force which dimi- 
 nishes in the same proportion as the square of the dis- 
 tances between them is increased. 
 
 (65.) Having thus determined the law of repulsion 
 
274< ELECTRO-STATICS. PART t. 
 
 where bodies are similarly electrified, a like inquiry was 
 extended to the attraction of bodies oppositely elec- 
 trified. An inconvenience occurs in the application of 
 the same method of experimental inquiry to this ques- 
 tion. When the two electrified bodies, in virtue of 
 their mutual attraction, approach each other, the force 
 with which they are attracted often increases faster than 
 the force of torsion by which that attraction is resisted, 
 so that, in spite of the resistance of torsion, the discs or 
 balls rush into contact. It is easy to assign the me- 
 chanical conditions which will prevent this, but the 
 inconvenience is readily obviated by extending a thread 
 of fine silk vertically between the top and bottom of the 
 case A B C D (fig. 4.), having its ends attached to 
 them by wax. This thread should be placed near the 
 fixed disc, and in the commencement of the experi- 
 ments the needle may be placed in contact with it. 
 When the two discs are oppositely electrified, the move- 
 able disc may be forced from the fixed disc by turning 
 the index L in a direction contrary to that in which it 
 was moved in former experiments. By such -means, 
 the angles of torsion, corresponding to any distances of 
 the electrified bodies within these limits, which will 
 allow of the arcs being taken to represent the dis- 
 tances, may be observed. And by experiments thus 
 conducted, it was found that electrical attractions were 
 governed by the same law as electrical repulsions, and 
 that their energy diminished in the same proportion as 
 the square of the distance between the electrified bodies 
 was increased. 
 
 (6*6.) Besides this method of determining the law 
 of electrical forces, Coulomb also applied a method 
 of inquiry similar in principle to that by which the 
 variations of the force of gravity have been deter- 
 mined on different parts of the earth's surface. When 
 a pendulous body is at rest, its centre of gravity must 
 be placed in a straight line, joining its point of sus- 
 pension with the centre of the earth by which it 
 is attracted. If it be drawn from this position, and 
 
CHAP. III. LAWS OF ATTRACTION AND REPULSION. 275 
 
 liberated, it will fall back, and with the momentum 
 acquired in its descent, it will swing to the opposite side, 
 and will thus continue to vibrate alternately from side 
 to side. The rate of its vibration will be more or less 
 rapid, according to the force with which it is attracted ; 
 and it is demonstrated in mechanics, that the energy of 
 the force with which the pendulum is attracted will 
 be diminished in the same proportion as the square of 
 the time occupied by a given number of vibrations of 
 the pendulum is increased. Thus, if in one place the 
 pendulum were found to make ten vibrations in forty 
 seconds, and in another it made ten vibrations in twenty 
 seconds, then the force which attracts it in the latter 
 case would be greater than that which attracts it in the 
 former case, in the proportion of the square of forty to 
 the square of twenty, or, what is the same, of the square 
 of two to one. 
 
 This principle was applied in the following manner 
 by Coulomb. A needle of gum lac, A B (fig. 10.), fif- 
 
 Fig. 10. 
 
 teen or sixteen lines in length, carried at one extremity 
 a small disc of gilt paper, and was suspended by a 
 single filament of raw silk seven or eight inches in 
 length. The upper end of this thread was fastened to 
 a slip of wood D., well dried in the oven, and coated 
 T 2 
 
276 ELECTRO -STATICS. PART I. 
 
 with a varnish of gum lac, by the non-conducting power 
 of which, and of the needle itself, the disc A was per- 
 fectly insulated. The slip of wood D supporting the 
 silk was inserted in a frame E, capable of sliding on a 
 horizontal rod F G, to which it could be fastened by 
 adjusting screws. The flexibility of the thread by 
 which the needle was suspended was such, that a force 
 equal to the 1 20,000th part of a grain, applied to the 
 extremity of the needle, was sufficient to turn it one 
 entire revolution. After having left the apparatus thus 
 arranged standing for several days, in order to give time 
 to the silk to untwist itself perfectly, the re- action of 
 torsion for several degrees on either side of its position 
 of equilibrium might be regarded as altogether in- 
 sensible. 
 
 A sphere H of wood, one foot in diameter, was coated 
 with tin- foil, and supported on three thin legs of 
 gum lac, by which it was insulated ; this sphere was 
 placed at any required distance from the needle, in 
 such a position that its centre stood precisely in the 
 direction of the needle A B. The distance of the disc 
 A from the globe was varied at pleasure, by sliding the 
 piece F on the horizontal arm F G. 
 
 These arrangements being made, the disc A was 
 electrified by an insulated pin-head, and a charge of 
 electricity of the opposite kind was given to the globe 
 H, by bringing into contact with it the prime conductor 
 of an electrical machine. The disc, being attracted to 
 the centre of the globe was, relatively to the globe H, 
 placed under the same mechanical conditions as a pen- 
 dulum is with respect to the globe of the earth ; and 
 fcrhen drawn a little aside from its position of rest, and 
 disengaged, it would therefore be made to vibrate like 
 a common pendulum, and the rate of its vibration would 
 become an indication of the force with which it is at- 
 tracted. 
 
 In the experiments made by Coulomb, he observed 
 the time by a seconds watch, in which the pendulum 
 performed a given number of oscillations. 
 
CHAP. III. LAWS OP ATTRACTION AND REPULSION. 277 
 
 When the disc A was placed at nine inches from the 
 centre of the globe H, it vibrated fifteen times in twenty 
 seconds. When the distance was increased to eighteen 
 inches, it vibrated fifteen times in forty seconds ; and 
 when the distance was further increased to twenty-four 
 inches, it vibrated fifteen times in sixty seconds. 
 
 In considering these experiments, it is necessary to 
 observe that the globe, in consequence of its geometrical 
 form, and of the uniform diffusion of electricity over its 
 surface, attracts the disc A in the same manner as if all 
 the attracting matter were collected at its centre. This 
 is a principle which is demonstrated in mechanics. It 
 must also be observed, that although, strictly speaking, 
 the different points of the surface of the disc A, in any 
 given position of the disc, are at different distances from 
 the centre of the globe, yet the disc .is so small, and the 
 range of its vibration so limited, compared with its dis- 
 tance from the centre of the globe, that the change of 
 distance arising from these causes may be disregarded. 
 The attractive force of the globe, therefore, on every 
 point of the disc, and in every position of it, may be 
 regarded as invariable. These circumstances render the 
 pendulum A in all respects analogous to a common pen- 
 dulum attracted by the earth. The formula which ex- 
 presses the relation between the attracting force, the time 
 of vibration, and the length of the pendulum in the one 
 case, will, therefore, be equally applicable to the other. 
 
 Let L then express the length of the pendulum ; let 
 T express the time occupied by one of its oscillations, 
 and let E express the attraction exerted upon it by the 
 globe H. Finally, let TT express the ratio of the cir- 
 cumference of a circle to its diameter or the number 
 3" 141 5. Then, by the principles established in me- 
 chanics, we shall have, 
 
 2 2 
 
 TT? __ T 
 
 j-j 7T AJ 
 
 A 
 
 T 3 
 
78 ELECTRO-STATICS. PART I, 
 
 Thus the force will always be proportional to the 
 length of the pendulum divided by the square of the 
 time of one of its oscillations ; but, as in the present 
 case, the length of the pendulum is the same in all the 
 experiments, the attracting force will increase in the 
 same proportion as the square of the time of one vibra- 
 tion is diminished. Now, the time of a single vibra- 
 tion is proportional to the time of any given number of 
 vibrations, and consequently it follows that the energy 
 of the attractive force will diminish in the same propor- 
 tion as the square of the time taken to make any given 
 number of vibrations increases. If the attractive force 
 diminish in the same proportion as the square of the 
 distance increases, then it is evident that the variation 
 of the distance would be proportional to the variation of 
 the time taken to make a given number of vibrations ; 
 and if this correspondence be found to exist between 
 the variation of the distance and the variation of the 
 time of vibration, it will supply a demonstration of the 
 law which establishes the relation between the energy of 
 the attraction and the distances of the electrified bodies. 
 
 In the three experiments which have just been de- 
 scribed, the distances are proportional to the numbers 
 3, 6, and 8, while the times of the vibrations are pro- 
 portional to the numbers 3, 6, and 9- There is, there- 
 fore, in the last of the three experiments a departure 
 from the law to be established. The actual time taken 
 to make fifteen vibrations, at the distance of twenty- four 
 inches, was sixty seconds; whereas, if the attraction 
 rigorously diminished as the square of the distance in- 
 creased, fifteen vibrations ought to have been made in 
 fifty-four seconds. The rate of the pendulum being 
 slower than it ought to be, is an indication (supposing 
 the law to be true) that the electrical attraction at the 
 last distance is more feeble than it ought to be. But 
 this is only what might have been anticipated, for Cou- 
 lomb found by other experiments, made on the same 
 day, that an electrified body lost by dissipation in tVie air 
 about a fortieth part of its attractive power per minute* 
 
CHAP. III. LAWS OF ATTRACTION AND REPULSION. 279 
 
 Now, between the first and third of the above experi- 
 ments, there elapsed an interval of four minutes, in 
 which time the globe H would lose a tenth of its whole 
 attractive power. If a correction be made corresponding 
 to this loss of electricity, the result of the last experi- 
 ment \vill be found in sufficiently near accordance with 
 the law of attraction already established by means of 
 experiments made with the electrometer. This method 
 of the electrical pendulum will equally afford an experi- 
 mental proof of the law of electrical repulsion. The 
 method of experimentising will be the same, the disc 
 A being, however, turned in the opposite direction, and 
 the needle being placed so that A and B shall inter- 
 change places, 
 
 (67.) By means of the law of electrical attraction 
 and repulsion the energies of the attraction or repul- 
 sion of two electrified bodies can always be found 
 for any distance whatever, when its energy has been 
 observed at any known distance ; for let F' be its energy 
 observed by experiment at the known distance D', and 
 let it be required to calculate its force F at any other 
 distance D ; since the force increases as the square of the 
 distance diminishes, we shall have the following propor- 
 
 tion : 
 
 12 2 
 
 F : F'::D : D 
 
 from which the following formula follows : 
 D" 
 
 To find, therefore, the force at the distance D, mul- 
 tiply the observed force by the square of the distance 
 at which it is observed, and divide the product by the 
 square of the distance at which the energy of the force 
 is sought. 
 
 As the forces are measured by the torsion whose 
 reaction holds them in equilibrium, the angles which 
 measure the torsion may always be taken to represent 
 the forces in such calculations. It may therefore be 
 
280 ELECTRO-STATICS. PARTI. 
 
 assumed in the use of the electrometer of torsion, that 
 the angle of torsion increases in the same proportion as 
 the square of the distance between the electrified bodies 
 diminishes. 
 
 (68.) By such means, the whole amount of the 
 attractive or repulsive force of two electrified bodies 
 placed at any given distance from each other, may be 
 determined. But, as both bodies unite in contributing 
 to the production of this effect, it still remains to in- 
 quire what proportion of the total attraction thus deter- 
 mined is due to each of them. We shall arrive at the 
 solution of the problem if we can increase or diminish 
 in any known proportion the quantity of electricity on 
 either of the bodies, and then observe the change in the 
 attractive or repulsive force thus produced. Now, it is 
 easy to take away from either of those electrified bodies 
 half of its electricity. To effect this, it is only neces- 
 sary to bring it into contact with another conducting 
 body of the same magnitude, form, and nature, and 
 similarly insulated. Thus, if an electrified gilt pith 
 ball, of a certain magnitude, being insulated, be touched 
 by an equal gilt pith ball, also insulated, but not pre- 
 viously electrified, then the electricity will be shared 
 equally between the two balls ; and when they are sepa- 
 rated, the first will possess exactly half its former 
 quantity of electricity. By repeating the same process, 
 the electricity upon the first ball may be reduced to a 
 fourth or an eighth of its electricity. 
 
 This equal partition of electricity may be verified 
 experimentally in the following manner : In the 
 electrometer of torsion, let a small pith ball be attached 
 to the end of the needle, and another of equal magni- 
 tude be introduced at the opening T, and placed in 
 contact with it. The balls being electrified by the head 
 of a pin, let us suppose they are repulsed to a distance of 
 48, and that, by turning the thread of suspension, the 
 moveable ball is forced back to the distance of 28 ; and 
 let the angle of torsion necessary to keep it at this dis- 
 tance be 148. The needle being stationary, let the 
 
CHAP. III. LAWS OF ATTRACTION AND REPULSION. 281 
 
 fixed ball be touched by another ball exactly equal and 
 similar,, and similarly insulated. After the ball, which 
 has just touched it, is withdrawn, the moveable ball will 
 approach it in consequence of the diminished repulsion, 
 and to restore the moveable ball to its former distance 
 of 28 from the fixed ball, it will be necessary to 
 untwist the thread of suspension by turning the index 
 L of the micrometer. When the index is so adjusted 
 that the moveable ball remains at rest, the angle of tor- 
 sion will be found to be a little less than 74. It would 
 be exactly 74, being the half of its former value, 148, 
 but for the small quantity of electricity dissipated by 
 contact with the air in the interval between the two 
 experiments. 
 
 (69.) By the same kind of experiment, varied by 
 changing the quantities of electricity and the distances 
 at which the moveable ball is brought to rest, a similar 
 result may be obtained ; and the same effect is produced, 
 whatever be the form of the bodies used, provided they 
 be such, that, in the calculation of their mutual attrac- 
 tion, they may be regarded as points. Coulomb sub- 
 stituted for the fixed ball an iron disc, ten lines in dia- 
 meter, keeping the pith ball attached to the needle. 
 He electrified these two bodies by the head of a pin, and 
 a repulsion was produced, such that, to keep the move- 
 able ball at 30 from the fixed ball, a torsion of 140 
 was necessary. He then touched the disc of iron 
 by another of the same diameter, after which he un- 
 twisted the wire until the moveable ball was again 
 brought to rest at 30 from the iron disc. The angle 
 of torsion was then found to be 70, or half its former 
 amount. 
 
 These and similar experiments lead to the conclusion 
 that the diffusion of electricity on such conducting 
 bodies brought in contact with each other depends only 
 upon the dimensions of the bodies, and is entirely in- 
 dependent of the substance out of which they are formed 
 Whether an electrified pith ball be touched by another 
 pith ball of equal magnitude, or by a metallic ball, or 
 
282 ELECTRO-STATICS. PART I. 
 
 in fine, by a ball of equal magnitude formed of any 
 conducting substance, it will equally lose half its 
 electricity,, and the effect will be the same., whether the 
 ball which touches it be solid or hollow. 
 
 (70.) It appears, therefore, that between the electri- 
 city which is diffused over a conductor and the matter 
 composing that conductor, there exists no peculiar at- 
 traction analogous to chemical affinity which would 
 give the electricity a greater hold on one kind of matter 
 than on another ; for, were it so, it would be found that, 
 when balls of equal magnitude, but of different materials, 
 one being electrified, the other not, and both being in- 
 sulated, are brought into contact, the electricity would 
 not be equally diffused over them, but would collect in 
 greater quantity on that for which it would have the 
 stronger affinity. It may also be inferred that the in- 
 ternal dimensions of a body have no effect on its con- 
 ducting power, since, whether a ball be solid or hollow, 
 it will take the same quantity of electricity from an 
 electrified body, with which it is brought in contact. 
 
 (71.) It appears, also, from the above experiments, 
 that the mutual attraction or repulsion of each of two 
 electrified bodies depends conjointly on the quantity of 
 electricity on each of them. Thus, if the quantity of 
 electricity on either be doubled, or halved, or augmented, 
 or diminished in any other proportion, then their mutual 
 attraction or repulsion will be augmented or diminished 
 in the same proportion. If the quantity of electricity 
 on one be double, while on the other it be halved, then 
 their mutual attraction or repulsion would remain the 
 same, and, in general, their attraction or repulsion at a 
 given distance will be found by multiplying together 
 the numbers expressing the quantities of electricity 
 upon them respectively. 
 
 By combining this law with that which expresses the 
 variation of the electrical force depending on the change 
 of distance of the electrified bodies, we arrive at the 
 following general law : 
 
 The mutual attraction or repulsion of two electrified 
 
CHAP. III. LAWS OF ATTRACTION AND REPULSION. 283 
 
 bodies is directly proportional to the quantity of electri- 
 city on the one, multiplied by the quantity of electricity 
 on the other, and inversely proportional to the square 
 of the distance between them. 
 
 If R express the quantity of electricity on one of two 
 electrified bodies, and R 7 the quantity of the other,, then 
 that mutual attraction or repulsion at any distance D 
 
 T>TO/ 
 
 will be expressed by the formula- -, which is nothing 
 
 more than the above law expressed in mathematical 
 symbols. 
 
 It is remarkable, that this law is in all respects iden- 
 tical with that which governs the mutual gravitation of 
 masses of matter. Thus the mutual attraction is in the 
 direct proportion of the product of their masses, and the 
 inverse proportion of the square of their distances. 
 
284 ELECTROSTATICS. PART I. 
 
 CHAP. IV. 
 
 DISSIPATION OF ELECTRICITY. 
 
 (72.) THE law determined in the last chapter, by which 
 the intensity of electrical attraction and repulsion varies 
 with the variation of distance,, is only observed so long 
 as the electrical state of the bodies between which these 
 forces are exhibited remains unaltered. But in all cases 
 where such experiments are made, the electricity dif- 
 fused over the bodies is exposed to continual diminution, 
 being dissipated partly by the contact of the surround- 
 ing atmosphere, and partly by the imperfect insulation 
 afforded by the supports. In order, therefore, to de- 
 duce exact conclusions, it is necessary to know the extent 
 to which, and the law according to which, electricity is 
 dissipated or lost during the experiment. 
 
 (73.) When an insulated conductor has been elec- 
 trified, the electricity has a tendency to escape from it 
 to its insulating supports, and from these last to the 
 earth. Although, in practice, certain substances are 
 usually called insulators, or non-conductors, because 
 they afford a great obstruction to the passage of elec- 
 tricity, yet, in an absolute sense, there is no substance 
 in nature which does not allow electricity to be propa- 
 gated upon its surface in a greater or less degree. Glass, 
 sealing-wax, and gum lac are bad conductors, especially 
 the last, and offer a great obstruction to the transmission 
 of the electric influence ; but still they do transmit it, 
 and even in a sensible degree. To render this manifest, 
 let the end of a rod formed of any of these substances 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 285 
 
 be held in contact for some time with the conductor of 
 an electrical machine,, the machine being kept in opera- 
 tion. After withdrawing it from the conductor, let the 
 same extremity be presented to the needle of Coulomb's 
 electrometer, and it will be found to be charged with 
 the electricity of the conductor. It may be objected, 
 that in this case the electricity has been imparted to it 
 by immediate contact with the electrified surface of the 
 conductor, and that there is no evidence of the electric 
 fluid moving along its surface, and, therefore, that there 
 is no proof of any conducting power. If, however, a 
 small piece be cut from the end of the rod which was 
 in contact with the conductor, and the rod be then pre- 
 sented to the electrometer, the side of the rod contiguous 
 to the extremity from which the piece was cut will be 
 found to be electrified, but with less force than the ex- 
 tremity itself which was in contact with the conductor. 
 In the same manner, if slice after slice be cut from 
 the rod, and the state of its sides examined with the 
 electrometer, it will be found that the sides, through a 
 certain length of the rod, are electrified with a continu- 
 ally decreasing intensity. 
 
 (74.) The effect which is thus rendered manifest is 
 produced in a similar manner upon the supports by 
 which electrified conductors are insulated. The elec- 
 tricity upon them is gradually absorbed by the insu- 
 lating supports along which it slowly, but continually, 
 steals. If the length of these supports be less than 
 the distance through which the electricity can force its' 
 way, then there will be a constant and slow escape 
 of the electricity from the conductor to the earth ; but 
 even if the support be too long to allow of this, there 
 will still be a continual escape of electricity to the 
 insulating support so long as the latter continues to 
 absorb it. 
 
 (75.) When the atmosphere holds in suspension any 
 considerable quantity of vapour, as is the case in warm 
 weather, the vapour has a tendency to be condensed on 
 
286 ELECTRO-STATICS. PART I. 
 
 the surface of all bodies exposed to the air, and it col- 
 lects upon them in a greater or less quantity, according 
 to the attraction which they severally have for it. 
 When a film of moisture is thus collected on the sur- 
 face of a non-conductor, such a body loses, in effect, 
 its non-conducting power; or, to express more correctly 
 the phenomena, the body becomes enveloped in a thin 
 covering of water, along the surface of which, and not 
 the surface of the body itself, the electricity passes. 
 And water being a conducting body, the deposition of 
 such moisture on the insulating supports of an electrified 
 conductor must give a free passage to the electricity, and 
 therefore destroy the effect of the insulator. In elec- 
 trical experiments, therefore, besides selecting the best 
 non-conducting substances for insulators, it is necessary 
 to keep them constantly dry by rubbing them occasion- 
 ally with a dry cloth. 
 
 (76.) But even supposing an electrified conductor to 
 be perfectly insulated, a gradual loss of electricity would 
 take place by means of the atmosphere surrounding it. 
 This atmosphere, like all other non-conductors, possesses 
 the insulating power only in an imperfect degree, and 
 this imperfection varies according to the quantity of 
 vapour suspended in it, and probably, also, according to 
 physical modifications produced by variations of heat 
 and other causes which affect the properties of its con- 
 stituent elements. 
 
 In general, the air may be considered as being com- 
 posed of an infinity of different atoms, which possess the 
 conducting power in different degrees. Each molecule 
 of air which touches the electrified conductor receives 
 a portion of its electricity, and becomes itself elec- 
 trified. The charge of electricity which it receives 
 depends conjointly on its magnitude and its conduct- 
 ing power, and having an electricity similar to that of 
 the conductor, it is repelled and replaced by another 
 molecule, which is likewise electrified, and in its turn 
 repelled. Thus, by the continued effects of such par- 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 28? 
 
 tides, the electricity of the conductor is progressively 
 diminished. 
 
 Since, in practice, the gradual loss of electricity sus- 
 tained by any electrified conductor is due to the com- 
 bined effect of the two causes above mentioned, namely, 
 the contact of the surrounding air and the imperfect in- 
 sulation of the supports, a difficulty is presented in de- 
 termining how much of the total loss of electricity is due 
 to the one, and how much to the other, of these causes. 
 The means selected by Coulomb to remove that diffi- 
 culty was the selection of supports composed of the best 
 non-conducting substances, and so reducing their dimen- 
 sions that the surface which they expose to the humidity 
 of the air is so small that the electricity escaping by the 
 imperfect insulation shall bear so small a proportion to 
 the total loss of electricity sustained, that it may be 
 disregarded, and that the whole loss may, without sen- 
 sible error, be ascribed to the air. 
 
 (77.) After various trials, he found that when the 
 conductor was not very strongly electrified, a rod of 
 sealing-wax or gum lac, about twenty lines in length and 
 half a line in diameter, perfectly insulated a pith ball of 
 five or six lines in diameter. For he found that, whe- 
 ther the ball was supported by a single rod of this kind 
 or by several, the loss of electricity was the same. Now 
 it is evident that the loss produced by the imperfection 
 of the insulating power of these rods would be greater 
 in proportion to the number of them used to support or 
 suspend the ball. If, then, the actual loss when the ball 
 was suspended by six such rods was sensibly the same as 
 when it was suspended by only one, it follows that the 
 loss produced by the imperfect insulating power of a 
 single rod was so small that no sensible difference existed 
 between it and a loss six times greater. 
 
 (78.) He found also, that when the atmosphere was 
 dry a thread of fine silk drawn through boiling sealing- 
 wax, so as to have a diameter not exceeding a quarter 
 of a line, was an equally good insulator when its length 
 
288 ELECTRO-STATICS. PART I. 
 
 was not less than six inches. A thread of glass, drawn 
 at the blow-pipe, five or six inches in length, was 
 a sufficiently good insulator when the electricity of 
 the conductor was very feeble, and the air dry. A 
 human hair or a thread of silk, coated with sealing-wax, 
 or a varnish of gum Jac, was found to answer the same 
 purpose. 
 
 (79-) Having by these preliminary experiments been 
 enabled to select the best insulators, Coulomb attached 
 a pith ball to the extremity of a thread of very pure 
 gum lac twenty lines in length, and the other ex- 
 tremity of this thread he attached to a filament of 
 very fine silk coated with sealing-wax. He considered 
 the ball thus suspended to be perfectly insulated. This 
 ball, so suspended, he introduced at the opening T 
 (fig. 4.) in the top of his electrometer, and used it as 
 the fixed ball in his experiments. The mov cable ball 
 being, as in the experiment formerly described, attached 
 to a needle of gum lac, and that needle being suspended 
 by a fine silken thread, was also perfectly insulated. 
 In the first instance he submitted to experiment two 
 balls of equal diameter in an electrometer of such sen- 
 sibility that the torsion produced by one revolution of 
 the needle did not exceed the 340th part of a grain. 
 The two balls being brought into contact were electri- 
 fied by the head of a pin, as in the former experi- 
 ments, and the moveable ball was repelled to the dis- 
 tance of 40 from the fixed ball. By turning the thread 
 of suspension the moveable ball was then forced nearer 
 to the fixed ball, and brought to rest, for example, at the 
 distance of 20. To maintain it at this distance, 
 the angle of torsion was found to be 160. The mo- 
 ment at which the ball was brought to rest in this 
 position was observed by a watch* having a seconds 
 hand, and was found to be fifty minutes after six 
 o'clock. 
 
 The apparatus being allowed to remain undisturbed, the 
 gradual loss of electricity by the surrounding air produced 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 289 
 
 a progressive diminution of the repulsive force by which 
 the balls were separated, and, accordingly, the distance 
 of the moveable from the fixed ball was observed to be 
 gradually diminished, and to have become less than 20. 
 To bring back the moveable ball to its former position, 
 the suspended thread was untwisted by moving the 
 index of the micrometer of torsion through 30. The 
 moveable ball now, however, was driven beyond its first 
 position, and came to rest at a distance from the fixed 
 ball greater than 20. The apparatus was again left 
 undisturbed, until, by the further loss of electricity, the 
 distance of the moveable ball was again diminished. 
 The moment at which that distance became 20 was 
 observed, and was found to be fifty-three minutes after 
 six o'clock. 
 
 It appears, therefore, that a torsion of l6'0 was 
 necessary to keep the moveable ball 20 from the fixed 
 ball at fifty minutes after six o'clock ; and that a 
 torsion of 30 degrees less was sufficient to maintain it 
 in the same position at fifty- three minutes after six. 
 Thus in three minutes the repulsive force was dimi- 
 nished by an amount corresponding to an angle of 
 torsion of 30 ; and as by further experiments it was 
 found that in small intervals of time, the loss is sensibly 
 proportional to the time, it follows that in this case the 
 electricity was lost at the rate of 1 per minute. Now, 
 since the mean repulsive force during the above expe- 
 riment was 145 (which is a mean between the initial 
 and final repulsive force), it follows that the loss of 
 electricity by the contact of the air was in this case J, y 
 or the twenty-ninth part of the whole mean repulsive 
 force. 
 
 By experiments conducted in this manner, Coulomb 
 obtained the results in the following table : 
 
 VOL. i. 
 
290 
 
 ELECTRO-STATICS. 
 
 PART I. 
 
 Time of 
 Experiment. 
 
 Distance of 
 Balls. 
 
 Torsion of 
 Micrometer. 
 
 Time elapsed 
 between two 
 
 Observations. 
 
 111 
 
 Mean 
 Force 
 between 
 two Ob- 
 servations. 
 
 jjlljj 
 
 First Experiment, May 28. Saussure's Hygrometer, 75. 
 
 Thermometer 60. Barometer 30'08. 
 
 Exp. H. M. S. 
 
 
 
 min. 
 
 
 
 
 1 6 32 30 
 
 30 
 
 150"! 
 
 3 
 
 C)f\ 
 
 1 dC\ 
 
 r. 
 
 2 6 38 15 
 3 6 44 30 
 
 id. 
 id. 
 
 130J 1 
 
 nol / 
 
 81 
 
 zu 
 id. 
 id. 
 
 1 4U 
 
 120 
 100 
 
 1 
 
 4 6 53 
 5730 
 6 7 17 
 
 id. 
 id. 
 
 id. 
 
 90 / 1 
 701 j 
 
 50 J 
 
 2 
 
 10 
 14 
 
 id! 
 id. 
 
 80 
 60 
 
 I 
 
 Second Experiment, May 29. Hygrometer 69. Thermo- 
 
 meter 60. Barometer 30'16 inches. 
 
 1 5 45 3 
 2 5 53 
 
 30 
 id. 
 
 160\ 
 140 J "1 
 
 7| 
 
 20 
 id. 
 
 150 
 130 
 
 * 
 
 3 6 2 30 
 
 id. 
 
 120\ J 
 
 | 
 
 i 
 
 
 1 
 
 4 6 12 15 
 5 6 33 
 6 6 51 
 
 id. 
 id. 
 id. 
 
 100J 1 
 70 \S 
 50/ 
 
 20} 
 18 
 
 ia. 
 30 
 id. 
 
 75 
 60 
 
 i 
 
 Third Experiment, June 22. Hygrometer 870. Thermo- 
 
 meter 601. Barometer 29,75 inches. 
 
 1 11 53 '45 
 2 11 56 45 
 3 11 59 45 
 
 20 
 id. 
 id. 
 
 100 \ 
 80 h 
 60\ / 
 
 3 
 3 
 
 20 
 id. 
 
 ;j 
 
 90 
 70 
 
 i; 
 
 4 12 5 
 5 12 16 15 
 
 id. 
 id. 
 
 40 M 
 25 / 
 
 111 
 
 ia. 
 25 
 
 50 
 
 28 
 
 S 
 
 Fourth Experiment, July 2. Hygrometer 80. Thermo- 
 
 meter 60|. Barometer 30 inches. 
 
 1 7 43 40 
 2 7 49 
 
 20 
 id. 
 
 1001 
 80 J\ 
 
 51 
 
 20 
 j 
 
 90 
 
 A 
 
 3 7 57 20 
 4 8 9 20 
 
 id. 
 id. 
 
 60J{ 
 
 40/1 
 
 8| 
 
 "f 
 
 la. 
 id. 
 
 70 
 50 
 
 Q 
 
 re 
 
 5 8 17 30 
 
 id. 
 
 30 / 
 
 8 ' 
 
 10 
 
 OO 
 
 * 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 291 
 
 (80.) From the mere inspection of this table it will 
 be evident, that, on the same day, and in the same state 
 of the air, the loss of the electricity which takes place 
 in equal small intervals of time, bears the same ratio 
 to the whole intensity of the electric force of the body. 
 Thus, in all the six experiments of the 28th of May, 
 the loss per minute was very nearly the same fraction 
 of the total mean repulsive force, the greatest observed 
 amount of loss being ^Jg-, and the least ^V> part of the 
 whole ; and a like correspondence will be observed be- 
 tween the result of all the experiments made on any 
 one day. 
 
 But on comparing the rate at which electricity was 
 lost on different days, a striking difference will be ob- 
 served: thus, on the 22 d of June the loss per minute 
 was at the rate of about the 13th part of the whole 
 electric force of the bodies, while on the 29th of May 
 the loss was little more than the 60th part. On com- 
 paring the state of the hygrometer on the different days 
 with the results of the experiments, it will be found that 
 in proportion as the hygrometer rose, the loss of elec- 
 tricity increased. On the 22d of June, when the hy- 
 grometer was at 87, a 13th part of the electricity was 
 lost per minute, while on the 29th of May, when the 
 hygrometer stood at 69., the loss per minute was scarcely 
 a 60th part. It is apparent, therefore, that the loss of 
 electricity increases in some proportion not yet deter- 
 mined, with the quantity of aqueous vapour suspended 
 in the air. It will be observed that, on the four days 
 on which the experiments here recorded were made, 
 the barometer and thermometer had nearly the same 
 altitude. 
 
 (81.) If experiments on this subject were extended 
 and varied, they would doubtless lead to the discovery 
 of the exact relation between the quantity of aqueous 
 vapour suspended in the air, and the rate at which 
 electricity is lost. It would also be apparent, whether 
 the loss of electricity is owing exclusively to the 
 aqueous vapour suspended in the air, or in part due to 
 
292 ELECTRO-STATICS. PART I. 
 
 the pressure and temperature of the air itself. The 
 exact solution of these problems would convert the 
 electrometer into the most delicate and precise meteoro- 
 logical instrument, and the determination of the meteoro- 
 logical state of the atmosphere as to its humidity, tem- 
 perature, and pressure, would probably afford sufficient 
 data to enable us to calculate the rate at which the 
 electricity is dissipated on any given day, independently 
 of experiments. 
 
 (82.) In the absence of such data, it is necessary to 
 determine by direct experiment, in the manner above 
 explained, the rate at which electricity is lost by the 
 contact of the air on each particular day on which any 
 experimental enquiry in electricity is made. 
 
 A single experiment, however, is in general sufficient 
 for this purpose, since the indications of the hygrometer, ' 
 thermometer, and barometer do not generally suffer any 
 considerable change in the time occupied by such ex- 
 periments on any given day, and while they remain the 
 same, the rate at which electricity is dissipated remains 
 unaltered. 
 
 (83.) The law which governs the loss of electricity 
 being thus known, it only remains to establish such 
 formulae as will enable the observer, when the electric 
 state of body, or in other words the intensity of attrac- 
 tion or repulsion at a given distance, is known at any 
 one epoch, to determine what it would be at any other, 
 and thus to enable him to introduce into his calculations 
 those corrections which the loss of electricity by the 
 contact of the air renders necessary. For this purpose 
 let A be the torsion which measures the electric force 
 at the time T , and let the successive times be T l} Tg, 
 T 3 , &c. at which, by the gradual waste of electricity, 
 the torsion becomes A l} A 2 , A 3 , &c. 
 
 Tn the successive intervals of time expressed by 
 
 T,-T O , 
 
 T 2 -T,, 
 T 3 -T 2 , 
 T 4 -T 3 , 
 
AP. IV. DISSIPATION OF ELECTRICITY. 2Q3 
 
 the losses of force expressed by the corresponding ele- 
 ments of torsion will he 
 
 A **w 
 
 "1 *2) 
 
 A 2 A 3 , 
 A 3 -A 4 . 
 
 If we suppose the time expressed as before in minutes, 
 and the torsion in degrees, the first series will express 
 the number of minutes in which are lost the forces equi- 
 valent to the degrees of torsion expressed in the second 
 series. Thus,, in the number of minutes expressed by 
 Tj T , the loss of electricity is proportional to the 
 number of degrees expressed by A AI ; and as the loss 
 of electricity in one minute will be found by dividing the 
 total loss in any number of minutes by that number,, it 
 follows, that the loss of electricity per minute is propor- 
 tional to 
 
 and as the loss per minute has been proved to be always 
 in the same ratio to the total mean electric force, let 
 this proportion in the present case be expressed by a, the 
 mean electric force being half the sum of the initial and 
 final electric forces, and we shall have 
 
 1 1 A 
 From which we infer 
 
 T,- To)' 
 
 If we suppose the same interval to elapse between each 
 successive pair of observations, and this interval to be 
 expressed by t, we shall have 
 
 u 3 
 
ELECTRO-STATICS. PAKT I. 
 
 T -T 2 =* 
 
 and by extending the preceding formula to every suc- 
 cessive pair of observations,, we shall have 
 
 Ai~mA , A 2 =mA L , A 3 =wA 2 ; 
 where, for brevity, m= - ^ . From these it follows, 
 
 that 
 
 A L =mA , A 2 =m 2 A , A 3 
 
 By which, if m and the torsion A at the time T be 
 known, the torsion at any succeeding epoch may be 
 immediately computed. If, in general, n express the 
 order of any observation counted from the first, we shall 
 have A n =m n A . 
 
 Since the whole interval from the time T to the time 
 T n of the n th observation is T n T , and since t is 
 the interval between two succeeding observations, the 
 number of observations n will be found by dividing 
 Tn T by t } and we shall therefore have 
 
 T n -T 
 
 n= - . 
 t 
 
 But we have A n =m n A , log. A n =n log. 
 Therefore 
 
 log. m 
 and therefore we have 
 
 T,.-T = (log. A. -log. Ao). 
 
 log. m 
 
 The relation here expressed between the time T n T , 
 and the quantity of electricity lost, has an apparent de- 
 pendence on the interval of time t between two succes- 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 9' 
 
 sive observations, of which, however, it ought evidently 
 to he independent. It is easy, however, to show that 
 
 the quantity - is really a constant quantity inde- 
 log.w 
 
 pendent of t. Substitute for m its value, and we have 
 
 log.m=log.l^i=log. (1 -la*) -lg. (1 + JflO- 
 i ~r t fl 
 
 Developing these logarithms in series of powers of t, by 
 the theorem of Taylor, we have 
 
 in which M=2'302585 ; from whence we infer 
 t M 
 
 But in order that the conclusion at which we have 
 arrived be rigorously true, the interval t must be taken 
 =0. Hence 
 
 M 
 
 - - = , 
 log. m a 
 
 which is independent of t, and dependent only en the 
 rate at which electricity is lost by the contact of the air. 
 Hence we have 
 
 M 
 
 T n -T =-(log. Ao-log. A n ) . . . . (i), 
 
 
 or log. A n =log. A - ^(T n - T ) . . . (2). 
 
 3y the formula (1) we can calculate the interval 
 u 4 
 
296 ELECTRO-STATICS. PART I. 
 
 T n T , after which the torsion shall have any given 
 value A n ; and by the formula (2) we shall find the 
 torsion, which will be produced by the electrical force 
 after any given interval, T B T . 
 
 As an example of the practical application of the pre- 
 ceding formula, let us take the observations of the 28th 
 May, given in the table (79-)- ^ n tn ^ s case tne an gl e 
 of torsion at the moment of the first observation was 
 150, and the loss of force was at the rate of ^ T of the 
 whole mean force per minute. Let it be required to 
 calculate what the angle of torsion would be after the 
 lapse of 45 minutes. We have then 
 
 A =150, a= , T n -T =45. 
 Hence we have 
 
 log. A =2-1760913. 
 Hence we shall have 
 
 log. A a =2-1760913-0'4766625==l-6994288. 
 which gives 
 
 A n =50 3' 10". 
 
 Now the angle of torsion given by observation was 50, 
 as appears by the table. The accordance of the calcu- 
 lated and observed angles is perfect, since, in this species 
 of observation, small fractions of a degree cannot be 
 estimated. 
 
 (84.) The amount of torsion being thus reduced to 
 computation, it remains to explain the method of de- 
 ducing from it the actual intensity of the electric force 
 to which this torsion is proportional. If F express the at- 
 tractive or repulsive force exerted by the electrified bodies 
 
 F 
 
 at the unit of distance, then - will express the force 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 297 
 
 exerted at the distance D ; and since this bears an in- 
 variable ratio to the torsion which measures it, we shall 
 have 
 
 where A is the angle of torsion corresponding to the 
 force F. 
 
 Let F he the force, and A the corresponding angle 
 of torsion at any proposed epoch, and after the lapse of 
 any given interval of time t, let F* be the force and A, the 
 angle of torsion ; we shall then have 
 
 Hence we have 
 
 Fo A A, 
 
 _ =1? orF ( =F - 
 
 which is nothing more than the statement in mathema- 
 tical symbols, that the electric forces are proportional to 
 angles of torsion. Taking the logarithms we have 
 
 log. F<=rlog. F + log. A, log. A . 
 But by (2) we have 
 
 log.A,= log.A - 
 Hence we have 
 
 log. F,=log. F. J. . . . (3). 
 
 By which the force F, may be computed when F is 
 known. 
 
 The forces which are here submitted to calculation, 
 being the actual attractions or repulsions exerted between 
 two electrified bodies, their total effect is the compound 
 result of the actions of each body on the other ; arid if 
 these forces separately be expressed by R and R', we 
 have already shown that F will be equal to their pro- 
 
298 ELECTRO-STATICS. PABT I. 
 
 duct. Retaining a notation analogous to the preceding 
 one, we shall therefore have 
 
 F = R R o) F* = R, R',. 
 
 (85.) In the experiments of Coulomb., tabulated in 
 (79-)j the two electrified bodies were equal balls of the 
 same substance simultaneously and equally electrified. 
 Hence, after the lapse of any time, their electric forces 
 must have been necessarily equal. 
 
 Thus we should have R=R', and therefore F =R 2 
 and F,=R, 2 . Making these substitutions in the equation 
 (3), we shall have 
 
 log. R,=log.R -~ 
 
 The electric force of each ball, therefore, decreases ac- 
 cording to the same law as the angle of torsion. But 
 the co-efficient la of this proportion is only half that of 
 the decrement of torsion. For example, in the experi- 
 ments of the 28th of May, tabulated in (7.9.), the decre- 
 ment per minute of torsion is -^ of the whole mean 
 torsion, while the decrement per minute of the electric 
 force of each ball is only the -g 1 ^ part of the whole mean 
 electric force of that ball. 
 
 It is easy to extend this problem to the case of balls 
 of unequal magnitude. If R R', be the electric forces of 
 the balls respectively at any moment, the decrement 
 which each suffers per minute will follow the law just 
 explained ; so that we shall have 
 
 at at 
 
 log. R,=log. Ro-gjj, log. R',=log. R'.-^- 
 
 From which we deduce by addition 
 
 log. R,R',=log. RoR'o-^- .... (4). 
 
 Substituting for the products of the separate forces of 
 the balls their values, we have 
 
CHAP. IV. DISSIPATION OF ELECTRICITY. 299 
 
 and therefore 
 
 log. A,= log. AO ^ (6'), 
 
 since the forces of attraction or repulsion, compared, as 
 they are here understood to be, at the same distance, are 
 always proportional to the angles of torsion. 
 
 It appears therefore, by the result of this investigation, 
 that in the case of unequal balls, the decrement of tor- 
 sion, by reason of the dissipation of electricity, takes 
 place in the same manner, and according to the same 
 proportion, as in the case of equal balls. The results of 
 the experiments are altogether in accordance with this 
 whatever be the proportion of the magnitude of the 
 fixed to the moveable ball whatever be the quantity 
 of electricity first imparted to them, whether they be 
 electrified simultaneously or at different times, equally 
 or unequally the rate of decrease of the force with 
 which they mutually attract or repel each other, will 
 be invariably proportional to the intensity of that 
 force. It is further to be observed, that the ratio which 
 this rate of decrease bears to the whole amount of the 
 attractive or repulsive force, is the same, whatever be 
 the nature of the electrified bodies. The nature of the 
 matter of which they are formed has therefore no dis- 
 coverable influence on the rate at which electricity is 
 lost by the contact of the air. This result is in ac- 
 cordance with what has been already stated, that bodies 
 appear to retain the electricity imparted to them not by 
 any peculiar affinity or attraction for it, but only by 
 the mechanical resistance opposed to its escape by the 
 surrounding air. For example, on the day on which 
 the electricity decreased on each of the balls at the rate 
 of -g 1 ^ part of its whole amount per minute, the same 
 rate of decrease was observed when a ball of copper was 
 substituted for one of the pith balls. A ball of sealing 
 
300 ELECTRO-STATICS. PART I. 
 
 wax charged with electricity hy contact with a strongly 
 electrified conductor being substituted for the fixed ball 
 in the electrometer, the same rate of decrease was ob- 
 served. It appears, therefore, that even in the case of 
 non-conducting bodies, whose surface opposes an ob- 
 struction to the transmission of electricity over them, 
 there is no corresponding obstruction to its escape by 
 the contact of the surrounding air ; but that, on the other 
 hand, such a body loses its electricity as fast as the 
 most perfect conductor. 
 
 The experiments which have been referred to were 
 made on small balls, such as could be conveniently in- 
 troduced into the electrometer ; but the law which has 
 been deduced from them is universal, provided the 
 charge of electricity given to them is not very consider- 
 able. Whatever be the figure of the electrified body, 
 whatever be its magnitude, and whatever be the amount 
 of its repulsive force, if only the atmosphere be dry, 
 and the degree of electricity imparted to the body 
 be not considerable, the decrement of the repulsive 
 force per minute will always be the same fractional part 
 of its whole amount, supposing the hygrometric state 
 of the air not to change. Coulomb extended this in- 
 vestigation to bodies of various magnitudes and forms. 
 He submitted to experiment a globe of a foot in dia- 
 meter with cylinders of various magnitudes and lengths. 
 He substituted for the balls discs of various sub- 
 stances: he tried also the effect of a copper wire ten 
 lines in length and one-fourth of a line in diameter, 
 and found that the decrease of all these different bodies 
 electrified on the same day was the same. It is neces- 
 sary to observe, however, that this uniform decrease in 
 bodies differing much in figure is only observed when 
 they are charged with a very feeble electricity^ for when 
 a strong charge of electricity is given to bodies having 
 an angular form, the rate of its decrease is much more 
 rapid than is the case . with round bodies, for reasons 
 which will be explained hereafter. But even these 
 bodies will conform to the law of the uniform decrease 
 
CHAP. IV. DISSIPATION OP ELECTRICITY. 301 
 
 of electricity, when their electricity has been so far en- 
 feebled that the angles and points lose their influence. 
 
 Biot, to whom the mathematical analysis of the loss 
 of electricity by the contact of the air is due, instituted 
 a series of experiments with a view to determine whether 
 any difference existed at a given time between the rates 
 at which positive and negative electricity were lost. 
 The result of his enquiry was., that these rates were the 
 same. 
 
 (86.) It is evident from the results of the numerous 
 and careful experiments instituted by Coulomb, that 
 the quality of the atmosphere which produces the most 
 material effect on the dissipation of electricity, is that 
 which is indicated by the hygrometer ; but, notwith- 
 standing the elaborate investigation of that philosopher, 
 he could discover no law of electrical dissipation de- 
 pending exclusively on the combined indications of the 
 barometer, thermometer, and hygrometer. It appears, 
 therefore, that there is some source of the absorption of 
 electricity independent of variation of the pressure, tem- 
 perature, and dryness of the air; or, as is perhaps more 
 probable, these instruments are not so immediately affected 
 by atmospheric changes as an electrified body is. Hence 
 it may have happened that on different days, when the 
 barometer, thermometer, and hygrometer indicated the 
 same state of the atmosphere, the quantity of electricity 
 lost per minute would not be the same. On a sudden 
 change of weather, when the hygrometer indicated in- 
 creased dryness, the quantity of electricity lost per 
 minute was not diminished so much as it ought to have 
 been by calculation. Coulomb accounts for this by the 
 supposition, that there is an adhesion between the air 
 and the vapours which interferes with the indications of 
 the hygrometer, so that the substance, by the qualities 
 of which the indications of this instrument are obtained, 
 can only be affected by that portion of moisture which 
 is entirely disengaged from any combination with the 
 molecules of air ; and that, as the separation is effected 
 only by degrees, the dissipation of electricity is pro- 
 
302 ELECTRO-STATICS. PART I. 
 
 moted by those particles of aqueous vapour remaining 
 in combination with the air, of which the hygrometer is 
 not sensible. 
 
 (87.) The same method of experiment which con- 
 ducted Coulomb to the discovery of the law according 
 to which electricity is dissipated by the contact of the 
 air, would naturally be resorted to ascertain the loss of 
 electricity proceeding from imperfect insulation. 
 
 Such a method would consist in the adoption of sup- 
 ports which would produce a loss of electricity bearing 
 a very considerable proportion to the loss sustained by 
 the contact of the air. This would, however, be at- 
 tended with great practical inconvenience and difficulty. 
 Each time the apparatus is touched, whether it be to 
 electrify the balls or to adjust the torsion of the sus- 
 pended thread, the needle would continue to oscillate for 
 a considerable time before it would come to rest ; and in 
 that interval the intensity of the electricity would suffer 
 great variation ; the insulator, therefore, should be at least 
 sufficiently perfect to enable the observer to complete 
 a series of several observations without renewing the 
 electricity of the balls. In the experiments instituted 
 by Coulomb for this purpose, the fixed ball, instead of 
 being suspended by a small rod of gum lac which per- 
 fectly insulated it, was suspended by a single fibre of 
 raw silk fifteen inches in length. The moveable ball 
 was, as before, equal to the fixed ball, and perfectly in- 
 sulated. The apparatus being thus arranged, the expe- 
 riments were made in the same manner as in the former 
 case, and the decrement of torsion which took place in 
 known intervals of time was observed. As these expe- 
 riments were made on the 28th and 29th of May, two 
 of the days on which the former experiments were made, 
 the loss of electricity by the atmosphere alone was 
 known, which being deducted from the total loss of 
 electricity, the remainder would be that due to the im- 
 perfect insulation of the supports. 
 
 The results of the experiments are exhibited in the 
 following table : 
 
CHAP. IV. 
 
 IMPERFECT INSULATION. 
 
 303 
 
 Time of 
 Experiment. 
 
 Distance of 
 Balls. 
 
 Torsion of 
 Micrometer. 
 
 Time elapsed 
 between two 
 consecutive 
 Observations. 
 
 tm. 
 
 foP 
 
 Mean 
 Force 
 between 
 two Ob- 
 servations. 
 
 llrll, 
 j|IJ|l 
 
 First Experiment, May 28. 
 
 Exp. H. M. S. 
 
 
 
 M. S. 
 
 
 
 
 1 10 
 
 30 
 
 180 
 
 
 
 
 
 
 
 
 2 30 
 
 30 
 
 165 
 
 A 
 
 2 10 2 30 
 
 id. 
 
 150 
 
 
 
 
 
 
 
 
 5 30 
 
 40 
 
 130 
 
 T5 
 
 3 10 8 
 
 id. 
 
 110 
 
 
 
 
 
 
 
 
 5 
 
 20 
 
 100 
 
 iz 
 
 4 10 13 
 
 id. 
 
 90 
 
 
 
 
 
 
 
 
 16 30 
 
 40 
 
 70 
 
 55 
 
 5 10 29 30 
 
 id. 
 
 50 
 
 
 
 
 
 
 
 
 21 
 
 20 
 
 40 
 
 A 
 
 6 10 50 30 
 
 id. 
 
 30 
 
 
 
 
 
 
 
 
 16 30 
 
 10 
 
 25 
 
 3 ! T 
 
 7 11 7 
 
 id. 
 
 20 
 
 
 
 
 
 Second Experiment, May 29. 
 
 1 7 34 
 
 30 
 
 180 
 
 
 
 
 
 
 
 
 2 40 
 
 20 
 
 170 
 
 la 
 
 2 7 36 40 
 
 id. 
 
 160 
 
 
 
 
 
 
 
 
 4 50 
 
 id. 
 
 150 
 
 55 
 
 3 7 41 30 
 
 id. 
 
 140 
 
 
 
 
 
 , 
 
 
 
 6 50 
 
 id. 
 
 130 
 
 1 
 
 4 7 48 20 
 
 id. 
 
 120 
 
 
 
 
 
 
 
 
 7 25 
 
 id. 
 
 110 
 
 A 
 
 5 7 55 45 
 
 id. 
 
 100 
 
 
 
 
 
 
 
 
 11 45 
 
 id. 
 
 90 
 
 J 
 
 6 8 27 30 
 
 id. 
 
 80 
 
 1 
 
 
 
 ' 
 
 
 
 
 17 30 
 
 id. 
 
 70 
 
 A 
 
 7 8 25 C 
 
 id. 
 
 60 
 
 
 
 
 
 
 
 
 17 30 
 
 15 
 
 52 
 
 A 
 
 8 8 42 3C 
 
 id. 
 
 45 
 
 
 
 
 
 
 
 
 22 30 
 
 14 
 
 38 
 
 A 
 
 9 9 5 id. 
 
 31 
 
 
 
 
 
 On inspecting this table it will be evident, that in the 
 commencement, when the repulsive force of the elec- 
 
304 ELECTRO- STATICS. PART I. 
 
 tricity is considerable, the loss of electricity is much 
 more rapid than by the air alone ; but, according as 
 the repulsive force is diminished and the electricity 
 becomes more feeble, the rate at which it is lost is 
 diminished, until at last it becomes equal to the rate at 
 which it is lost by the contact of the air alone. Thus, 
 on the 28th of May, in the first experiment, the total 
 loss per minute was a fourteenth of the whole electric 
 force ; but when the repulsive force was diminished, so 
 as to be balanced by a torsion of 40, the quantity lost 
 per minute was equal to that which was lost by the 
 contact of the air alone. When the electricity, there- 
 fore, did not exceed this intensity, the insulators became 
 perfect. In the same manner, in the experiments on 
 the 2Qth of May, the insulation became perfect when 
 the repulsive force was measured by the angle of torsion 
 of 70. 
 
 In these experiments, the moveable ball, being per- 
 fectly insulated, loses its electricity only by the con- 
 tact of the air. The electric force of this ball, there- 
 fore, for any time, may be calculated by the formulae 
 already established for the case of bodies perfectly insu- 
 lated; and as the experiments supply the value of the 
 whole repulsion of the two balls, that of the fixed ball 
 may be immediately inferred. Let R be the electric 
 force of the moveable, and R X that of the fixed ball at 
 the commencement of the observations, and let R, and 
 R', be their forces after the interval t, and let F and 
 F, be the total electric force exerted by the balls on each 
 other at the commencement, and at the end of the time t. 
 We have then 
 
 F =R R / , F,=R,R'f. 
 Therefore 
 
 R', 
 
CHAP. IV. IMPERFECT INSULATION. 305 
 
 But according to what has been already proved with 
 respect to balls perfectly insulated, we have 
 
 R, at 
 
 the number a expressing the decrement per minute, 
 which the electricity suffers by the air alone. The 
 
 F 
 
 quantity is determined by the experiment, being 
 
 F o 
 
 equal to the ratio of the torsions at the commencement 
 and at the end of the time t. Let these torsions, as be- 
 fore, be expressed by A and A,, and we shall have 
 
 R' t A t at , . 
 
 This formula will therefore be sufficient to determine 
 the electrical state of an imperfectly insulated body after 
 the lapse of any given time. 
 
 As an example of the practical application of this 
 formula, let it be required to determine the electrical 
 state of the fixed ball at the time when the silk thread 
 of suspension beg- an to insulate it perfectly, in the expe- 
 riments of the 28th May. Let it be assumed that this 
 perfect insulation commenced after an interval of forty 
 minutes, when the torsion was reduced from 180 to 40. 
 The decrement of torsion per minute by the operation 
 of the air alone was on that day -fa part of the mean 
 torsion. Hence we have 
 
 A =180, A,=40, *=40. a= 1 1 T - 
 Since M= 2-302585, we shall then have 
 
 -^-=0-0105925, 4--= 0<0052 962, -^- =0'2118480. 
 M 2M 2M 
 
 By the tables then we find 
 
30t) ELECTRO- STATICS. PART I. 
 
 log.A,=l-6020600, 
 
 log. ~'=r-S467875, 
 
 ^=0-2118480, 
 
 T>/ __ 
 
 log. -J =1-5586355, 
 HO 
 
 ^=0-36194. 
 
 tt 
 
 Hence it appears that the thread of silk fifteen inches 
 long "became a perfect insulator when the electricity of 
 the ball was reduced to -ffy of its original amount. 
 
 The original electric force of the fixed ball may be 
 easily determined. We have F =R R / . But since 
 the balls are equal, and similarly and simultaneously 
 electrified, R U =R / . Therefore 
 
 F =R' 2 , andR' =VF . 
 
 F 
 But if 2=&A we shall have 
 
 But since A =180, we obtain by taking the square 
 root 
 
 R' = 
 
 The state of the air remaining the same,, Coulomb re- 
 peated this experiment with a thread of the same silk 
 five feet long, being four times the length of the former, 
 and he found R' =971 1 84D\/6~~. As the experiment 
 was made with the same apparatus, and the distance D 
 was the same in both cases, it is evident, that by in- 
 creasing the length of the insulating thread in the ratio 
 
CHAP. IV. IMPERFECT INSULATION. 3(37 
 
 of four to one, its insulating power was increased in the 
 ratio of two to one. 
 
 (88.) This principle may be considered as appli- 
 cable to all cylindrical supports of very small diameter,, 
 whose insulating power is imperfect. In a given state 
 of the atmosphere, the intensity of the electric force at 
 which they begin to insulate perfectly is proportional 
 to the square root of their length, their nature and 
 diameter being the same. When the insulating powers 
 of different substances are required, the ratio must be 
 deduced by means of the formula itself from direct ex- 
 periment. By calculating thus, the electric forces at 
 which threads of gum lac and silk of equal length and 
 the same diameter begin to insulate perfectly, Coulomb 
 found that its value was ten times greater for gum lac 
 than for silk. In the same manner, the insulating 
 power of all substances may be compared and reduced 
 to numerical estimation. 
 
 In such experiments it is not necessary that the 
 balls of the electrometer should be observed at the same 
 distance in the two series of experiments : it will be 
 sufficient that this distance be maintained constant in 
 each series, and that its value be substituted in each 
 case in the formula. 
 
 It is also indifferent what degree of electricity is im- 
 parted to the balls, provided only equal balls similarly 
 suspended, and simultaneously electrified, be used in 
 all those parts. If this condition were not fulfilled, 
 the co-efficient B would not be the same in the different 
 series, which would render their comparison difficult 
 and indirect. 
 
 x 2 
 
SOS 
 
 ELECTRO-STATICS. 
 
 PART I. 
 
 CHAP. V. 
 
 DISTRIBUTION OF ELECTRICITY ON CONDUCTORS. 
 
 (89.) BY what has been explained, it is apparent that 
 when electricity is communicated to any point of an 
 insulated conductor, it is immediately diffused over its 
 entire surface ; but whether on every part of that sur- 
 face its distribution is the same, or if not, according to 
 what law its energy at different parts of the surface 
 varies, and whether it penetrates beyond the surface 
 to the internal parts of the body, are questions which 
 still remain to be investigated. 
 
 Several of the facts which have been already ex- 
 plained render it highly probable, that the electricity 
 which is imparted to conductors is confined to their 
 surface, and that none of their particles, sensibly below 
 the surface, are affected by, or produce any effect upon, 
 it. A ball formed of any material will be equally elec- 
 trified, whether it be solid or hollow ; and if it be 
 hollow, the charge which it receives from any source 
 of electricity will be the same, whether the shell of 
 matter of which it is formed be thin or thick. 
 
 To reduce this question to direct experiment, let A 
 (fy. 11.) be a conductor of an oval or spheroidal form ; 
 
 Jet two thin hollow covers B B' be formed to corre- 
 
CHAP. V. DISTRIBUTION OP ELECTRICITY. 309 
 
 spond with the shape and magnitude of the conductor 
 A, so that, when applied to it, they shall form a coating, 
 which shall completely cover it, and be in close contact 
 with it. Such cover may be formed of paper, gilt on 
 the concave side. Let C C' be two handles of gum lac 
 attached to these covers, by which they may be applied 
 to the body A, and removed from it without either 
 imparting electricity to them, or allowing any to escape 
 from them. Let the body A be now placed on an in- 
 sulating support, or suspended by a thread of fine silk, 
 varnished with gum lac, and let it be electrified in the 
 usual way. After having touched the two inner covers 
 B B' to deprive them of any electricity they may have, 
 let them be applied to the conductor A, holding them 
 by the insulating handles C C'. After withdrawing 
 them from A, they will be found, if tested by the 
 electrometer, to be electrified with the same electricity 
 as was previously communicated to A ; and if A be 
 similarly tested, it will be found to have lost all its 
 electricity. Thus, by mere superficial contact, all the 
 electricity which had been previously imparted to A 
 has been taken away upon the covers B B'. To this 
 experiment it may, however, be objected, that the at- 
 traction of the surface of the covers B B' may have 
 been sufficient to draw the electricity from a depth 
 more or less considerable within the surface of the con- 
 ductor A. 
 
 Fig. 12. (90-) The following experiment made by| 
 Coulomb indicates in a manner still more direct 
 the superficial distribution of electricity ; and! 
 the result is more general, being applicable to 
 bodies of any form whatever. Several small 
 round holes, of less than half an inch in dia- 
 meter, and of various depths, are made in dif- 
 ferent parts of the surface of a conductor. A 
 thread of pure gumlac AB (fig. 12.), several 
 inches in length, is then attached to a small disc 
 C of gilt paper, the diameter of which is about a 
 third of the diameter of the holes. 
 x 3 
 
310 ELECTRO-STATICS. PART I. 
 
 The conductor being insulated, is then strongly elec- 
 trified by sparks taken from the prime conductor of an 
 electrical machine. Holding the insulating handle of 
 gum lac at A, the disc C is then introduced into one of 
 the holes so carefully as not to touch their edges, and 
 being brought into contact with the surface of the con- 
 ductor at the bottom of the hole, it is withdrawn with 
 the same precaution. This disc C being then tested by 
 an electrometer, will be found to have acquired no elec- 
 tricity whatever ; and the same result will be obtained 
 by introducing it in the same manner into each of the 
 holes. The successful result of this experiment may 
 be rendered more certain by providing a tube of glass, 
 corresponding in magnitude with the holes, which being 
 introduced into them before the disc, the latter may be 
 inserted in the tube, so as to be defended from contact 
 with the edges of the hole. 
 
 If the disc C be applied to any part of the surface of 
 the conductor not perforated, or even to the very edges 
 of the holes, it will be found, on withdrawing it from 
 the surface, to be strongly electrified. 
 
 From this experiment it appears, that the distributior 
 of electricity upon bodies is confined to their surfaces 
 but an effect is sometimes observed, which might lead 
 to the supposition that the surface at the bottom of the 
 hole is electrified with electricity of a contrary kind to 
 that of the surface of the conductor ; for if the con- 
 ductor be positively electrified, the disc, after being 
 withdrawn from the hole, is sometimes found to have 
 feeble charge of negative electricity ; and, if the con- 
 ductor be negatively electrified, the electricity of the 
 disc is found in similar cases to be positive. This ife 
 observed to take place more especially when the atmo- 
 sphere is humid, the gum lac not pure, and when in the < 
 performance of the experiment the disc is allowed tc 
 remain a considerable time in the hole. The electricity 
 of the disc in such cases does not proceed from any 
 electricity taken up from the surface of the bottom oi' 
 the hole, but is an effect produced by the electricity 
 
DISTRIBUTION OF ELECTRICITY. 
 
 Sll 
 
 diffused on the surface of the conductor acting in a 
 peculiar manner, which will be explained hereafter, on 
 ihe natural electricity of the gum lac. 
 
 (91.) The superficial distribution of electricity on 
 conductors, may be also illustrated by the following ex- 
 periment: Let A B (fig. 13.) be a cylinder of metal, 
 
 Fig. 13. 
 
 insulated, and capable of being turned on a horizontal 
 axis by a handle M formed of glass, so that, when held 
 by the hand, no electricity shall pass from the con- 
 ductor to the hand. Let R be a piece of metallic leaf 
 or foil sufficiently strong to be raised without being 
 broken, and let this be rolled several times round the 
 cylinder, and terminated in a semi-circle, which shall 
 hang from the side of the cylinder. To the lowest 
 point of this semi-circle let a thread of silk F be 
 attached. To the end of the conductor let an electro- 
 meter be fixed, consisting of two pith balls, suspended 
 by metallic wires or threads of linen, or other conduct- 
 ing substance. Let the conductor be now electrified. 
 The electricity having free passage to the pith balls, 
 they will repel each other, and the extent of their 
 divergence will be proportional to the intensity of the 
 electricity of the conductor. If the metallic covering 
 R be now unrolled from the cylinder by taking hold of 
 the silk thread F, the divergence of the balls will gra- 
 . x 4 
 
312 ' ELECTRO-STATICS. PART I. 
 
 dually diminish; and, when the covering is entirely 
 unrolled, the balls will exhibit no sensible divergence. 
 The silk thread by which the metallic covering is sus- 
 tained being an insulator, the electricity will be retained 
 on the surface of the metal, and will not escape to the 
 hand. If the metallic covering be now again rolled 
 upon the cylinder, the balls will gradually diverge, their 
 divergence increasing as the covering is successively 
 roiled up ; and when it is restored to its first position, 
 the balls will exhibit the same divergence as in the 
 first instance, provided the interval suffered to elapse be 
 not so great as to allow any sensible loss of electricity 
 by the contact of the air. 
 
 In this experiment it is evident, that when the con- 
 ductor and metallic ribbon are first electrified, the electric 
 fluid is diffused on the external surface of the ribbon. 
 As the ribbon is unrolled from the cylinder by the 
 thread F or by turning the handle M, a more ex- 
 tensive surface of the ribbon is successively exposed, 
 over which the electric fluid is diffused. Its intensity 
 diminishes as this surface increases ; and if the ribbon 
 be sufficiently long, the surface over which it is distri- 
 buted will become so extensive, and its intensity will be 
 consequently so diminished, that it will cease to produce 
 any sensible effect upon the balls. When the ribbon, 
 however, is again rolled upon the cylinder by turning 
 the handle M in the other direction, the electricity will 
 again be collected on the same surface on which it was 
 originally diffused, and it will recover its original in- 
 tensity, and the balls will again diverge. 
 
 (92.) Although these experiments satisfactorily prove 
 that the distribution of the electric fluid is either ri- 
 gorously confined to the surface of bodies or very nearly 
 so, there are some circumstances which render it pro- 
 bable that it penetrates to some depth below their 
 surface, but to so smah 1 a depth as to be inappre- 
 ciable by any known methods of measurement. At 
 so early a period of electrical investigation as the 
 middle of the last century, it occurred to Watson, 
 
CHAP. V. DISTRIBUTION OP ELECTRICITY. SIS 
 
 and about the same time to Lemounier, to try whe- 
 ther the electric fluid passed within the external 
 surface of a metallic rod; and for this purpose the 
 rod was coated through a portion of its length with 
 sealing wax, which,, being a non-conductor, would not 
 allow the electric fluid to pass either along its outer 
 surface or along its inner surface,, which was in contact 
 with the surface of the metal. A charge of electricity,, 
 however,, even of feeble intensity,, communicated to one 
 end of the rod,, passed as freely through its entire 
 length as if there was no coating of wax upon it. It 
 follows from this experiment,, that the electricity must 
 have passed sufficiently below the external surface of 
 the metal to be out of contact with the wax. 
 
 (93.) Having thus proved the distribution of electri- 
 city to be nearly superficial, it now remains to examine 
 whether its intensity on every part of the electrified sur- 
 face of a conductor is the same. There are some bodies 
 whose geometrical properties suggest so strongly the 
 necessity of uniform distribution as to render demon- 
 stration unnecessary. If the electrified body,, for ex- 
 ample, be a sphere of metal or any other conducting 
 substance, the symmetry of its figure alone renders the 
 uniform distribution of electricity upon it inevitable, 
 since no reason can be assigned why the electricity 
 should be stronger in any one part of the sphere than in 
 any other. If, then, electricity be regarded as a material 
 fluid diffused over the surface of the sphere, having a 
 uniform but extremely small depth on every part of it, 
 as a liquid might be diffused over the surface of the 
 globe, then the total quantity of electricity upon the 
 sphere will be found by multiplying the surface of the 
 sphere by the depth of the fluid. It is proved in 
 geometry that the surface of a sphere is four times the 
 magnitude of the area of one of its great circles, that is, 
 of a section of the sphere made by cutting it through 
 its centre. If, therefore, four times the area of such a 
 circle be multiplied by the supposed depth of the elec- 
 tricity, the total quantity of electricity will be obtained. 
 
314 ELECTRO-STATICS. PART I. 
 
 Let R be the radius of the sphere, and TT =3*141 5, 
 then the area of the great circle of the sphere will be 
 R 2 7r; and if x express the thickness or depth of the 
 electric fluid, the quantity of electricity upon the sphere 
 will be expressed by 4<#R 2 TT ; so that,, if E be the 
 quantity of electricity, we shall have 
 
 And if E y be the quantity of electricity on another 
 sphere whose radius is R x , and on which the thickness 
 of the fluid is x' 9 we shall have 
 
 Hence it follows, that the electricities of two spheres 
 will be proportional to the squares of their radii mul- 
 tiplied by the depths of the electric fluid upon them, 
 which is expressed mathematically by 
 
 E #R 2 
 
 It follows also that the quantities of electricity on 
 spheres similarly electrified, will be in the proportion of 
 the squares of their radii or diameters, for in that 
 case #=#' 
 
 (94.) It has been here assumed, that, by imparting 
 an increased charge of electricity to a sphere, the thick. 
 ness of the shell of electric fluid by which it is enve- 
 loped is proportionally increased, which involves the 
 supposition that the density of the electric fluid remains 
 the same. If it be supposed, on the other hand, that 
 each addition made to the electricity leaves the depth 
 of the fluid the same, it must be assumed that its density 
 is increased in the same proportion as the quantity 
 of electricity imparted to it is augmented. On such a 
 supposition the symbol x in the above formula would 
 express the density of the electric fluid and not its 
 depth ; and, subject to this modification, the preceding 
 reasoning and the formula will still hold good. It is, 
 in a word, indifferent whether the increased energy or 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 315 
 
 intensity of the electricity be ascribed to the increased 
 depth or increased density of the fluid; for the depth of 
 the fluid, whether variable or not, always bearing an in- 
 finitely small proportion to the magnitude of the surface 
 on which the electricity is diffused, the attraction or 
 repulsion it exercises is the same as if all its particles, 
 superposed on one another, were condensed into a single 
 point. 
 
 (95.) The electrometer of Coulomb supplies the 
 means of determining by immediate experiment the 
 distribution of electricity on the surfaces of electrified 
 conductors. For this purpose, the disc of gilt paper 
 attached to the handle of gum lac already described is 
 used. This -disc is applied to any part of the surface 
 of an electrified conductor, where the intensity of the 
 electricity is to be ascertained. It is then introduced 
 into the electrometer of torsion, and placed opposite the 
 ball or a similar disc electrified with the same kind of 
 electricity attached to the extremity of the needle. The 
 intensity of the electricity with which it is charged is 
 estimated by the torsion necessary to bring the move- 
 able ball or disc of the electrometer within a given 
 distance of it. Let us suppose, for example, that after 
 applying the disc C (fig. 12.) (which we shall call the 
 proof plane) to any point of an electrified conductor, 
 it is introduced into the electrometer, and is observed to 
 repel the moveable disc similarly electrified. Let the 
 micrometer of torsion be now turned until the moveable 
 disc is brought within 20 of the proof plane, and let 
 the angle of torsion necessary to keep it there be 80. 
 Let the proof plane be now removed from the elec- 
 trometer and applied to another part of the conductor, 
 and let it be again introduced into the electrometer. 
 Let the micrometer of torsion be adjusted, so that the 
 moveable disc shall be maintained at a distance from 
 the fixed disc less than 20 by the angle of torsion, 
 which would be lost by the contact of the air in the 
 inter /al between the two experiments j and let the tor- 
 sion necessary to maintain it in this position be again 
 
31 6 ELECTRO-STATICS. PART I. 
 
 observed, and suppose its amount to be 120. It will 
 follow then that the intensities of the electricity at the 
 two points of the conductor to which the proof disc has 
 been applied, or, what is the same, the depths or densi- 
 ties of the electric fluid at those points will be in the 
 ratio of 80 to 120, or of 2 to 3. If therefore it can 
 be assumed, that the electricity of the proof plane after 
 touching the conductor is really proportional to the 
 electricity of the conductor at the point of contact, the 
 manner in which electricity is distributed over the 
 surface of different conductors may always be determined 
 by thus applying the proof plane to various points upon 
 them, and determining the force of its electricity in the 
 electrometer. 
 
 (96.) The first important conclusion to which such 
 an experimental inquiry has led is, that on a conductor 
 of any given form and magnitude the distribution of 
 electricity is always the same, whatever be the depth or 
 density of the electric fluid with which the conductor is 
 charged. This is easily rendered manifest. An insu- 
 lated conductor being strongly electrified, let the proof 
 plane be successively applied at several points, P, P', P /x , 
 P //x , &c. &c. and let it be successively tried in the 
 electrometer after each contact. Let the torsions repre- 
 senting the repulsive force it exerts at a given distance 
 after each contact be T, T x , T", T'", &c., the loss of 
 electricity by the contact of the air upon the moveablo 
 disc of the electrometer being duly allowed for. Let 
 the conductor be now discharged by touching it, or 
 otherwise putting it in communication with the earth, 
 and let it be again electrified with a greater or less force 
 than before. Let the proof plane be again brought into 
 contact with the same points P, P', P' x , P x ", &c. &c. as 
 before, and let it, as before, be tested in the electrometer. 
 Let the torsion corresponding to those points be now 
 t, t', t", t"' ', &c. These several angles of torsion will ex- 
 press, as before, the electric force which the proof plane 
 has acquired by contact with the conductor at the several 
 points. 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 317 
 
 Now, if the quantities T, T', T", T"", &c. be compared 
 severally with the quantities t, t', t", t'", &c., they will 
 be found to be exactly proportional, each to each, so 
 that the following mathematical condition will be ful- 
 filled: 
 
 Thus, if it were observed that in the second electrical 
 state produced upon the conductor the torsion t were 
 only one-third of the torsion T, it would follow that the 
 depth or density of the electricity at the point P was 
 three times greater in the first case than in the second ; 
 and it would accordingly be found that the torsions 
 T', T", T"', &c., expressing the depths of the electricity 
 at the points P, P', P", P'", &c., would be three times 
 the torsion t', t" > t'" , &c., expressing the depths of elec- 
 tricity at the same points in the second case ; and, in 
 general, that the depth of the electricity on every part 
 of the conductor in the first case would be three times 
 its depth on the same parts of it in the second. 
 
 (97.) It follows, therefore, that the total quantity of 
 electricity with which any given conductor is charged at 
 different times is always proportional to the quantities 
 of electricity found by the proof plane at these times on 
 any given point of it. 
 
 (98.) If an electrified conductor be tried at a given 
 point by the proof plane after equal successive intervals 
 of time, it will be found that its electricity is subject to 
 a gradual diminution ; and if this diminution per mi- 
 nute be calculated, it will be found to be precisely the 
 same as would be produced by the contact of the air, 
 provided the conductor is well insulated, which is there- 
 fore evidently the cause of the gradual diminution of its 
 electric force. This may be verified in the following 
 manner : Let a proof plane be applied to any point of 
 the conductor, and introduced into the electrometer ; 
 after remaining there any given time, for example five 
 
I 8 ELECTRO-STATICS. PART I. 
 
 minutes, it will be observed that the moveable disc will 
 gradually approach it, by reason of the loss of electrical 
 force by the contact of air. Let the moveable disc be 
 restored to its proper distance by diminishing the torsion 
 which retains it. Let another similar proof disc be now 
 applied to the same points of the conductor, and let it be 
 introduced into the electrometer in place of the former. 
 It will be found that the moveable disc will be held in 
 the same position by its repulsion, without any change 
 in the torsion. It follows, therefore, that the loss of 
 electricity sustained in any given time by any part of 
 the surface of the conductor is exactly the same as the 
 loss which would be sustained in the same time by the 
 proof disc electrified by contact with the conductor. In 
 other words, the conductor itself, and the discs elec- 
 trified at any point of it, undergo the same changes of 
 electrical force in the same intervals of time. 
 
 (99) If it be desired to obtain the exact ratio of the 
 electric densities at any other point of an electrified con- 
 ductor, independently of any computation of the loss of 
 electricity by the contact of the air, it may be done in 
 the following manner: Let the proposed points be P 
 and P', and let the proof plane be first applied to the 
 point P, and its repulsive force be determined by the 
 electrometer. Let it then be applied to the point P', 
 observing the interval between the two contacts by a 
 watch having a seconds hand, and let its repulsive force 
 be again observed. After the lapse of an equal interval 
 of time, let it be again applied to the point P, and let 
 the electric force be again observed in the electrometer. 
 If half the sum of the two angles of torsion obtained 
 after the two contacts with the point P be taken, it will 
 be the angle of torsion which would have been obtained 
 if the two points P and P' had been touched by the 
 proof plane at the same moment. The ratio of the tor- 
 sions, and therefore of the electric densities^ at the points 
 P and P', will thus be obtained independently of any 
 computation of the loss by atmospheric contact, or of 
 any other supposition, save that the loss of electricity in 
 
CHAP. V. DISTRIBUTION OP ELECTRICITY. 
 
 the interval between the first contact with the point P 
 and the contact with the point P' was equal to the loss 
 in the equal interval between the contact with the point 
 P' and the second contact with the point P. 
 
 (100.) The following experiment will further cor- 
 roborate the proposition, that the quantities of electricity 
 taken by the proof plane from the same point of a con- 
 ductor at different times are proportional to the whole 
 quantities of electricity with which the conductor is 
 charged at those times. Let two equal cylinders of 
 metal or of metallic surface, whose lengths are much 
 greater than their diameters, be placed on insulating 
 supports, and let one of the two be strongly electrified. 
 Let the proof plane be applied to it first at a point near 
 the middle of its length, and next at a point near its 
 extremity, it will be found by the electrometer, that 
 the electrical forces at these two points are very dif- 
 ferent. Let them, however, be exactly observed. Let 
 the second insulated cylinder, which was not electrified, 
 be now placed beside the other, so that their extremities 
 shall exactly coincide, and let them thus be brought 
 into lateral contact. From the perfect regularity and 
 symmetry of the two cylinders and from their conducting 
 power it follows, that the electricity with which the one 
 was previously charged will now be diffused uniformly 
 over the surfaces of both, and that the first will be only 
 charged with half its original quantity. Let them be 
 separated, and let the first be touched at the same points 
 as before by the proof plane, and it will be found, by 
 the indications of the electrometer, that the electrical 
 force of the proof plane will in each case be exactly 
 ahalf what it ws before. This result may be further 
 verified by touching the other cylinder at corresponding 
 points, when the electric forces manifested by the proof 
 plane will be the same. 
 
 (101.) To determine by means of the proof plane 
 and the electrometer the actual quantity of electricity 
 with which a conductor at any point is charged, it will 
 be necessary to determine the ratio of the electricity with 
 
S20 ELECTRO-STATICS. PART T. 
 
 which the proof plane becomes charged in touching a 
 conductor to the electricity with which the portion of 
 the surface of the conductor so touched was charged. 
 With a view to the solution of this question,, Coulomb made 
 the following experiment: he placed a globe eight inches 
 in diameter upon insulating supports, and charged it, as 
 well as the moveable disc of the electrometer, with posi- 
 tive electricity. To determine the electric force of this 
 globe^ he brought into contact with it another of an 
 inch in diameter insulated by a handle of gumlac. This 
 latter being introduced into the electrometer, it was 
 found that an angle of torsion 144 was necessary to 
 hold the moveable disc at a given distance from it. He 
 next brought an insulated circular disc of metal, sixteen 
 inches in diameter, in contact with it; after removing 
 which, he again brought the small globe into contact as 
 before with the great one, and again introduced it into 
 the electrometer, when he found that an angle of torsion 
 of 47 was sufficient to keep the moveable disc at the 
 same distance from it. 
 
 The insulated disc, sixteen inches in diameter, had a 
 surface on each of its faces equal to four times the area 
 of a circle eight inches in diameter, and therefore a total 
 surface equal to eight times the area of such a circle ; 
 but the surface of a globe eight inches in diameter is 
 proved in geometry to be equal to four times the area 
 of a circle of the same diameter. It consequently 
 follows, that the surface of each face of the great disc 
 is equal to the whole surface of the greater globe ; 
 and therefore, that the total surface of both faces is 
 double the surface of the globe. When this disc is 
 therefore brought into contact with the globe, the elec- 
 tricity which was previously diffused upon the globe is 
 diffused over the united surfaces of the globe and the 
 disc ; that is, over three times the surface to which it 
 was before confined. If it be diffused uniformly on 
 these surfaces, it will be therefore three times less dense 
 on the greater globe after the contact of the disc, than 
 it was before. In the second experiment made with 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 321 
 
 the inch ball, the repulsive force was found to corre- 
 spond to an angle of torsion of 47, which would be a 
 third part of 141, which is little different from the 
 actual angle of torsion 144 obtained by the first expe- 
 riment. Since the repulsive force of the smaller globe 
 in the two cases is proportional to the depth of electri- 
 city on the greater globe., it follows that this depth is 
 in the ratio of three to one ; and therefore, that by the 
 contact,, the disc has taken from the globe two-thirds of 
 its charge of electricity, and consequently that the depth 
 of electricity on the plane after contact is the same as 
 the depth of electricity remaining on the globe. 
 
 Had the disc been electrified and the globe not so, 
 the result would have been the same. In that case, the 
 globe would have received one- third of the charge of 
 electricity upon the disc. Coulomb verified this by a 
 great variety of experiments, and found in general, that 
 whatever the magnitude of the disc might be, the quan- 
 tity of electricity it required always bore to the quan- 
 tity remaining on the globe the ratio which the area of 
 the two sides of the disc bore to the area of the surface 
 of the globe. It follows, therefore, that when the disc be- 
 comes so small that its total surface bears a very minute 
 proportion to that of the globe, the quantity of electri- 
 city it takes from the globe bears no sensible proportion 
 to the quantity remaining on the globe ; the disc, there- 
 fore, carries away upon each of its faces as much elec- 
 tricity as covers the portion of the globe with which it 
 is brought into contact, and consequently the total quan- 
 tity of electricity taken upon the disc at each contact must 
 be double the quantity which covers an equal part of the 
 surface of the globe. The angle of torsion, therefore, 
 produced by the repulsive power of this electricity, will be 
 double that which would be produced by the electricity 
 diffused upon a portion of the globe equal in magnitude 
 to the area of the disc. 
 
 (102.) By the frequent repetition of such experi- 
 ments, the electricity taken from the globe by the proof 
 
22 ELECTRO-STATICS. FAULT 1. 
 
 plane would diminish in a slight degree the absolute 
 quantity of electricity ; and consequently, to be enabled 
 to compare together the results of a succession of ob- 
 servations so made,, it would, in strictness,, be necessary to 
 take into account this gradual diminution of the electric 
 force. This precaution is, however, rendered needless 
 if the magnitude of the proof disc bears so small a pro- 
 portion to the magnitude of the surface of the con- 
 ductor, that the whole quantity of electricity which it 
 takes away in any one series of experiments bears an 
 insignificant proportion to the quantity which remains 
 on the conductor ; but if this precaution be considered 
 insufficient, it is only necessary to bring back the 
 proof disc to the conductor in each experiment without 
 discharging it. By such means the only diminution 
 of electricity which will take place will be that due to 
 the contact of the air, which can be computed and 
 allowed for as already explained. 
 
 (103.) In the practical application in this method 
 of experimenting the results of the experiments are 
 sometimes exposed to a source of error against which 
 it is necessary to provide. The rods of gum lac used 
 to support the proof discs are imperfect non-conductors, 
 and are capable of absorbing, in a sensible degree, the 
 electricity of the disc. The errors arising from this 
 source are greater in proportion to the humidity of the 
 air ; but they are also dependent on the nature of the 
 gum lac itself, which possesses the insulating power in 
 different degrees, according to its purity. That which 
 has the darkest colour is generally the best, but the 
 colour alone can hardly be depended on as a test of 
 purity. The gum lac must be tested by experiment, 
 before it is employed in investigations requiring such 
 extreme delicacy. With this view, when the threads are 
 formed of the requisite thickness and length, it is ne- 
 cessary to bring the extremity intended to bear the 
 proof disc in contact with a strongly electrified con- 
 ductor, after which it should be presented to the move- 
 
CHAP. V. DISTRIBUTION OP ELECTRICITY. 3 C 23 
 
 able disc of an electrometer charged with electricity of 
 the same kind. If it produces a sensible repulsion, it 
 must be rejected, and such threads only retained as 
 produce no repulsion. For if threads be used which are 
 not almost perfect insulators, the succession of contacts 
 which are made by the disc with the electrified conductor 
 would at length impregnate a portion of the handle of 
 the gum lac by which they are supported with the elec- 
 tricity of the conductor, and when once the gum lac 
 should thus absorb any portion of electricity, it would 
 be extremely difficult to remove it, and a permanent 
 source of error would be produced, which would aug- 
 ment the electric force of the proof disc after each 
 contact. 
 
 This effect would be greater for feeble electricities, 
 and would entirely vitiate and render useless all expe- 
 riments made on parts of the conductor weakly electri- 
 fied. Considering this source of error, there is an ad- 
 vantage in the employment, in such experiments, of 
 conductors of some considerable magnitude, for, in that 
 case, a proportional magnitude may be given to the 
 proof plane, and the proportion will be lessened in which 
 its electricity will be affected by the electricity, if any, 
 which may be absorbed by the handle of the gum lac. 
 
 (104.) The following practical example of the ap- 
 plication of this species of experimental investigation 
 has been extracted by Biot, from the unpublished manu- 
 scripts of Coulomb. 
 
 He proposed to determine how the electricity was 
 distributed upon a thin insulated plate of metaL For 
 this purpose he insulated a plate of steel eleven inches 
 in length, one inch in width, and half a line in thick- 
 ness. To be able to touch it in its whole width, he 
 made the proof plane an inch long and three lines 
 wide. He first applied this plane at the centre C of 
 the plate A B (fig. 14.), and then at P and Q points, 
 one inch from its extremities, and he obtained the fol- 
 lowing results : 
 
 Y 2 
 
324 
 
 ELECTRO-STATICS. 
 
 
 Torsions 
 observed. 
 
 Mean tor. 
 sion at 
 centre. 
 
 Mean torsion 
 at 1 inch from 
 extremity. 
 
 Ratio of the 
 mean torsions. 
 
 At centre 
 
 370 
 
 
 
 
 At 1 inch from 
 
 
 
 
 
 extremity - 
 At centre 
 
 440 
 350 
 
 360 
 350 
 
 440 
 4] 7'5 
 
 1-22 
 1-20 
 
 At 1 inch from 
 
 
 
 
 
 extremity - 
 At centre . - 
 
 395 
 320 
 
 335 
 
 395 
 
 1-18 
 
 Mean 
 
 1-20 
 
 (105.) Thus, on equal spaces taken across the whole 
 width of the plate, at the centre, and at points one inch 
 from its extremities, the quantities of electricity are as 
 10 to 12, and, consequently, nearly equal, the excess 
 being in favour of the electricity towards the extremi- 
 ties. Coulomb repeated these experiments, but instead 
 of applying the proof plane at points an inch from the 
 extremities, he applied it close to the extremities, so as 
 to obtain the quantity of electricity covering a portion 
 of the plate extending three lines from the end. The 
 results of these experiments are given in the following 
 table : 
 
 
 Torsions 
 observed. 
 
 Mean tor- 
 sion at 
 centre. 
 
 Mean torsions 
 at extremity. 
 
 Ratio of mean 
 torsions. 
 
 At extremity - 
 
 400 
 
 
 
 
 At centre 
 
 195 
 
 195 
 
 395 
 
 2-02 
 
 At extremity - 
 
 390 
 
 190 
 
 390 
 
 2'05 
 
 At centre 
 
 185 
 
 185 
 
 370 
 
 2-00 
 
 At extremity 
 
 350 
 
 
 
 
 Mean 
 
 2-02 
 
 
 i 
 
 It will be observed that, in this case, the excess of 
 electricity at the ends, compared with the centre, is 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 
 
 325 
 
 much greater than in the former case. In fact, while 
 the quantity of electricity upon the plate is nearly uni- 
 form from the centre to within an inch of the borders, 
 it is augmented rapidly towards the edges, where it he- 
 comes more than double its amount at the centre. 
 
 (106.) Coulomb extended this inquiry still further, 
 by applying the proof plane, not on the surface of the 
 plate itself, but in the space corresponding to the pro- 
 longation of the plate beyond its extremities A and B, 
 in such a manner that the edge of the proof plane coin- 
 cided with the extreme edges of the ends of the plate, the 
 proof plane, however, lying beyond the limits of the sur- 
 face of the plate. The results of these experiments are 
 given in the following table : 
 
 
 Observed 
 torsions. 
 
 Mean tor- 
 sions at 
 centre. 
 
 Mean bodies 
 beyond the 
 "edge. 
 
 Ratio of mean 
 torsions. 
 
 Centre - 
 
 305 
 
 
 
 
 Beyond edge - 
 
 1175 
 
 295 
 
 1175 
 
 3'98 
 
 Centre - 
 
 285 
 
 285 
 
 1156 
 
 4*05 
 
 Beyond edge - 
 
 1137 
 
 
 
 
 Mean 
 
 4-01 
 
 It appears thus that the proof plane, beyond the limits 
 of the plate, acquired an electricity four times that which 
 it acquired at the centre, and double that which it ac- 
 quired at the extremities. 
 
 The experiment being repeated with a plate twenty- 
 two inches long, or double the former length, and of the 
 same dimensions in other respects, exactly the same re- 
 sults were obtained. 
 
 (107.) Coulomb concluded, first, that the proof plane 
 in these experiments only acquired the electricity of that 
 side of the plate to which it was applied ; and, secondly, 
 that no increase of length of the plate beyond a certain 
 limit of length, sufficient to enable the electricity to be- 
 come uniform on the middle portion of its surface, pro- 
 Y 3 
 
326 
 
 ELECTRO-STATICS. 
 
 duced any sensible effect on the ratio of the quantity of 
 electricity at the extremity, and at the middle. The 
 first being always double the second. 
 
 (108.) To render the result of these experiments 
 more apparent, let A B (fig. 14.) represent a plate 
 
 Fig. 14. 
 
 -IB 
 
 whose length surpasses the limit necessary to insure a 
 uniform depth of electricity at the centre C. Let per- 
 pendiculars, such as P and Q, be drawn to represent the 
 depths of the electricities at the different points of the 
 plate, and let the curve formed by the ends of such per- 
 pendiculars be A' P E Q B', the curve between the points 
 P and Q will hold its course parallel to the surface of 
 plate A B, all its points being equally distant from the 
 surface of the plate ; but from P to A', and from Q to 
 B', the curve rises by the increased length of the per- 
 pendiculars until A A' and B B' are double the magni- 
 tude of the perpendiculars between P and Q. Now, 
 since the ratio of these extreme particulars to the centre 
 perpendicular C E is the same in all plates whose length 
 is considerable compared with their width, and since the 
 intermediate ordinates between P and Q are the same in 
 all, it follows that the curves A' P and B' Q have the 
 same magnitude in all plates, whatever be their length ; 
 and that they consequently designate the varying 
 thickness or density of the electricity at the extremities 
 of all such plates. We are thus enabled to predict 
 perfectly the electric state of any part of such plates 
 when the electricity of the centre has once been ob- 
 served. 
 
 (109.) This rapid increase of electricity towards the 
 extremities of long and narrow plates of metal is not 
 peculiar to them. An analogous effect is observed in all 
 bodies of a prismatic or cylindrical form, whose lengths 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 327 
 
 are considerable in proportion to their thickness, and the 
 increase of electricity at their extremities is found to be 
 so much the more rapid as they are thinner. Coulomb 
 insulated a circular cylinder, two inches in diameter and 
 thirty inches in length, of which the ends were hemi- 
 spherical, and on comparing, as in the former case, the 
 quantities of electricity collected at the centre, and at 
 points near the extremities, he obtained the following 
 results. At two inches from the extremity the electricity 
 was to that at the centre as 1^ to 1; at one inch from 
 the extremity it was as 1 J- to 1 ; and at the extremity 
 it was as 2-f 1 ,- to 1. 
 
 (110.) The manuscripts of Coulomb supplied ana- 
 logous experiments to determine the distribution of 
 electricity upon thin circular plates, which have likewise 
 been published by Biot. He touched these plates at 
 different distances from their centre with the proof plane, 
 and ascertained by the electrometer, as in the former 
 experiments, the quantity of electricity at different places 
 upon them. In the following table, the result of expe- 
 riments made in this way upon a plate of copper ten 
 inches in diameter are given. 
 
 Distances from the edge. 
 
 Depths of electricity. 
 
 5 centre of the disc. 
 
 1 
 
 4 
 
 1-001 
 
 3 
 
 1-005 
 
 2 
 
 1-17 
 
 1 
 
 1-52 
 
 0-5 
 
 2-07 
 
 
 
 2-90 
 
 (111.) To express, in mathematical language, the re- 
 lation between the quantities of electricity, and the dis- 
 tances from the edge of the plate, let y express the in- 
 tensities of the electric forces, the unit being the force 
 at the centre, and let x express the distances from the 
 edge of the disc in the first column of the above table. 
 Y 4 
 
328 
 
 ELECTRO-STATICS. 
 
 M.Biot proposes the following formula obtained empi- 
 rically to express this relation : 
 
 y = 1 + A (m* w 2M ) 
 
 In which r is the radius of the disc, and A and m 
 constants, whose value are to be determined by observa- 
 tion. To obtain the value of A, let x O, and for y 
 substitute 2 '90, which is the value of y at the edge of 
 the disc, and we have 
 
 2-90= 1+ A (1-w 2 *-). 
 
 But since r = 5, this becomes 
 
 2-90 = 1 + A(l-ra 10 ). 
 
 It will appear presently that m is a fraction, not ex- 
 ceeding three-tenths, and consequently that its tenth 
 power may be neglected ; hence we have by the above 
 equation, 
 
 A = 1-9. 
 
 To determine the value of m let %=. 1, and the corre- 
 sponding value of y being 1'52, we shall find m = 0'27 
 But, on comparing the other observations, it will be 
 found that 0*3 is a value of m more in accordance 
 with them. The value m being thus determined, the 
 values of x and y may be found by calculation, and com- 
 pared with those obtained with experiment. In the 
 following table, the observed and calculated values of y 
 are given, and their near accordance sufficiently proves, 
 that the empirical formula of M. Biot represents with 
 great accuracy the phenomena. 
 
 Distances 
 from the 
 centre. 
 
 Depths of electricity. 
 
 4 
 Difference. 
 
 By formula. 
 
 By observation. 
 
 5 
 
 1-000 
 
 1-000 
 
 o-ooo 
 
 4 
 
 1-014 
 
 1-001 
 
 + 0-01 3 
 
 3 
 
 1-051 
 
 1-005 
 
 + 0-046 
 
 2 
 
 1-170 
 
 1-170 
 
 o-ooo 
 
 1 
 
 1-570 
 
 1-520 
 
 + 0-050 
 
 0-5 
 
 2-041 
 
 2-070 
 
 -0-029 
 
 
 
 2-900 
 
 2-900 
 
 o-ooo 
 
 
 
 
 
CHAP. V. DISTRIBUTION OF ELECTRICITY. 329 
 
 (112.) It appears in general, from the experiments 
 of Coulomb, that the depth of the electric fluid on a 
 conductor always increases in a rapid proportion in ap- 
 proaching the edges, and that this increase commences 
 at a certain small distance from those edges ; thus, when 
 a circular disc or rectangular plate has any considera- 
 ble magnitude, the density of the electricity is uniform 
 throughout those parts of its area not contiguous with 
 its borders, and whatever be the figure of the conductor, 
 whether round or square, if only it be terminated by sharp 
 angular edges, the density of the electricity will be found 
 to increase at all parts contiguous to those edges. 
 
 (113.) If a conductor be terminated not by sharp 
 angular edges, but rounded sides or ends, then the dis- 
 tribution of electricity will again become more uniform. 
 Thus, if a cylindrical conductor of considerable diame- 
 ter have hemispherical ends, the distribution of electri- 
 city on it will be uniform : but if its ends be flat, with 
 sharp circular edges, then an accumulation of electricity 
 will take place contiguous to them, as already described. 
 If the sides and ends of a flat plate of considerable 
 thickness be rounded or receive a semicylindrical form, 
 then the accumulation of electricity at the borders will 
 cease. 
 
 (114.) The increase of electrical density at the an- 
 gular edge of a conductor produces still more aug- 
 mented effects at its corners, where the increase of 
 density due to two edges are in fact combined, and 
 this effect is still further increased if any part of a con- 
 ductor have the form of a point. 
 
 (115.) It has been shown that the pressure of the 
 surrounding air is the principal, if not the only, force 
 which retains the electric fluid upon a conductor. Now 
 it is evident, that if at the edges, corners, or angular 
 points of a conductor, the electric depth be so much in- 
 creased that the elastic force of the electric fluid shall 
 exceed the restraining pressure of the atmosphere, the 
 electricity must escape. Let P (fig. ,25.) be a metallic point 
 attached to a conductor C, and let the perpendicular n 
 
330 ELECTROSTATICS. PART J. 
 
 express the thickness or density of the electric fluid at 
 that place, this thickness will increase in approaching 
 the point P, so as to be represented hy perpendiculars 
 drawn from the respective points of the curve n n' n" 
 to A P, so that its density at P will he expressed by the 
 perpendicular n" P. Experience shows, that, in ordi- 
 nary states of the atmosphere, a very moderate charge of 
 electricity given to the conductor C will produce such 
 a density of the electric fluid at the point P, as to over- 
 come the pressure of the atmosphere, and to cause the 
 spontaneous discharge of the electricity. The following 
 experiments will serve to illustrate this escape of electri- 
 city from points. 
 
 (116.) Let a metallic point, such as AP (^.15.), be 
 
 r . , - attached to a con- 
 
 Jbig. 15. 
 
 ductor, and let a me- 
 tallic ball of two or 
 three inches in dia- 
 meter, having a hole 
 in it corresponding 
 to the point P, be 
 struck upon the point. 
 If the conductor be 
 now electrified, the 
 electricity will be diffused over it, and over the ball 
 which has been struck upon the point P. The electric 
 state of the conductor may be shown by a quadrant elec- 
 trometer being attached to it. Let the ball now be drawn 
 off the point P by a silk thread attached to it for the pur- 
 pose, and let it be held suspended by that thread. The 
 electricity of the conductor C will now escape by the 
 point P, as will be indicated by the electrometer, but the 
 ball suspended by the silk thread will be electrified as 
 before. 
 
 (117.) Let two wires AB and C D (fg. 16.), placed 
 at right angles, be supported by a cap E upon a fine 
 point at the top of an insulating stand, and let them 
 communicate by chain F with a conductor kept con- 
 
DISTRIBUTION OF ELECTRICITY. 
 
 331 
 
 Fig. 1 6. stantly electrified by a machine. 
 
 Let each of the four arms of the 
 wires be terminated by a point 
 in a horizontal direction at right 
 angles to the wire, each point 
 being turned in the same direc- 
 t tion as represented in the figure. 
 When the electricity comes from 
 the conductor to the wires, it 
 will escape from the wires at 
 these four points respectively, 
 and the force with which it leaves 
 
 them will be attended with a proportionate recoil, which 
 will cause the wire to spin rapidly on the centre E. 
 
 (118.) An apparatus supplying another illustration 
 of this principle is represented in fig. 17- A square 
 
 Fig. 17. 
 
 wooden stand T 
 has four rods of 
 glass inserted at 
 its corners, the rods 
 at one end being 
 less in height than 
 those at the other. 
 The tops of these 
 rods having metal 
 wires A B and C D stretched between them, across 
 these wires another wire E F is placed, having at- 
 tached to it at right angles another wire G H having 
 two points turned in opposite directions at its extre- 
 mities, so that when G H is horizontal, these two points 
 shall be vertical, one being presented upwards, and the 
 other downwards. A chain from A communicates with 
 a conductor, kept constantly electrified by a machine. 
 
 The electricity coming from the conductor by the 
 chain passes along the system of wires, and escapes at 
 the points G and H. The consequent recoil causes the 
 wire G H to revolve round E F as an axis, and thereby 
 causes E F to roll up the inclined plane. 
 
ELECTRO-STATICS. 
 
 (119.) An apparatus called the electrical orrery is 
 represented in fig. 18. A metallic ball A rests upon 
 
 Fig. 18. 
 
 an insulating stand by 
 means of a cap within it, 
 placed upon a fine me- 
 tallic point, forming the 
 top of the stand. From 
 the ball A, an arm D A 
 proceeds, the extremity 
 of which is turned up 
 at E, and formed into a 
 fine point. A small ball, 
 B, rests by means of a 
 cap on this point, and 
 attached to it are two arms, extended in opposite direc- 
 tions; one terminated with a small ball C, and the 
 other by a point, P, presented in the horizontal di- 
 rection at right angles to the arm. Another point P', 
 attached at right angles to the arm DA, is likewise 
 presented in the horizontal direction. By this arrange^- 
 ment the ball A, together with the arm D A, is capable 
 of revolving round the insulating stand, by which 
 motion the ball B will be carried in a circle round the 
 ball A. The ball B is also capable, at the same time, 
 of revolving on the point which supports it, by 
 which motion the ball C will revolve round the ball B 
 in a circle. If electricity be supplied by the chain to 
 the apparatus, the balls A and B, and the metallic rods, 
 will be electrified, and the electricity will escape at the 
 points P and P x . The recoil produced by this escape 
 will cause the rod D A to revolve round the insulating 
 pillar ; and, at the same time, the rod P C, together 
 with the ball B, to revolve on the extremity of the arm 
 D A. Thus, while the ball B revolves in a circular 
 orbit round the ball A, the ball C revolves in a smaller 
 circle round the ball B. The motion resembling that 
 of the moon and earth with respect to the sun. 
 
INDUCTION. 333 
 
 CHAP. VI. 
 
 INDUCTION. 
 
 (120.) IN all the phenomena which have heen hi- 
 therto considered, the electricity developed on bodies 
 has heen educed either by the contact of two bodies, 
 accompanied by friction, or by the mere contact of an 
 electrified body with one not electrified. Among the 
 earliest manifestations, however, of the phenomena of 
 electricity, effects were rendered apparent, which proved 
 that the agency of electricity did not require absolute 
 contact ; but, on the contrary, that it operated at de- 
 finite distances, producing distinct mechanical effects. 
 Thus an electrified body, presented to bits of paper, 
 feathers, or other light substances, caused these latter 
 to move towards it If it be presented to a small ball 
 not electrified suspended by a thread, it will draw the 
 ball aside from the vertical position in which it would 
 naturally rest. 
 
 Since a body, not electrified, presented to these 
 various substances, will manifest no attraction for 
 them, it is evident, that the attractive and repulsive 
 forces exhibited in the effects above referred to are due, 
 not to the matter of the electrified body itself, but to 
 ihe electricity diffused upon it. If these attractions or 
 repulsions were only manifested between electrified 
 bodies, the natural inference would then be, that these 
 forces are exerted between the electric fluid on one 
 body, and the electric fluid on the other ; and that the 
 apparent attractions and repulsions, exhibited in the 
 motion of the bodies, are due to the electric fluid with 
 which these bodies are enveloped ; but since an elec- 
 trified body exhibits an attraction on bodies not elec- 
 trified, it would seem that the electric fluid, diffused 
 
334} ELECTRO-STATICS. PART I. 
 
 over an electrified body, exerts an attraction on the 
 matter of bodies not electrified. Such an inference, on 
 the other hand, is discountenanced by the ascertained 
 fact, that the electricity diffused on an electrified body 
 is retained there, not by any proper attraction existing 
 between the electric fluid and the matter of the body, 
 but merely by the pressure of the surrounding atmo- 
 sphere. If the electric fluid have no attraction for the 
 matter of the body which it envelopes, it would be un- 
 reasonable to suppose that it would have an attraction 
 for the more distant matter of another body. The 
 electric fluid diffused upon the surface of an electrified 
 pith ball, suspended by a silken thread, is proved to 
 have no attraction for the matter composing that pith 
 ball. How then, it may be asked, can it be assumed 
 to have an attraction for another precisely similar pith 
 ball, similarly suspended in its neighbourhood? Yet 
 the effects of such an attraction are produced, and ought 
 to be accounted for. 
 
 (121.) To reduce these questions to a rigorous ana- 
 lysis, it will be necessary, first, to ascertain, by imme- 
 diate experiment, whether the electric fluid diffused on 
 one electrified body can produce any effect on the elec- 
 tric fluid diffused upon another body in its neighbour- 
 hood, independently of the bodies themselves. As the 
 bodies themselves may be fixed while the electric fluid 
 upon them is free to move (as will be the case if the 
 bodies selected be conductors), it will be easy to bring 
 this question to the immediate test of experiment. Let 
 A A' (fig. 1 9) be a cylindrical conductor, with hemi- 
 
 Fiff.l9. 
 
 
CHAP. VI. INDUCTION. 335 
 
 spherical ends, supported by a pillar of glass, or other 
 insulating substance ; let S be a sphere formed of a con- 
 ducting substance, and likewise supported on an in- 
 sulating pillar; let the centre of the sphere S be so 
 placed, that it shall be in the direction of the geome- 
 trical axis of the conductor A A'. Let the conductor 
 and the sphere be both strongly electrified with the 
 same kind of electricity, for example, with positive 
 electricity. If the conductor be examined by means of 
 the proof plane and the electrometer, it will be found 
 that the electricity is not uniformly diffused upon it, 
 but that its depth is least at the extremity A, nearest 
 the sphere, and that it increases gradually towards the 
 electrical A', most remote from the sphere. 
 
 If the sphere S be now gradually removed from the 
 conductor, its centre however being still kept in the 
 direction of the axis of the conductor, the distribution 
 of the electricity upon the conductor will undergo a 
 gradual change, its density towards the extremity A 
 being progressively increased while its density towards 
 the extremity A' is progressively diminished. If, on 
 the other hand, the sphere S be moved towards the con- 
 ductor, the density of the electricity towards the ex- 
 tremity A will be diminished, while the density towards 
 the extremity A' will be increased. 
 
 If the sphere S be removed altogether from the 
 neighbourhood of the conductor, or if the sphere be 
 deprived of its electricity, the electricity of the con- 
 ductor A A' will instantly resume that uniform distri- 
 bution which, under ordinary circumstances, it takes, 
 and every part of its surface examined by the proof 
 plane will indicate electricity of the same density. If 
 the sphere S be again electrified, and brought near to 
 A A', the uniform distribution of the electricity on A A' 
 will again be destroyed, and the electricity will, as before, 
 be accumulated towards the extremity A', most remote 
 from the sphere S. 
 
 (122.) The result of this experiment indicates a re- 
 pulsive action, exercised by the electric fluid of the 
 
336 ELECTRO-STATICS. PART I. 
 
 pphere S, upon the electric fluid diffused upon the con- 
 ductor A A'. If the sphere S, instead of being charged 
 with positive electricity, were equally charged with ne- 
 gative electricity, and, instead of being presented to the 
 extremity A_, were presented to the extremity A', pre- 
 cisely the same effects would ensue. The electricity on 
 A A' would be accumulated towards A" , and its densi- 
 ties on different points of the conductor would increase 
 from A to A'. The result of such experiment would in- 
 dicate an attractive force exerted by the negative elec- 
 tricity of the sphere upon the positive electricity of the 
 conductor. 
 
 (123.) We have supposed that the conductor A A' 
 was in this case charged with positive electricity. The 
 same results however would be obtained,, mutatis mu- 
 tandis, if it were charged with negative electricity. In 
 that case, the negative electricity would be accumulated 
 towards the extremity most remote from the sphere 
 charged with negative electricity, or towards the ex- 
 tremity nearest the sphere when charged with positive 
 electricity. 
 
 (124.) All these experiments lead to the conclusion, 
 that the attractive and repulsive forces which are mani- 
 fested between electrified bodies belong to the electric 
 fluid diffused upon bodies, and that similar electric fluids 
 repel each other, while dissimilar electric fluids attract 
 each other. This principle will be further illustrated 
 by phenomena, which we shall hereafter explain. 
 
 (125.) If the attractions and repulsions which have 
 been referred to be ascribed to the electric fluids with 
 which bodies be invested, and not to the bodies them- 
 selves, the motions imparted to bodies not electrified by 
 the near approach of electrified bodies still demand ex- 
 planation ; for if it be true that the electric fluids only 
 attract and repel themselves, and that the bodies neither 
 exercise nor suffer attraction or repulsion, how, it may 
 be asked, can the electric fluid with which any body is 
 invested, produce the effects of attraction on a body 
 which is free from all electricity ? 
 
CHAP. VI. INDUCTION. 33J 
 
 (126.) To arrive at an explanation of this difficulty, 
 it will only be necessary to examine carefully what will 
 happen to an insulated conductor, not electrified, when 
 an electrified body is brought near it in such a position, 
 that, were it free to move, and sufficiently light, it 
 would be attracted in the same manner as a pith ball, 
 not electrified, is attracted by an electrified body. For 
 this purpose, let S and S' (fig. 20.) be metallic spheres 
 
 supported on pillars of glass, and let A A' be a con- 
 ductor also insulated ; let these bodies be in the first 
 instance free of all electricity, which may be insured by 
 putting each of them for a moment in communication 
 with the earth by touching them ; in addition to which, 
 each of them may be tested by the proof plane, and the 
 electrometer in the usual manner. Let the spheres S and 
 S' be now strongly and equally charged, the one S with 
 positive, and the other & with negative electricity, and 
 let the conductor A A' be placed between them, as re- 
 presented in the figure, so that the centres of the spheres 
 shall be placed in the prolongation of the geometrical 
 axis X X' of the conductor, and at equal distances from 
 its middle point O. If the conductor be now examined 
 by the proof plane and the electrometer, it will be found 
 to be no longer in its natural state, or free from electri- 
 city, as it was before the spheres S and & were electri- 
 fied. On applying the proof plane to different points 
 
338 ELECTRO-STATICS. PART 3. 
 
 throughout its length, it will be found, that the only 
 part which is free from electricity is its centre O ; that 
 the half of the conductor, extending from O to A, is 
 electrified, and that its electricity is negative; and that 
 the other half of the conductor, extending from O to Af, 
 is also electrified, and that its electricity is positive. The 
 intensity of the opposite electricities at the extremities 
 A and A' will be equal, and the intensities will gradually 
 diminish from the extremities to the centre O, where 
 all electricity disappears. At any points such as P and 
 P', equally distant from the centre O, the depths of the 
 electric fluids will be equal. In fact, the electric state 
 of each half will be represented by the ordinates PM, 
 P / M / , of two branches of a curve, which are precisely 
 similar and equal. 
 
 (127.) If the electrified spheres S and & be gradually 
 removed from the conductor, their centres being main- 
 tained in the line X X x , at equal distances from the 
 middle point O, it will be found, that the electric state 
 of the conductor will vary with the varying distance of 
 the centres of the spheres from its middle point. The 
 middle point O will still be free from all electricity, 
 while the conductor on each side of it will still be 
 electrified, with contrary electricities, and the electric 
 depths at equal distances from O will still be equal; 
 but the actual electrical depth at any given distance 
 frcm O will be diminished by the increase of distance 
 of the spheres from O. In fact, as the spheres recede 
 from O, in opposite directions, being kept however at 
 equal distances from it, the curve, whose ordinates re- 
 present the densities, will become less and less concave, 
 taking successively the forms in figures 21. and 22., until 
 at length, when the electrified spheres are removed in- 
 definitely, the conductor recovers its natural state, and 
 is free of all electricity. 
 
 On the other hand, if the sphere S and S' be moved 
 in the contrary direction, and made to approach the 
 conductor, the accumulation of electricity towards the 
 extremities of the conductor will be increased, and the 
 
CHAP. VI. INDUCTION. 339 
 
 curve representing the electrical densities will take the 
 form represented in figure 23. 
 
 Fig. 21 
 
 (128.) It is impossible not to perceive how strongly 
 these phenomena suggest the existence of the two electric 
 fluids in equal quantities, and uniformly distributed over 
 a body in its natural state. In the experiments here 
 described, no electricity has been imparted to the con- 
 ductor A A' by either of the spheres S and S' ; for if 
 these spheres be examined after the experiment, by 
 means of the proof plane and the electrometer, they 
 will be found to have lost no electricity, except what 
 was dissipated by the air. Neither has the permanent 
 electrical state of the conductor been changed, for in 
 removing the spheres from it, it will be found to be in 
 its natural state, and free from all electricity. On the 
 other hand, the result of the experiment is exactly what 
 would have been anticipated, if we could have conceived 
 the conductor A A' to be at the same time charged with 
 equal quantities of positive and negative electricity ; for, 
 when it was charged with positive electricity, the repul- 
 sion of the electricity of the sphere S caused the elec- 
 tricity to accumulate towards the extremity A', and 
 the attraction of the negative electricity of the sphere 
 S' produced a like effect. When the conductor was 
 charged with negative electricity, the attraction of the 
 positive electricity of the sphere S caused the negative 
 electricity of the conductor to accumulate towards the 
 
340 ELECTRO-STATICS. PAKT I. 
 
 extremity A, while the repulsion of the negative electri- 
 city of the sphere S' produced the same effect. It is 
 evident, therefore, that if equal quantities of both kinds 
 of electricity be at the same time diffused upon the 
 conductor, the presence of the two electrified spheres 
 would cause the positive electricity to accumulate to- 
 wards the end nearest the negative sphere, and most 
 distant from the positive sphere, and the negative elec- 
 tricity to accumulate towards the end nearest the positive 
 sphere, and most distant from the negative sphere. If 
 the action of these electrified spheres increase with the 
 diminution of distance, this accumulation of the oppo- 
 site electricities towards the extremities will be aug- 
 mented by bringing the spheres closer to the conductors, 
 and will be diminished by removing them farther from 
 it. These are precisely the effects which take place, and 
 the conductor A h.' in its natural state, therefore, com- 
 ports itself under the various circumstances above men- 
 tioned exactly as a conductor would do charged with equal 
 quantities of the contrary electricities. 
 
 (129.) In these experiments we have supposed that 
 the two spheres S and S' are maintained throughout 
 all their changes of position at equal distances from 
 the conductor A A', and on the supposition that the 
 two spheres under like circumstances produce like effects 
 on the positive and negative electrical principles dif- 
 fused upon the conductor, it might be expected that 
 the neutral point O would be placed, as in fact it 
 has been found to be, exactly in the middle of the con- 
 ductor ; but if the two spheres equally charged with 
 opposite electricities be not symmetrically placed with 
 respect to opposite ends of the conductor, this simi- 
 lar distribution of opposite electricities on either side 
 of the centre of the conductor will not be expected. 
 If one of the spheres, S' for example, be gradually 
 removed from the conductor, the distribution of the 
 contrary electricities will no longer be symmetrical 
 with respect to the centre of the cylinder, and the neu- 
 tral point O will be removed from the centre of the 
 
INDUCTION. 
 
 341 
 
 cylinder to a position nearer the extremity A ; and that 
 portion of the cylinder on which negative electricity is 
 educed, will be less than half its length. 
 
 (130.) To carry this supposition to its extreme 
 length, let it be supposed that tte sphere $' is removed 
 altogether from the conductor, which is therefore left 
 under the sole influence of the sphere S, in the present 
 case supposed to be positively electrified. If the state 
 of the conductor A A' be now examined by the proof 
 plane and the electrometer, the neutral point O will be 
 found, as represented in figure 24., to be between the 
 
 Fig. 24. 
 
 centre of the cylinder and the extremity A ; the cylinder 
 from O to A will be negatively electrified, the electric 
 density increasing in like manner. The electric densities 
 being as before represented by the ordinates of a curve, 
 the state of the cylinder will be shown by the curve in 
 the figure. 
 
 If the sphere S, acting thus alone upon the conductor 
 A A', be gradually moved towards X, so that its dis- 
 tance from the conductor is progressively increased, the 
 effects of its attraction and repulsion are gradually 
 diminished, and accordingly it is found by experiment 
 that the neutral point O, which by this inequality was 
 transferred from the centre towards A, is again removed 
 towards the centre, and approaches it more nearly as 
 the distance of S from the conductor is augmented. 
 z 3 
 
342 ELECTRO-STATICS. PART I. 
 
 We have here supposed that the sphere S is charged 
 with positive electricity. Corresponding results would 
 be obtained if it had been electrified negatively. In that 
 case,, the electricity from A to O would have been posi * 
 tive, and the electricity from A' to O would have been 
 negative : in all other respects the effects would have 
 been the same. 
 
 (151.) All these results tend to establish the position 
 that a conductor, such as A A x in its natural state, 
 and apparently divested of all electricity, holds never- 
 theless upon it the two opposite electric principles 
 which have been denominated positive and negative ; 
 and that these two principles are decomposed, and sepa- 
 rated, and driven to opposite ends of the conductor by 
 the influence of either or both of the electrified spheres 
 S and S'. It has been already shown, that neither of these 
 spheres have parted with any electricity, and that none 
 has been received by the conductor A A' ; and that, con- 
 sequently, whatever electricity may be on A A' when 
 the spheres are brought near to it, must have been also 
 on it in their absence. That the electricity thus educed 
 on A A' by the presence of the spheres is a principle in 
 all respects identical with that produced by excitation 
 or communication, admits of easy demonstration. When 
 the opposite extremities of the conductor A A' are en- 
 veloped with contrary electricities by the proximity of 
 the sphere S, let an insulated conductor, such as a 
 sphere of metal supported on a glass pillar, be brought 
 into contact with A', it will immediately become charged 
 with positive electricity, and when removed will produce 
 the same effect as the electrified positive conductor of an 
 electrical machine. If, after this has been done, the sphere 
 S be removed from the neighbourhood of the conductor 
 A A', it will be found that the conductor A A' is charged 
 with negative electricity, and will produce the same 
 effects as the negative conductor of an electrical ma- 
 chine. 
 
 If the sphere, which by contact with the extremity 
 A' became positively electrified^ be now again brought 
 
CHAP. VI. INDUCTION. 343 
 
 in contact with the conductor A A', the positive elec- 
 tricity which had been taken from the conductor will 
 be restored to it, and it will return to its natural state. 
 
 (132.) Since an electrified insulated conductor, brought 
 near to another insulated conductor in its natural state, 
 will cause the latter to become electrical, having con- 
 trary electricities on opposite sides of it, the electricities 
 thus developed in the conductor will be capable of act- 
 ing in the same manner on another conductor contiguous 
 to it, and the latter on a third, and so on. In the 
 illustrations of these principles which have been here 
 given, we have, for the convenience of explanation, as- 
 signed certain forms to the bodies under consideration ; 
 but the same effects will be produced, and the same rea- 
 soning may be applied, whatever may be the form of the 
 bodies. Let S, as before, be an insulated conductor 
 electrified positively, an4 let A, A/ A", A"', &c. be a 
 series of insulated conductors disposed contiguous to 
 each other, but not in contact, as 
 
 A A' A" A x// A"", 
 
 these being in their natural state. Now let S, electrified 
 positively, be placed to tbe left of A, and let the posi- 
 tive electrical state be expressed by -f , and the negative 
 by ; the electrical state of the whole series produced 
 by the position assigned to S will be as follows : 
 
 + S+, -A+, -A'+, -A"+, -A'"* 
 
 The positive electricity which envelopes S has educed 
 negative electricity on the side of A next to it, and 
 positive electricity on the opposite side. 
 
 The positive electricity thus educed on the opposite 
 side of A, educes negative electricity on the adjacent 
 side of A 7 , and positive electricity on the opposite side. 
 Again, the positive electricity thus educed on the oppo- 
 site side of A' has educed negative electricity on the ad- 
 jacent side, and positive electricity on the opposite side 
 of A", and so on throughout the series ; negative elec- 
 tricity being evolved on the sides of the conductors 
 z 4 
 
344 
 
 ELECTRO-STATICS. 
 
 nearest to S, and positive electricity on the opposite 
 sides. 
 
 (133.) As an electrified body, such as the sphere S, 
 produces so important an action on a body in its na- 
 tural state, it may be expected in conformity with the 
 general principles and analogies of physics, that a re- 
 ciprocal effect would be produced, and that the elec- 
 trified body S itself would suffer some corresponding 
 effect from the influence of the electricity which it has 
 evolved on the conductor contiguous to it. Experiment 
 accordingly confirms the anticipation suggested by these 
 analogies ; but in order to render the explanation of 
 this reaction more simple and more pointedly illus- 
 trative of the theoretical principles which have been 
 raised upon these phenomena, we shall here consider 
 the case in which the electrified body has a form si- 
 milar to that of the conductor gn which it acts. 
 
 Let us then suppose that B ft' (fig. 25.) is an in- 
 
 Fig. 25. 
 
 sulated cylindrical conduc- 
 tor, with hemispherical ends 
 strongly charged with posi- 
 tive electricity, and let a 
 pair of pith balls be sus- 
 pended from each extremity 
 of it, connected by linen 
 threads attached to rings 
 of wire surrounding the con- 
 ductor. Since the threads 
 and wire are conductors of 
 electricity, the pith balls will 
 share the electricity of the conductor, and will diverge 
 from each other to a distance proportionate to the elec- 
 tric density of the conductor at the points at which 
 they are attached. The balls will diverge equally, in- 
 dicating the uniform diffusion of the electricity of the 
 conductor. Let a similar and equal conductor, A A', in 
 its natural state, be so placed that its geometrical axis 
 shall be in the same straight line with that of the elec- 
 trified conductor B B', and let it be gradually moved 
 
CHAP. VI. INDUCTION. 345 
 
 towards B B'. As it approaches B B', the opposite 
 electricities will be educed upon its surface, and dis- 
 tributed in the same manner as already described with 
 respect to the conductor A A' (fig. 24.), when brought 
 near the electrified sphere S. The negative electricity 
 is accumulated at the end A., and the positive electricity 
 at the end A', the neutral point O being nearer to A 
 than to A'. If the state of the pith balls upon the 
 conductor B B' be examined, it will be found that, as 
 soon as any sensible evolution of electricity has taken 
 place upon A A', the divergence of the balls near the 
 extremity B' will be increased, while the divergence of 
 those near the extremity B is diminished, and according 
 as the distance of the conductor A A' from BB' is lessened, 
 the divergence of the balls near B' is augmented, and 
 the divergence of those near B is diminished. These 
 effects indicate a gradually increasing accumulation of 
 the positive electricity of BB' towards the extremity B'. 
 
 This is nothing more than might have been expected. 
 The electricity which has been educed on A A', acts 
 upon the electricity on B B' in the same manner as an 
 electrified sphere does. 
 
 The negative electricity accumulated between O and 
 A has a tendency, by its attraction, to produce an ac- 
 cumulation of the positive electricity of the conductor 
 B B' towards the extremity B. The positive electricity 
 accumulated from O to A' has a contrary tendency, by 
 reason of its repulsion; but its effect is diminished by its 
 greater distance from B', and the two electricities of the 
 conductor A A' produce an attraction on the positive elec- 
 tricity of the conductor BB' proportionate to the excess of 
 the attraction exercised by the negative electricity accu- 
 mulated from O to A above the repulsion of the positive 
 accumulated from O to A'. This excess of attractive 
 force, acting on the positive electricity with which BB' 
 is charged, produces the accumulation towards B', which 
 causes the increased divergence of the one pair of balls 
 and the diminished divergence of the other. 
 
 (134.) But another important phenomenon con- 
 
346 
 
 ELECTRO-STATICS. 
 
 tributes to increase this effect. The excess of the at- 
 traction of the negative electricity on the conductor A A' 
 affects not only the positive electricity with which it has 
 been charged, but, in addition to this, is attended with 
 the same effect as would be produced upon it by that 
 attraction if it were in its natural state. These effects 
 would be, as has been already explained, the evolution 
 of positive electricity towards the extremity B', and the 
 negative electricity towards the extremity B. The 
 former of these effects tends to increase the quantity of 
 positive electricity drawn to the end B x , while the latter 
 diminishes the depth of the portion which remains at 
 the end B. In the present case, this part of the effect 
 of the electricity educed on the conductor A A' cannot be 
 rendered apparent, since the negative fluid evolved to- 
 wards the extremity B is neutralized by a portion of the 
 positive fluid with which the conductor has been charged. 
 (135.) To render it apparent, let the conductor 
 BB X (fig. 26.) be feebly charged with positive electricity. 
 
 Fig. 26. 
 
 In the absence of the conductor A A', the depth of elec- 
 tricity on every part of B B' will be the same, and the 
 two pairs of pith balls will be equally divergent. Let 
 the conductor A A' in its natural state be now made to 
 approach B B' slowly from a distance, being moved in 
 the direction X'X. As it approaches the divergence of 
 the balls near the extremity, B' will gradually increase, 
 while the divergence of the other balls will gradually 
 diminish. When it has come within a certain distance 
 of B B', the balls near B will have collapsed, while the 
 divergence of the balls near B will be still more aug- 
 mented. In this position of the conductors, therefore, 
 the extremity B is reduced to its natural state, while 
 the conductor extending towards B' is electrified po- 
 
CHAP. VI. INDUCTION. 34? 
 
 sitively, the electric density continually increasing, as 
 represented by the curve (fig. 26-), from B to B'. This 
 effect is produced not merely by the attraction of the 
 electricity developed on the conductor A A' acting on 
 the positive electricity with which the conductor B B' 
 is charged ; for if that were the case, the further ap- 
 proach of the conductor A A x towards the conductor 
 B B' would only produce a continued motion of the 
 positive electricity of B B' towards the extremity B', 
 and a portion of the conductor B B' of more or less 
 extent at the extremity B, would be reduced to its na- 
 tural state, so that pith balls suspended any where upon 
 it would collapse by their gravity. Such, however, will 
 not be the case ; for if the conductor A A' be moved 
 still nearer to B B', the balls near the extremity B, 
 which had before collapsed, will again diverge, and if 
 another pair of pith balls be moveable on the con- 
 ductor, a point O (fig. 2?.) will be found near the ex- 
 tremity B, where the balls will 
 collapse, and where the con- 
 ductor therefore is in its na- 
 tural state. The divergence 
 of the balls near B, proves that 
 the conductor between O and 
 A is electrified, and if it be 
 examined by means of the 
 proof plane and the electrometer, it will be found to be 
 charged with negative electricity of gradually increasing 
 density from O to A, while the conductor from O to A' 
 is charged with positive electricity of gradually increas- 
 ing density. 
 
 (136.) It is evident, therefore, that the effect of the 
 electricities developed on A A' is twofold ; first, it at- 
 tracts the positive electricity with which the conductor 
 B B' is charged from B towards B': secondly, it deve- 
 lopes on B B' the opposite electrical principles, in the 
 same manner as they are developed on a conductor in 
 its natural state by the proximity of a conductor posi- 
 tively electrified. The combination of these two effects 
 
348 ELECTRO-STATICS. PART I. 
 
 produces an electrical state of the conductor B B', which 
 is dependent on the distance of the conductor A A' from 
 it. Beyond a certain distance B B' is positively elec- 
 trified. When the conductor A A' is brought to that 
 particular distance, the end B of the other conductor is 
 reduced to its natural state,, the electricity from B to B' 
 being positive, and of gradually increasing density; 
 when the conductor A A' is brought nearer to B B' than 
 that particular distance, a neutral point O (fig. 27.) is 
 established between B and the middle of the conductor 
 where the conductor is in its natural state, and which 
 divides it into two portions oppositely electrified, O B 
 being negatively electrified, and O B x being positively 
 electrified. The position of the neutral point O will 
 depend on the distance of the conductor A A' from B B'; 
 the less this distance is, the nearer the point O will be 
 to the centre of the conductor. 
 
 (137-) It appears from what has been explained, that 
 the effect of the approach of the conductor A A' upon 
 the distribution and development of the electricity on 
 B B ' depends on the excess of the attraction of the ne- 
 gative electricity accumulated towards the extremity A 
 above the repulsion of the positive electricity accumu- 
 lated towards the extremity A'. It may therefore be 
 expected, that if by any means the quantity of positive 
 electricity collected at the extremity A' were diminished, 
 and still more if that electricity were altogether abstracted, 
 the effects of the conductor A A' upon B B' would be 
 augmented. This may be easily tested by experiment : 
 Let us suppose the two conductors to be placed under 
 the circumstances represented in fig. 35., in which the 
 conductor B B' is electrified positively throughout its 
 whole length, the electric density gradually increasing 
 towards the extremity B',, as indicated by the compara- 
 tive divergence of the pith balls. Let a small insulated 
 spherical conductor be now brought in contact with the 
 conductor A A' at the extremity A', and removed from 
 it. Immediately the divergence of the pith balls near 
 B' will be observed to be increased, and the divergency 
 
CHAP. VI. INDUCTION. 349 
 
 of those near B diminished, indicating a further move- 
 ment of the positive electricity from B towards B'. If 
 the spherical conductor which was brought into contact 
 with A' be examined, it will be found to be charged 
 with positive electricity. The result of the experiment 
 is therefore easily explained. A portion of the positive 
 electricity, which was collected near A', has been re- 
 moved. The negative electricity near A now exceeds 
 the positive electricity near A', and this has given in- 
 creased power to the electricity developed upon A A' 
 over the electricity diffused upon B B', and a greater 
 quantity of the latter has been consequently drawn from 
 B towards B', producing, as we have seen, an increased 
 divergence of one pair of balls, and a diminished di- 
 vergence of the other pair. 
 
 (138.) But the effects of this change in the electrical 
 state of A A' do not end here. The increased accumu- 
 lation of positive electricity towards B' is attended with 
 a corresponding means of attraction excited upon the 
 negative electricity of A A', and of repulsion upon its 
 positive electricity. An augmented development of 
 both these principles is the consequence, and this again 
 re-acts on the electricities of B B x ; and in this manner 
 a reciprocal series of effects is produced, which continue 
 until the mechanical equilibrium between the electricities 
 of the two conductors, disturbed by the abstraction of the 
 electricity from A' by the contact of the spherical con- 
 ductor has been re-established. In order to render this 
 process intelligible, the series of effects has been here 
 described as if a definite time were occupied in their pro- 
 duction. Such, however, is not the case. They are, 
 in fact, completed instantaneously, and the moment the 
 spherical conductor has touched the extremity A the 
 pith balls on B B' assume a new divergence, which they 
 maintain. 
 
 (139.) If the spherical conductor, having been dis- 
 
 * charged, be again brought in contact with A', like 
 
 effects ensue. The pith balls near B' further increase 
 
 their divergence, and the divergence of those near B is 
 
350 ELECTRO-STATICS. PART I, 
 
 diminished, and the same process may be repeated with 
 the same effects. To carry this principle to its extreme 
 limits, let the extremity A' be put for a moment in 
 communication with the earth, by being touched by a 
 conductor which is not insulated ; immediately the whole 
 of the positive electricity accumulated near A' will pass 
 away, and the negative electricity accumulated near A 
 will take effect with un diminished force on the elec- 
 tricity of the conductor B B'. An increased divergence 
 of the balls near B' will take place, and the balls near 
 B will either be diminished in their divergence, or will 
 collapse, or finally will diverge with negative electricity, 
 according to the quantity of positive electricity with 
 which the conductor B B x has been originally charged. 
 A reaction, similar to what has been already described, 
 takes place between the electricities on the two con- 
 ductors, until their mutual attractions and repulsions are 
 restored to a state of equilibrium. 
 
 To render these effects more easily intelligible, we 
 have supposed the conductor B B' to be originally 
 charged with positive electricity. The results, how- 
 ever, would have been the same, mutatis mutandis, had 
 it been charged with negative electricity. 
 
 (140.) The important electrical phenomena of the 
 development of electricities on bodies in their natural 
 state by the near approach of bodies already electrified, 
 which have formed the subject of the present chapter, 
 have been denominated by English electricians induction, 
 and when a body is rendered electrical by the presence 
 of an electrified body not in contact with it, it is said 
 to be electrified by induction, and the electricity deve- 
 loped upon it called induced electricity. The effects of 
 induction will be still more fully developed in the fol- 
 lowing chapter, when the theories which have been pro- 
 posed to connect together and explain the phenomena 
 of electricity will be discussed. In a series of con- 
 ductors (such as A A' A", described in 132.), in ' 
 which electricity is educed by the influence of a posi- 
 tively electrified conductor S placed before A, if the 
 
CHAP. VI. INDUCTION. 351 
 
 positive end of the conductor A"' be put in commu- 
 nication with the earth, so as to discharge its positive 
 electricity, an increased development of electricity will 
 take place on the preceding conductor A", upon the 
 principles already explained. This will produce, hy the 
 same principles, a corresponding increase of electricity 
 on the conductor which precedes it, and so on through- 
 out the series, so that the first conductor A, having an 
 increased development of negative electricity on the 
 side presented to S, will produce an increased accu- 
 mulation of positive electricity on the side of S pre- 
 sented towards it. 
 
032 ELECTRO-STATICS. PART I. 
 
 CHAP. VII. 
 
 THEORY OF ELECTRICITY. 
 
 (141.) IN any branch of natural science, where the 
 facts collected in the careful observation of the opera- 
 tions of nature, and by accurate and well-directed ex- 
 periments, become sufficiently numerous and varied to 
 show traces of general laws, it is the province of the 
 philosopher to form a theory, which, by assigning ade- 
 quate general causes, may embrace, under a few com- 
 prehensive theorems, all the various phenomena which 
 observation and experiment have supplied. By such 
 means, those who prosecute scientific researches are 
 placed in a condition to foretell what will happen under 
 any supposable physical conditions ; and the accordance 
 of the event with such predictions, supplies the most 
 legitimate proof of the validity of the theory on the 
 principles of which such predictions were made. 
 
 It has happened in almost every branch of natural 
 science, that more than one theory has been propounded 
 to account for the phenomena ; and, in many cases, 
 rival theories have maintained their ground, supported 
 by a body of partisans, during the progressive advance- 
 ment of the science, under the increasing labours of 
 those whose vocation is to observe and collect facts 
 and phenomena rather than generalise them. No hy- 
 pothesis can be expected to gain any general or per- 
 manent acceptance, which does not afford a satisfac- 
 tory explanation of the more striking phenomena, and 
 obvious appearances, for the explanation of which it has 
 been proposed. In cases, therefore, where the com- 
 munity of science has been divided between two con- 
 tending theories, more especially in modern times, when 
 inductive science is so well understood, it ought to 
 
CHAP. VII. THEORIES OP ELECTRICITY. 353 
 
 excite no surprise that both such theories afford expla- 
 nations equally plausible and satisfactory for all the 
 ordinary phenomena comprised in the department of 
 science to which they extend. It is not, then, by the 
 account which they render of these prominent effects 
 that the claims of conflicting hypotheses can be decided. 
 If they had not been adequate to the explanation of 
 such appearances, they never could have obtained such 
 an extensive assent as to raise any question respecting 
 their validity. Such hypotheses can only be tested in 
 two ways : first) by exacting from them a clear and 
 consistent account of phenomena developed after the 
 theory itself had been proposed, and which were not fore- 
 seen by those who propounded it ; and, secondly, by de- 
 ducing from it, not merely a general account of the phe- 
 nomena which will be produced under any given phy - 
 sical conditions, but by exacting from it a rigorous 
 numerical and quantitative estimate of the effects, and 
 by comparing such estimate, so deduced from the theory, 
 with the actual numerical and quantitative account as 
 obtained from experiment and observation. If the dis 
 crepancy between the numbers and quantities furnished 
 by the theory and by observation exceed the possible 
 amount of the errors of observation, and still more, if 
 the principles of the theory afford no means whatever 
 of reducing the effects to numerical calculation, such 
 theory must be rejected as insufficient. If, on the 
 other hand, it be found that an hypothesis, capable of 
 affording a clear and satisfactory explanation of the 
 general nature of all the phenomena, as well those 
 which were known to its proposers, as those which 
 observation and experiment subsequently developed ; 
 if it also supply the means, by calculation and reason- 
 ing, of predicting other phenomena to which experi- 
 ment and observation have not yet been directed, and that 
 the effects already produced under the prescribed con- 
 ditions have been found to be in strict accordance with 
 such predictions ; if, moreover, by the application of 
 the principles of analytical calculation, the numerical 
 
854) ELECTRO-STATICS. PART I. 
 
 and quantitative amount of all the phenomena are 
 capable of being deduced from such hypothesis, and 
 the difference between such numerical results, and the 
 actual numerical quantities obtained by observation and 
 experiment, do not exceed the possible amount of the 
 errors of observation ; - then such theory must be re- 
 garded as proved, and ought, by the principles of 
 inductive philosophy, to be assented to and received, 
 until some phenomena shall arise of which it is in- 
 capable of giving a satisfactory account. 
 
 (142.) Two theories have been proposed for the 
 explication of the phenomena of electricity. That which, 
 until within a few years, has been most generally em- 
 braced in this country, originated with Dr. Franklin, 
 by whose labours this department in physics has been 
 so highly enriched. The other hypothesis is usually 
 ascribed to Dufay ; but it may be more properly attri- 
 buted to Mr. Robert Symmer. INT. (9^-) 
 
 Both these theories agree in ascribing the phenomena 
 of electricity to a material substance, endowed with the 
 most perfect fluidity, the molecules of which are distri- 
 buted on the surfaces of bodies. The properties of a 
 fluid are irresistibly suggested by all the most striking 
 phenomena of electricity. The extreme facility with 
 which electricity diffuses itself on conductors, its rapid 
 escape when relieved from the pressure of the surround- 
 ing air, the perfect mobility with which its particles 
 transfer themselves from conductor to conductor, and by 
 which they combine and separate, all concur in suggest- 
 ing the notion of fluidity. 
 
 In the theory of Symmer, however, the existence of 
 two independent fluids is assumed, the vitreous and the 
 resinous, or, according to the other nomenclature, the 
 positive and the negative. Each of these fluids is self- 
 repulsive, its particles tending to separate from each other 
 by an elastic force like that of atmospheric air. Thus, 
 if a certain quantity of either of those fluids, the posi- 
 tive for example, be introduced upon a metallic sphere, 
 where its motion is unobstructed, it will instantly dif- 
 
CHAP. VII. THEORY OF TWO FLUIDS. 355 
 
5) ELECTRO-STATICS. PART I. 
 
 state ; and, when a body is negatively electrified, it is 
 considered as having a quantity of the negative electric 
 fluid diffused upon it, having likewise the combination 
 of electricities which characterise the natural state. 
 
 (145.) When an electrified body is brought near an 
 insulated conductor, which is in the natural state, the 
 electric fluid with which the former is charged acts by 
 attraction and repulsion upon the two fluids which are 
 naturally combined upon the conductor. It acts by re- 
 pulsion on the molecules of fluid of its own name, and 
 by attraction on those of the contrary name. Thus, if 
 the electrified body be charged with negative electricity, 
 it attracts the molecules of the positive fluid, and repels 
 those of the negative fluid on the conductor. By this 
 means, it separates or decomposes a quantity of the 
 combined fluids on the conductor, drawing the positive 
 molecules to the side next itself, and repelling the nega- 
 tive molecules to the opposite side. The quantity of na- 
 tural electricity thus decomposed is proportional to the 
 force excited by the electrified body upon the conductor, 
 which force depends partly on the quantity of the elec- 
 tricity with which the electrified body is charged, and 
 partly on its distance from the conductor. 
 
 It will be easy, by following out these views, to per- 
 ceive how the various effects of induction detailed in 
 the last chapter are accounted for by this theory. 
 
 When electricity is excited by the friction of two 
 bodies, the natural electricity upon one or both of them 
 is decomposed by some forces or powers brought into 
 operation by the friction, and the molecules of the po- 
 sitive fluid adhere to one of the two bodies so submitted 
 to friction, while those of the other fluid adhere to the 
 other body. 
 
 When an insulated conductor charged with positive 
 electricity is brought near to another conductor not in- 
 sulated, that other conductor is regarded as a part of 
 the entire globe of the earth, forming one great con- 
 ductor. The positive electricity with which the insu- 
 lated conductor is charged, as it approaches the con- 
 
CHAP. VII. THEORY OF TWO FLUIDS. 357 
 
 ductor not insulated, decomposes by its attraction and 
 repulsion a portion of the combined electricity of the 
 entire globe, and attracts to the part of the conductor 
 nearest to it a portion of negative electricity. When 
 the conductors are brought so near together that their 
 mutual attraction produces such an accumulation of 
 contrary electricities at the parts contiguous to each 
 other that the pressure exceeds that of the atmosphere, 
 they rush towards each other, and combine so as to 
 neutralise each other, and it may be considered that the 
 positive electricity of the electrified conductor escapes to 
 the earth, where it combines with an equal portion of 
 negative electricity. 
 
 It may be here observed that the supposition that 
 the total quantity of negative electricity existing on the 
 globe is equal to the total quantity of positive electricity, 
 involves the consequence that whenever there is any 
 quantity of either electricity uncombined, as is the case 
 when an insulated conductor is charged with either 
 fluid, then there will be on the earth, or on some body 
 upon it, an equal quantity of the other electricity also 
 uncombined, which by combination with the elec- 
 tricity of the conductor, will restore the electrical equi- 
 librium and reproduce the natural state. 
 
 (146.) That the combination of the two opposite 
 electricities will manifest no attraction on a body which 
 would be attracted by either acting alone, may be 
 proved by direct experiment in the following manner. 
 Let two circular discs of plate glass be formed about 
 six inches in diameter, and let insulating handles of 
 glass be attached to them. Being brought into contact 
 face to face, as represented in fig. 28., let them be 
 Fig. 28. rubbed one against the other, being 
 
 held by the insulating handles E and F. 
 If without separating them they are 
 _L now presented to a pith ball sus- 
 pended by a filament of raw silk, 
 they will not attract it ; but if they 
 be separated, and be presented se- 
 A A 3 
 
358 
 
 ELECTRO-STATICS. 
 
 parately to it, they will attract it. They are there- 
 fore both electrified, and it is easy to show that they 
 are oppositely electrified; for if they be presented 
 separately to an electrified pith ball, one of them 
 will attract and the other will repel it. Since, how- 
 ever, the discs, when in contact, produce no effect 
 upon the electrical pendulum in its natural state, 
 it is clear that the two electricities, though developed 
 on the surfaces of the glass discs, and not naturally 
 combined, nevertheless neutralise each other's effects ; 
 the attraction which either would exert on the pith 
 ball being counteracted by the effect of the other. The 
 positive electricity developed upon the discs has a ten- 
 dency to decompose the natural electricities of the pith 
 ball, drawing its negative electricity to the side next to 
 the discs, and driving its positive electricity to the op- 
 posite side ; while on the contrary the negative electri- 
 city of the discs has a tendency to draw the positive 
 electricity of the ball to the side next the discs, and to 
 drive the negative electricity to the opposite side. When 
 the faces of the discs are in contact, the two electricities 
 being equally distant from the ball exert equal forces 
 tending to produce these effects, and they consequently 
 neutralise each other; but it is easy to show that if 
 they were presented to the ball at different distances, 
 they would produce an effect in virtue of the difference 
 of their attractions, and that this effect would gradually 
 diminish as the discs would approach each other. Let 
 G (fig. 29.) be a pith ball suspended in the usual manner, 
 
 Fig. 29. 
 
 \ 
 
 fl" 1 
 
 Mr 
 
 JA cj 
 
IAP. VII. FBANKLIMAX THEORY. 359 
 
 and let A B be one of the discs presented with its insulat- 
 ing handle E towards the pith ball ; the electricityexcited 
 upon its surface acting through the glass will draw the 
 ball from the vertical position G to the position G'. Let 
 the other disc C D be now presented with its face to- 
 wards A B. As it is advanced towards AB, the ball 
 will gradually descend from the position G' to the po- 
 sition G, arriving at the position G at the moment when 
 the face of the disc C D comes into contact with that of 
 the disc A B. 
 
 (14?.) Such, in general terms, is the hypothesis of 
 two fluids propounded by Symmer and Dufay, and 
 which, as we shall presently show, has by the aid of the 
 higher principles of mathematical analysis afforded sa- 
 tisfactory solutions for all the phenomena of electricity 
 to which the present powers of mathematical science 
 allow it to be applied. We shall now briefly explain 
 the Franklinian theory. 
 
 This theory assumes the existence of a single electric 
 fluid, self-expansive and possessing perfect fluidity and 
 mobility. The molecules of this fluid are supposed to 
 attract and be attracted by the constituent particles of 
 all material substances. The attractions however exerted 
 between it and different substances are different, and its 
 attraction even for the same substance is variable, accord- 
 ing to the various changes which the physical properties 
 of such substance may undergo. A body is supposed 
 to be in its natural state when it contains upon it as 
 much of the electric fluid as satisfies its attraction. If 
 it contain more, such excess is free, and the body is po- 
 sitively electrified. If it contain less, the body is said 
 to be negatively electrified. Excitation, therefore, takes 
 place when there is either more or less electricity on the 
 body than is adequate to the saturation of its existing 
 attraction ; but not to incur the risk of unintentionally 
 misstating a theory to the validity of which we do not 
 assent, we shall borrow from a modern author, a par- 
 tisan of the theory, an outline of it, together with a few 
 experimental illustrations in support of it 
 A A -i 
 
360 ELECTRO-STATICS. PART I. 
 
 tc Electrical excitation may be thus effected : the bo- 
 dies employed have each a certain quantity of the elec- 
 trical fluid proportionate to their natural attraction for 
 it. This they retain, and appear unelectrified so long as 
 they remain in their natural state. Now, if two such 
 bodies are brought into contact, their natural attractions 
 are altered ; one of them attracts more than in its sepa- 
 rate state, and the other less, the electric fluid diffuses 
 itself amongst them in quantities proportionate to their 
 relative attractions, and they consequently appear un- 
 electrified. But if they are suddenly separated, the new 
 distribution of the electric fluid remains whilst the origi- 
 nal attractions are restored ; and as these are not equal to 
 each other, the bodies will appear electrical : that whose 
 natural attraction was increased by contact, having re* 
 ceived an addition to its quantity of electric fluid, will 
 be positively electrified ; and that whose attraction was 
 lessened, having lost a portion, will be negative/' 
 
 fe Take as an instance the electrical machine : let the 
 attraction of the cushion for the electric fluid be repre- 
 sented by 20, and that of a similar surface of glass by 
 30 : the sum is 50. Bring the bodies in contact, their 
 attractions alter ; that of the glass becomes 40, and that 
 of the cushion 10; the sum of these is still 50: the 
 natural electricity, therefore, though unequally distri- 
 buted, is still equal to the sum of its attractions, and 
 does not appear, for the cause of its unequal distribution 
 (the contact) is still active. Separate the glass from 
 the cushion, its original attraction of 30 will now only 
 operate, but it has acquired 40 of electricity by contact 
 with the cushion ; the glass is therefore positive with 
 force equal to 10. The cushion will now also have its 
 original attraction of 20; but its electricity amounts 
 only to 10. It is therefore negative with a force equal 
 to 10. And here is seen the reason why positive and 
 negative bodies act more powerfully on each other than 
 on indifferent matter, for their mutual difference is often 
 twice as great as their individual, since if the latter be 
 10, the former may be 20. 
 
CHAP. VII. FRANKLINIAN THEORY. 361 
 
 " The effects now described continually recur during 
 the revolutions of the cylinder, every part of which is 
 successively brought in contact with the cushion, and 
 passes forward with the electricity it thus progressively 
 acquires. The silk flap may be considered as a con- 
 tinuation of the rubber, which, by partially maintaining 
 the altered attraction of the glass, counteracts the ten- 
 dency of the acquired electricity to pass back into the 
 cushion. The surface of the glass, when it passes from 
 beneath the silk flap, has not this compensation. Hence 
 the acquired electricity is there uncombined, and has a 
 tendency to diffuse itself among the surrounding bodies : 
 the conductor, with its row of points, is the nearest 
 reservoir, and into this it passes, and the conductor 
 becomes thereby positively electrified. During this pro- 
 cess the cushion and its attached conductor constantly 
 furnish electricity to the glass, and they are conse- 
 quently negative in the same degree j but they have only 
 a limited surface and a certain quantity of natural elec- 
 tricity, and if perfectly insulated can furnish only a 
 definite portion ; but if they are connected with the 
 ground whose surface is comparatively unlimited, they 
 operate upon an extensive store, to the supply of which 
 there appears no assignable bound. It is for this reason 
 that the electricity of either conductor separately is more 
 apparent when the opposite one is insulated." * 
 
 Such is the view of the theory of a single fluid mo- 
 dified, as it has been since it was first proposed by its 
 illustrious author, to adapt it to the phenomena of the 
 science in its modern state. Different partisans of this 
 theory accept it with different qualifications and con- 
 ditions ; but the above may be considered in the main 
 as a fair statement of the hypothesis by a recent elec- 
 trician who advocated its validity. 
 
 Considering what was observed respecting physical 
 theories in general at the commencement of this chapter, 
 it is needless to state that a theory which has been so 
 long and so extensively accepted as the true one, is 
 
 * Singer's Electricity. 
 
362 
 
 ELECTRO-STATICS. 
 
 quite sufficient to afford an explanation of all the or- 
 dinary phenomena of electricity. It is not, therefore, by 
 such means that the question can be decided between 
 these two celebrated theories. That question can only 
 be settled by requiring from each of them an account 
 of some phenomena which have more recently attracted 
 the attention of electricians, but above all by demand- 
 ing from each of them a rigorous numerical estimate of 
 the state of bodies brought under each other's influence 
 in an electrified state in cases which we shall presently 
 explain. 
 
 Meanwhile it may be right here to give the particular 
 details of some striking and beautiful experiments, which 1 
 are much relied on by the advocates for the theory of a j 
 single fluid, as being decisive tests in favour of the reality 
 of that theory. 
 
 EXPERIMENT- I. 
 
 (148.) To the conductor of a machine so arranged 
 as to be charged with negative electricity present a 
 pointed metal rod held in the hand, in a darkened 
 room. A cone, formed of rays of light, will be seen, 
 having its vertex at the point and its base towards the 
 conductor ; but if a similar point be presented to a po- 
 sitively electrified body, instead of the cone of light there 
 will be seen a brilliant star at the extremity of the me- 
 tallic point. The light in these experiments is con- 
 sidered to indicate the course of the electric fluid. The 
 point is a sort of pipe capable of emitting or receiv- 
 ing it. The negative conductor is supposed to have a 
 deficiency, and the point presented towards it is illumi- 
 nated by a diverging pencil of rays, which indicates that 
 the cause of that light moves from the point to the ne- 
 gative body. The positive conductor is supposed to 
 have an excess of electric fluid, and the point presented 
 to it is merely illuminated by a globular spot of light, 
 
CHAP. VII. FRANKL1NIAN THEORY. 363 
 
 an appearance that may be conceived to attend the en- 
 trance of a subtle fluid into it. 
 
 Reversing the experiment, by connecting the points 
 with the conductors respectively, the converse series of 
 phenomena is produced. 
 
 EXPERIMENT II. 
 
 (149-) Take the transfer plate of an air-pump, and 
 affix to its centre, by a wire three inches long, a brass 
 ball one inch in diameter (fig. 30.) ; connect a similar 
 Fig. 30. ball* by a sliding wire, to the top of 
 a receiver, and place this over the 
 transfer plate, so that the one ball 
 may be vertically over the other, and 
 at the distance of about one inch. 
 Exhaust the receiver accurately, and 
 then connect the plate with the nega- 
 tive conductor, and the upper wire 
 and ball will be positive. Upon turn- 
 ing the machine, a torrent of beauti- 
 ful light will pass from the positive to 
 the negative ball, on which it breaks 
 and divides into a luminous atmosphere, entirely sur- 
 rounding the ball and stem, and conveying most strik- 
 ingly the idea of a fluid running over the surface of a 
 resisting solid, which it cannot enter with facility. No 
 appearance of light occurs on the positive ball, but the 
 straight luminous line that passes from it ; but if it be 
 rendered negative and the lower ball positive, these ef- 
 fects are entirely reversed. 
 
 EXPERIMENT III. 
 
 (150.) Fig. 31. represents two hollow metal balls, 
 about | of an inch in diameter, insulated upon sepa- 
 rate glass pillars, by which they are supported two inches 
 
364 
 
 ELECTRO-STATICS. 
 
 PART I. 
 
 Fig. 31. 
 
 apart ; the upper part of each ball is indented, so as to 
 form a small cup, in which a fragment of phosphorus 
 is to be placed. A small 
 candle or lamp has its 
 flame situated midway be- 
 tween the balls ; one of 
 them is connected with 
 the positive, the other with 
 the negative conductor of 
 the electrical machine by 
 means of a wire. When 
 the balls are electrified the 
 flame is agitated, and in- 
 clines to that which is negative. This it soon heats 
 sufficiently to fire the phosphorus, whilst the posi- 
 tive ball remains perfectly cold, and the phosphorus 
 solid. If the connecting wires be now reversed, so 
 that the ball which was before negative shall become 
 positive, and that which was positive be rendered ne- 
 gative, the phosphorus in the latter will soon take fire. 
 So that electricity passes from the positive to the nega- 
 tive, and transmits with it the heat of any intervening 
 ignited body. 
 
 We shall not now enter into any observations or ar- 
 gument drawn from these experiments, in favour of the 
 theory of a single fluid. There are other experiments, 
 of the same kind, which we shall have occasion to ex- 
 plain hereafter, when it will appear that they may be 
 accounted for in a very different manner. 
 
 (151.) About the year 1760, JEpinus, INT. (82.), 
 brought the Franklinian theory to the test of strict mathe- 
 matical statement, and obtained numerous results in gene- 
 ral accordance with the phenomena. He found, however, 
 that the mere hypothesis of an elastic fluid, whose mole- 
 cules attracted and were attracted by the particles of 
 bodies, even admitting this attraction to vary in different 
 bodies, and in different states of the same body, was 
 insufficient to explain the phenomena, and that to bring 
 the theory into accordance with the facts, a further as- 
 
CHAP. VII. FRANKLINIAN THEORY. 
 
 sumption must be made, that masses of matter exer- 
 cise upon each other,, at great distances, a reciprocally 
 repulsive force of sensible amount, similar to the re- 
 pulsion which exists among the molecules of the electric 
 fluid itself. 
 
 If no other reason could be assigned for rejecting the 
 theory of a single fluid as untenable, than this assump- 
 tion of a mutual repulsion, of the existence of which 
 there is not only no evidence whatever, but which is in 
 immediate opposition to all the laws developed by the 
 motions of the great bodies of the universe, we should 
 be justified in its rejection. The illustrious proposer 
 of this theory did not himself see the necessity for so 
 unphilosophical an assumption. In the loose manner 
 in which the theory was in his time applied, it was 
 sufficient to explain in a general manner the limited 
 body of phenomena then observed ; and it was not until 
 further observation and inquiry had multiplied the facts, 
 and a more rigorous account of those facts was exacted, 
 and it became necessary to reduce the theory to a strict 
 mathematical form, that the necessity of this additional 
 assumption of a repulsion between matter and matter 
 at a distance, so completely opposed to the most striking 
 phenomena in nature, became apparent. 
 
 But supposing this hypothesis, objectionable as it is, 
 to be admitted, would the theory even then be sufficient 
 for the due explication of the phenomena ? That it would 
 afford a reasonable explanation of the ordinary attractions 
 and repulsions, and the chief electrical effects upon bodies 
 by induction, may be admitted; for no theory which could 
 fail in giving a reasonable account of effects so conspi- 
 cuous, could have obtained any acceptance. It does 
 not appear, however, in what way a theory, in which 
 different kinds of matter must be admitted to exert 
 different attractions for the electric fluid, can explain 
 the distribution of electricity on the surfaces of con- 
 ducting bodies, in a manner depending solely on their 
 form, and not at all on their chemical composition. If 
 different material substances have different attractions 
 
366 ELECTBO-STATICS. PART I. 
 
 for the electric fluid, how does it happen that the dis- 
 tribution of that fluid on an oblong plate of metal, of 
 given dimensions, will be the same, of whatever metal tha 
 plate be formed, or even though it be formed of pieces of 
 various different metals connected together? Neither 
 can that theory show why negative electricity, which it 
 views as a deficiency or absence of the electric fluid, 
 should, when developed on the surfaces of bodies, pro- 
 duce effects in conformity with the rigorous hydrostatical 
 laws, which an elastic fluid would obey, whose mole- 
 cules repel each other with forces which diminish in 
 intensity as the square of the distance between them 
 increases. If this theory of a single fluid fail in af- 
 fording a reasonable explanation of such striking phe- 
 nomena, its insufficiency becomes still more glaring when 
 by its principles an attempt is made to calculate the 
 depth of the electric fluid on bodies of various forms, 
 brought under each other's influence in given positions. 
 Thus, if two spheres composed of a conducting sub- 
 stance are brought into contact and electrified, and then 
 separated, it will be found, that round the point of con- 
 tact, on the smaller sphere, electricity will be diffused 
 of a kind contrary to that with which the spheres were 
 electrified ; and this electricity will extend to a certain 
 distance round the point of contact, beyond which the 
 electricity diffused over the remainder of the sphere will 
 be the same as that with which the two spheres were 
 electrified. The theory of a single fluid cannot account 
 for this general fact ; still less can it enable us to com- 
 pute the limits which separate the portions of the sphere 
 electrified by the one electricity from that electrified by 
 the other. On the other hand, the hypothesis of two 
 fluids, observing the conditions by which that hypothesis 
 is restricted, not only enables us to show that under 
 these circumstances the development of a contrary 
 electricity round the point of contact, and of the same 
 electricity over the remainder of the sphere, is a natural 
 and inevitable consequence of the properties, which, in 
 this theory, are assigned to the two fluids, but it en- 
 
CHAP. VII. THEORY OP TWO FLUIDS. 367 
 
 ables us to calculate with the most surprising precision 
 the exact limits which, in any given position, separate 
 the positive from the negative regions of the smaller 
 sphere. 
 
 The mathematical problems involved in the application 
 of the theory of two fluids to the explanation of electrical 
 phenomena require for their solution the last resources 
 of the most profound analytical researches of modern 
 science. It would not be consistent with the object of a 
 treatise like the present to enter into the details of such 
 investigations. The language itself, in which alone the 
 general theorems of electricity must be expressed, would 
 be unintelligible to the great majority of our readers. 
 Much may be done to popularise mathematics, and more 
 especially those parts of mathematics which express the 
 laws of physical phenomena; but this advantage has 
 practical limits beyond which it cannot be carried, and 
 in the whole range of mathematical physics it would 
 probably be difficult to find a portion of science which 
 would more decidedly forbid any attempt at popular 
 or elementary exposition than the results of the mathe- 
 matical investigation of electrical phenomena. Although 
 we cannot pretend, therefore, to deliver here even an 
 abridged view of the splendid labours of POISSON in this 
 department of physics, we shall endeavour to select a 
 few results at which he has arrived ; and by showing 
 their accordance with the phenomena, we may direct 
 and encourage those who have time and opportunity to 
 pursue experimental inquiry to those points which ana- 
 lysis has suggested, and thereby enable them to supply 
 new facts which will either fulfil the predictions of 
 theory, or show by their departure from these antici- 
 pations in what respects theory must be modified in 
 order more perfectly to represent the phenomena. 
 
 (152.) The first proposition which presents itself 
 for solution is to determine the distribution of the 
 electric fluid upon an insulated conductor of a given 
 form which is charged with electricity of any given in- 
 tensity. The analytical conditions immediately derivable 
 
368 ELECTRO-STATICS. PART I. 
 
 from the hypothetical properties of the electric fluid 
 ought to solve this problem. If we view the electric 
 fluid as heing of uniform density in every part, a 
 supposition which is suggested by the self-expansive 
 property of that fluid, we must regard its varying 
 intensity at different parts of the surface of the elec- 
 trified conductor, or what we have occasionally called 
 its varying density, to be the consequence of the vary- 
 ing depth of the fluid on different parts of the sur- 
 face. But as the atmospheric pressure keeps the supe- 
 rior surface of the electric fluid level with the surface of 
 the body itself, the problem proposed will resolve itself 
 to the determination of the inferior surface, or to the law 
 according to which the depth of the electric fluid varies on 
 different parts of the conductor. If it be more conso- 
 nant with the notions of some persons to view the elec- 
 tric, fluid as being of uniform depth, and to ascribe the 
 varying intensity which is found by experiment to pre- 
 vail at different parts of the same electrified conductor 
 to a variation in the density of the electric fluid, such a 
 supposition may be equally entertained, and the calcu- 
 lations will remain undisturbed, save that the numbers 
 which in the one case indicate the depths of electric 
 fluid in the other case express its densities. 
 
 Supposing, then, that the electric fluid were diffused 
 upon an insulated electrified conductor, the analytical 
 conditions by which its state is expressed will be those 
 by which the conditions of equilibrium are declared to 
 subsist between the attractions and repulsions exercised 
 by the various molecules of the electric fluid diffused 
 over the conductor on the natural electricities of each 
 particle of matter in the conductor. It is evident that 
 this is a condition, and the only condition, of electrical 
 equilibrium. If the combined attractions and repul- 
 sions of the electric fluid diffused over the conductor, 
 acting on the natural electricities of any one or more of 
 its particles, were not in equilibrium, the resultant of such 
 forces would produce a decomposition of the natural 
 electricities of such particle or particles, and an evolu- 
 
CHAP. VII. 
 
 THEORY OF TWO FLUIDS. 
 
 369 
 
 tion of electricity would be the consequence, which would 
 contradict the supposition of electrical equilibrium as- 
 sumed as the basis of the hypothesis. The analytical 
 formulae by which such conditions are expressed must 
 depend on and be derivable from the laws of attraction 
 and repulsion, assumed in the electrical theory conjointly 
 with the form of the conductor. 
 
 The exterior surface of the electric fluid diffused 
 over the conductor being assumed to be identical with 
 the surface of the conductor itself, these analytical con- 
 ditions will necessarily supply the means of obtaining 
 its interior surface. 
 
 The problems arising from the application of such 
 principles to particular forms of conductors offer ma- 
 thematical difficulties of the most formidable kind, and 
 in many cases their complete solution cannot in the 
 present state of mathematical science be effected. The 
 form of the conductors in some cases, however, renders 
 the solution comparatively simple. 
 
 (153.) It has been demonstrated by Newton that a 
 spherical shell of uniform thickness surrounding a 
 sphere composed of matter whose attraction increases 
 as the square of the distance is diminished, exercises 
 forces on any point within the sphere which are in me- 
 chanical equilibrium. This 
 is the most simple case of 
 the preceding problem, and 
 it follows from it that the 
 diffusion of the electric fluid 
 on the surface of a sphere 
 of conducting matter is uni- 
 form. Thus if A B (fig. 32.) 
 represent the section of a 
 sphere of conducting matter 
 made by a plane through its 
 
 centre, the concentric circle C D will represent the 
 inferior surface of a stratum of electric fluid diffused 
 upon it. 
 
 If an elliptical speroid be surrounded by a shell of 
 
 Fig. 32. 
 
3?0 ELECTRO-STATICS. PART I. 
 
 attracting matter governed by the same law of the 
 inverse square of the distances, the form which would 
 reduce its attractions on any particle within the sphe- 
 roid to equilibrium would be one bounded by similar 
 elliptical surfaces. If the section of such a spheroid, 
 therefore, be represented by A B C D (fig. 33.),, the 
 
 section of the interior surface of the electric shell will 
 be represented by A' B' C' D', being an ellipse having 
 the same centre and axes, and similar to it; that is to 
 say, one whose axes are in the same proportion. 
 
 Since the distance between the surfaces of two such 
 ellipsoids at their vertices is the difference between 
 their semiaxes, it follows that the depths of the elec- 
 tricity at the vertices of an ellipsoid formed of a con- 
 ducting substance are proportional to the axes ter- 
 minated at these vertices respectively ; for the axes 
 being proportional to each other, their differences A A' 
 and B B' will be in the same proportion, and the 
 depth of the electricity at A will be greater than the 
 depth of the electricity at B in the ratio of the length 
 of the axis A C to the length of the axis B D. 
 
 (154.) When an ellipsoid is very elongated, and 
 therefore its longitudinal axis bears a great ratio to its 
 lesser axis, the depth of the electricity will be propor- 
 tionably greater at its ends. For example, if we sup- 
 pose a spheroid to be formed by the revolution of a very 
 elongated ellipse, such as A B C D (fig. 34.), on its 
 major axis A C, the thickness of the electric fluid will 
 
THEORY OP TWO FLUIDS. 
 
 Fig. 34. 
 
 be greater at the ends A and C of its longer axis than 
 round its equator, formed by the revolution of the points 
 B and D in the proportion of A C to B D. 
 
 (155.) Since any part of a body which has a gra- 
 dually tapering form, and is terminated in a point, 
 may be considered to be part of an extremely elongated 
 elliptical spheroid, the point being one of the extremities 
 of its longer axis, the principle here established will 
 show that the depth of the electricity at such a point 
 would be very great, if it could be maintained there ; 
 but in all such cases, when the electricity has a sensible 
 force on other parts of the conductor, its intensity at 
 the point will greatly exceed the atmospheric pressure, 
 and it will escape from the point into the air, as it is 
 found by experience to do. 
 
 (156.) It would be easy to verify, in a general 
 manner, these conclusions by constructing elliptical 
 spheroids of metal, and electrifying them. On exa- 
 mining with the proof plane and the electrometer the 
 densities of the electricity at their vertices, the accord- 
 ance of the results with the theory will be apparent ; 
 but we shall presently state results of experiments made 
 by Coulomb, which will afford other satisfactory proofs 
 of such accordance. 
 
 (157-) The tendency of the electricity diffused upon 
 a body to overcome the atmospheric pressure increases 
 not merely in the proportion of the simple ratio of the 
 depth of the electricity, but is proportional to the square 
 of that depth. Thus, if the depth of the electricity in 
 the neighbourhood of an angular or pointed part of a 
 conductor be ten times greater than its depth at another 
 part of the surface, the pressure of the electricity against 
 
372 ELECTRO-STATICS. PART I. 
 
 the air will be a hundred times greater. This admits 
 of rigorous mathematical demonstration ; but, without 
 entering into such details, it will be easily comprehended 
 that since the outward pressure of an external mole- 
 cule of electricity is the result not merely of the re- 
 pulsion existing between itself and the molecules under 
 it, but also of the repulsion existing between each of 
 those molecules and those below it, the combined effects 
 of all the repulsions will be as the square of the depth. 
 Due consideration of these properties will explain the 
 great facility with which even feeble charges of elec- 
 tricity are dissipated at the angular points and edges of 
 bodies, and why conductors, which are intended to re- 
 tain upon them the electricity with which they are 
 charged, are every where made in a rounded form. 
 
 (158.) Having established the general analytical 
 conditions under which electricity is maintained in 
 equilibrium on a single conducting body, Poisson next 
 applied himself to the solution of the more difficult and 
 complicated problem, to determine the distribution of 
 electricity on two or more conductors, either brought 
 into actual contact, or so placed as to be affected by 
 their mutual electricities. The difficulties which this 
 general problem presented were happily surmounted by 
 showing that whatever might be the number or form of 
 electrified bodies brought under each other's influence, 
 the electricity diffused upon them when in a state of 
 equilibrium must be subject to a single general condition, 
 which has the double advantage of being self-evident, 
 and of being immediately expressed in mathematical 
 language. The following is this condition as an- 
 nounced by Poisson. 
 
 (159.) If two or more electrified conductors are 
 brought near each other, and the electric fluid upon them 
 has acquired a state of equilibrium, it will be necessary 
 that the resultant of the actions of all the molecules of 
 electric fluid diffused upon them, exerted upon any point 
 whatever assumed within the dimensions of any one of 
 these bodies, be nothing. For if this resultant were not 
 
CHAP. VII. THEORY OF TWO FLUIDS. 3J3 
 
 nothing, the natural electricities of the point on which 
 these forces would act would be decomposed, an evolution 
 of electricity would take place, and the electric state of 
 the body would be changed, contrary to the supposition 
 ivhich has been made of its equilibrium. 
 
 This principle, when reduced to mathematical ex- 
 pression,, supplies all the equations or conditions ne- 
 cessary to determine the distribution of electricity on 
 each of the hodies ; and if analytical science were 
 sufficiently advanced to supply the means of solving these 
 equations in every case, the phenomena of electrified 
 bodies could be as accurately calculated and as certainly 
 foreseen as the movements of the bodies of the solar 
 system. In most cases, the complete resolution of these 
 equations surpasses the power of analysis. Poisson has, 
 however, succeeded in surmounting the analytical diffi- 
 culties which they present in certain particular cases, 
 in which the electrical state of bodies had been long 
 before experimentally examined by Coulomb. The 
 comparison of the numerical results of the theoretical 
 calculations effected by Poisson, in those particular cases, 
 with experiments made before the theory which supplies 
 these results had been propounded, has afforded con- 
 clusive proofs of the soundness and truth of that theory, 
 and furnishes one of the most beautiful examples to be 
 found in the history of physical discovery of the co- 
 operation of experimental and theoretical inquiry in the 
 discovery of a great natural law. The bodies on which 
 Coulomb had experimented, and whose electrical state 
 he had minutely examined and recorded, were spheres; 
 and Poisson succeeded in rendering the general analy- 
 tical conditions capable of solution when applied to the 
 particular case of two spheres charged with given 
 quantities of electricity, and brought either into actual 
 contact or near each other. 
 
 (160.) According to the theory of Poisson, when a 
 body is in its natural state it contains equal quantities 
 of positive and negative electricity. Each of the two 
 fluids will be uniformly diffused through its dimensions; 
 
S?4 ELECTRO-STATICS. PART I. 
 
 and as one of them attracts whatever the other repels, 
 and these attractions and repulsions are equal, the me- 
 chanical forces thus exerted by the two fluids will be 
 in equilibrium. Hence, if two spheres be in their 
 natural state, their effect on each other will be nothing ; 
 and if they are not submitted to the influence of some 
 third body, they will continue in the natural state. But 
 if, besides the natural charge of electricity of the two 
 kinds, an excess of either electric fluid, as, for example, 
 the positive, be imparted to either sphere, it will diffuse 
 itself over the surface, forming a thin stratum upon it of 
 uniform depth. The attractions of this stratum on any 
 particle within the sphere on which it is diffused are in 
 equilibrium, and therefore it cannot, of itself, liberate 
 or decompose the combined electricities of any particle 
 of that sphere ; but this is not necessarily the case with 
 respect to the particles of the other sphere. On the con- 
 trary, by attracting the negative part of the combined 
 electricities, and repelling the positive, it will in general 
 effect a partial decomposition of them ; and the fluids 
 thus liberated will move to the surface, where they 
 will be retained in a thin stratum by the atmospheric 
 pressure. The depth of this stratum will vary in 
 different parts of the sphere, according to a law which 
 can be deduced from the theory. 
 
 (l6l.) The fluids thus decomposed and liberated on 
 the second sphere will react as well on the natural 
 electricities of the first sphere as on the excess of the 
 positive electricity with which it was charged, and a 
 new distribution of electricity on that sphere would be 
 the consequence. When the mutual decompositions 
 produced by these actions are completed, the electric 
 fluids which are free will dispose themselves in a certain 
 manner on the two spheres, and the depth and the 
 quality of the fluid at each point of each of the spheres 
 can be determined by theory, when the radii of the 
 spheres and their mutual position with respect to each 
 other are given. It may, however, be observed in ge- . 
 neraL that the reciprocal action of the two spheres can, 
 
CHAP. VII. MATHEMATICAL ANALYSES. 3J5 
 
 in no case, increase or diminish the difference between 
 the quantities of positive and negative fluids upon them. 
 This difference must always be equal to the excess of 
 positive or negative fluid which has been origin- 
 ally imparted to them by any third body or bodies by 
 which either or both of them may have been electrified ; 
 for since their natural electricities contain equal quanti- 
 ties of the positive and negative fluids, any decomposi- 
 tion which has been effected by their mutual action 
 must develop equal quantities of the positive and nega- 
 tive fluids. Such action, therefore, always adding 
 equal quantities to the positive and the negative fluids 
 with which the spheres are already charged, leaves their 
 difference unaltered. 
 
 (162.) If two conducting spheres, differently charged 
 with electricity, be brought into contact, the fluids will 
 distribute themselves between them, in a certain propor- 
 tion depending on the radii of the spheres. If, before 
 the contact, the electricities with which they are charged 
 happen to be in this proportion, no electricity will pass 
 from either to the other when brought into contact ; 
 but if the electricities with which they are charged 
 before contact be in any other proportion than when 
 contact takes place, it will distribute itself in the pro- 
 portion due to the relative magnitude of the spheres. 
 
 When the spheres have been separated after contact, 
 the total quantities of electricities upon them, estimat- 
 ing these quantities by the excess of the positive above 
 the negative, or, vice versa, will be the same as when 
 in contact ; but its distribution over their surfaces will 
 be different, being affected by the varying distance 
 between their centres. 
 
 (163.) We propose here to state some of the results 
 of the mathematical investigations of Poisson, and to 
 compare them with the results of the experiments of 
 Coulomb ; and, for the sake of clearness, we shall resolve 
 the theorems which are to be explained into a series of 
 distinct propositions. We shall first consider the cases 
 in which the spheres are brought into contact; secondly, 
 
#? ELECTRO-STATICS. PART I. 
 
 when after contact they are separated, no additional 
 electricity being imparted to them after contact ; and, 
 thirdly, the cases in which the two spheres are charged 
 with any quantities of electricity, and brought under 
 each other's influence without previous contact. 
 
 PROPOSITION I. 
 
 (164.) Two conducting spheres of given radii are charged 
 with electricity and brought^into contact : it is required 
 to determine the proportion in which the sum of their 
 electrical charges will be shared between them when elec- 
 trical equilibrium has been established after contact, and 
 also the mean depth of electricity on each sphere. 
 
 Let r and r be the radii of the spheres. 
 
 Let E and E' be the quantities of electricity which 
 each will have after contact. 
 
 Let D and D / be the mean depths on each after 
 contact. 
 
 From what has been explained in (93.) we have 
 
 By the formula established by Poisson we have 
 E - E'= - 4>v 2 mhr cot. (1 m)^ ; 
 
 /r- 1 - 1 
 - dt; 
 1 t 
 
 _ x 
 
 -* 1 
 
 ,.*-! 
 
 J l-t 
 
 
CHAP. VII. MATHEMATICAL ANALYSES. 377 
 
 ,cot.(l-> 
 
 E ft m ~ - 1 
 
 J L-t 
 
 where h is a constant quantity depending on the 
 absolute amount of the electric charge, and m= - 
 
 and the integral is taken from t=0 to t=l. 
 
 When the values of r and / are given in any par- 
 ticular case, the integral may be obtained by the usual 
 
 E' 
 
 methods of integration, and the value of may be in- 
 
 E 
 
 D' 
 
 ferred, and this being known may be computed. 
 
 (165.) Among the numerous experiments made by 
 Coulomb, are several in which, after bringing into 
 contact two electrified spheres, and separating them, he 
 examined their state by means of his electrometer, and 
 ascertained the depths of electricity upon them, and the 
 relation between the amount of their total charges. By 
 applying the formula [_3^\ obtained in Proposition I. to 
 these particular cases, and comparing the quantities and 
 depths of electricity thus derived from the formula with 
 the actual quantities and depths given by the experi- 
 ments, the theory from which the formula has been 
 derived may be tested. 
 
 The experiments of Coulomb, to which we now refer, 
 were conducted in the following manner : One of the 
 two spheres was first electrified by being applied to the 
 prime conductor of an electrical machine, or by any 
 other means. It was then introduced into the electro- 
 meter in the place of the fixed ball, the moveable ball 
 having been previously charged with similar electricity. 
 The moveable ball being repelled by it, was brought back 
 to a certain distance from it, suppose, for example, 30, 
 by turning the micrometer of torsion, and the angle of 
 
378 ELECTRO-STATICS. PART I. 
 
 torsion necessary to keep the moveable ball at this dis- 
 tance was observed. 
 
 The electrified sphere was now withdrawn from the 
 electrometer, and brought into contact with the other 
 sphere. By this means the charge of electricity origin- 
 ally given to the former sphere was shared between 
 the two. The spheres being again separated, the sphere 
 first electrified was again placed in the electrometer. 
 Having lost a part of its charge of electricity, it now 
 exerted a less repulsion than before on the moveable 
 ball ; and on bringing the latter to stand at the same dis- 
 tance of 30 from it, a less angle of torsion was neces- 
 sary. The difference between the first angle of torsion 
 and the second must represent the electricity lost by the 
 contact, and must therefore represent the electricity im- 
 parted to the other sphere. Let this difference be A'; 
 and let A express the angle of torsion which kept the 
 sphere at 30 from the moveable ball after the contact. 
 Also let E and E' express the quantities of electricity on 
 the spheres respectively after the contact. It is evident 
 then that we shall have 
 
 Having thus ascertained the ratio of the whole quan- 
 tities of electricity on the two spheres, their relative 
 depths would be given by the formula 2.], the radii 
 being known. 
 
 In the interval which elapsed between the first intro- 
 duction of the electrified sphere into the electrometer 
 and its second trial after the contact, a loss of electricity 
 must have taken place both on the sphere and on the 
 moveable ball of the electrometer by contact of the air, 
 which would affect the result of the experiment. This 
 loss, however, was ascertained upon the principles ex- 
 plained in Chap. IV., and the necessary correction was 
 made in the angles of torsion. The angles A and A' 
 
CHAP. VII. MATHEMATICAL ANALYSES. 379 
 
 may therefore be considered as free from any error due 
 to atmospheric dissipation. 
 
 (166.) In some cases this method cannot be conve- 
 niently practised. If the magnitude of the greater of 
 the two spheres be too considerable to allow of its being 
 introduced into the electrometer, it would be necessary 
 to electrify in the first instance the smaller sphere ; but 
 if in that case the spheres be very unequal in magnitude, 
 the latter would undergo too great a change in its charge 
 of electricity by contact with the greater. In such cases 
 Coulomb proceeded in the following manner : The 
 greater sphere C was first electrified, and being insu- 
 lated, the smaller sphere C' was placed in contact with 
 it, by which this latter became charged with electricity. 
 The lesser sphere C'was then placed in the electrometer, 
 and the an^le of torsion necessary to retain the move- 
 able ball at a given distance from it was observed. Sup- 
 pose this angle to be Aj. The sphere C x being then with- 
 drawn from the electrometer, was deprived of its electri- 
 city by being touched by the hand. It was then again 
 brought into contact with the greater sphere C, and 
 being withdrawn its electricity was again discharged by 
 touching it. This was repeated a certain number of 
 times, the electricity acquired by C' at each contact 
 with C being always discharged before the next contact. 
 After the last contact, the sphere C' was again placed 
 in the electrometer, and the angle of torsion necessary 
 to retain the moveable ball at the same distance from it 
 was observed. Suppose this angle to be A n . The pro- 
 portion in which the electricity was shared between the 
 two spheres was inferred from A l and A w , on the follow- 
 ing principles. 
 
 Whatever be the quantity of electricity with which 
 C may be charged, it will be shared with C' by contact 
 in the same proportion. 
 
 If then E and E' be the quantities with which the 
 spheres are charged after the first contact, and if 
 E=#E', then z will express the ratio of the quantities 
 
380 ELECTRO-STATICS. PART I. 
 
 of electricity with which the spheres are charged after any 
 succeeding contact. By the second contact the quantity 
 E, with which the first is charged,, is therefore shared 
 between the two spheres in the proportion of x to 1. 
 
 % 
 The charge of the sphere C will then he E - , and 
 
 that of the sphere C' will be E - . After the third 
 
 \+z 
 
 contact, the quantity of electricity E . -- will be as 
 
 i j~ z 
 
 before, shared between them in the same ratio of # to 1 . 
 
 z 2 
 
 Therefore the charge of C will be E . - - - , and that 
 
 - 
 
 of C' will be E . - - r-. In like manner, after the n th 
 (1+*) 2 
 
 gn-l 
 
 contact, the charge of C will be E . -- ^-^^ and that of 
 C' wiU be E . 
 
 But since the repulsion of C' after the first contact 
 was balanced by the torsion A l} and after the w th con- 
 tact by the torsion A n , we shall have 
 
 E *"" "A 'A' 
 
 P" " "' 
 
 A. 
 
 ' (1+*)- 1 A, 
 
 Let - =y, and take the logarithms, and we obtain 
 1 -\-z 
 
 (n- 1) Jog. #=log. A n -log. A! ; 
 log. A n log. A! 
 
 
 
 Hence, when A n and AI are known by observation, y 
 
CHAP. VII. MATHEMATICAL ANALYSES. 381 
 
 may be found, from which z may be derived, and there- 
 
 E' 
 
 fore the ratio of the electricities will be known. 
 
 E 
 
 In the practice of this latter method of experimenting, 
 it is especially necessary to take into account the loss 
 of electricity by the contact of the air, and even the 
 loss by imperfect insulators ; for so considerable a time 
 must elapse between the moments at which the lesser 
 sphere is placed in the electrometer, that these losses 
 must be expected to be of sensible amount. In an ex- 
 periment made by Coulomb by this method he touched 
 the greater sphere C twenty times with the lesser sphere 
 C', and found it necessary to allow for a loss of one 
 eighth part of the whole electricity by the contact of 
 the air in the interval. It does not appear that he made 
 any correction for imperfect insulation ; and this, as we 
 shall see hereafter, exposed this experiment to more 
 extensive limits of error. 
 
 (167.) Coulomb's experiments to determine the pro- 
 portion in which electricity is shared between two 
 spheres brought into contact were made with spheres 
 whose diameters were in the ratio of 2 to 1, 4 to 1, 
 and 8 to 1. In the first two cases he proceeded by 
 the method explained in (165.), but in the last case he 
 adopted the method described in (166.). The results 
 of these experiments are given in the following table, 
 where the radius of the lesser sphere is supposed =1, 
 and the quantity of electricity on the greater sphere also 
 1. The depths are obtained by the formula [2.], 
 Proposition I., the depth on the greater being =1. 
 
 Radius of greater 
 sphere. 
 
 Quantity on lesser 
 sphere. 
 
 Depth on lesser 
 sphere. 
 
 2 
 
 0-27 
 
 1-08 
 
 4 
 
 0-0909 
 
 1-33 
 
 8 
 
 0-02566 
 
 1-64 
 
382 ELECTRO-STATICS. PART I. 
 
 To compare these experimental quantities with those 
 deducible from the theory, it is only necessary to make 
 the necessary substitutions in the formulae \_%.~\ and [3.], 
 and execute the calculations. 
 
 1. In [4.], Proposition L, let r=2, r'=l ; /. 
 
 w=--- ; and the integral in the denominator becomes 
 
 Let t=0 3 , and therefore ^~l= . Hence the integral 
 is transformed into 
 
 This being integrated between the limits 6=0 and 
 6=1, gives 
 
 E' D' 
 
 2. In [3.] let r=4,, /=!, /.w= . If, as above, 
 
 o 
 
 we put t=6 5 and proceed as before, we shall obtain 
 
 E" W 
 
 =0*0823, =1-316*8. 
 
 3. In \jk.~] let r=8, r x =l, .*.m= ; and if we pul 
 
 t=6 9 , we shall obtain 
 
 "p x T)' 
 
 =0-0226, =1-4443. 
 
 Placing then these results of theory in juxtaposition 
 with the corresponding quantities obtained by expe. 
 riment, the comparison is exhibited in the following 
 table 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 383 
 
 Radius of 
 greater 
 sphere. 
 
 Quantity of electricity 
 on lesser sphere. 
 
 Depth of electricity 
 on lesser sphere. 
 
 Ratio of the 
 observed to the 
 computed 
 quantities. 
 
 Theory. 
 
 Observa- 
 tion. 
 
 Theory. 
 
 Observa- 
 tion. 
 
 2 
 
 0-29 
 
 0'27 
 
 1-1601 
 
 1-08 
 
 0-93 
 
 4 
 
 0-0823 
 
 0-0909 
 
 1-3168 
 
 1-33 
 
 1-01 
 
 8 
 
 0-0226 
 
 0-02566 
 
 1-4443 
 
 1-64 
 
 1-14 
 1-03 
 
 Mean ratio of the observed to the computed 
 values -...-._ 
 
 In the first two cases the coincidence of theory and 
 experiment is evidently within the limits of the errors 
 of observation, and the greater discrepancy which ap- 
 pears in the last case is satisfactorily accounted for 
 by the nature of the experiment, as already explained 
 (166.) 
 
 PROPOSITION II. 
 
 (168.) When two electrified spheres are brought into 
 contact, and then separated, the electric fluid is spread with 
 a greater depth on the lesser than on the greater sphere; 
 taking the depth (D) on the greater as the unit, it is re- 
 quired to find the limiting value of the depth (D') on the 
 less when its radius r is indefinitely diminished. 
 
 By making certain transformations on the formula for 
 
 , Poisson has shown that when - - = 0, it will 
 D r-\-r 
 
 become - 
 
 l s ' 7 
 
 D' f 
 J 
 
 D' 111 
 
 _ -I J __ i ___ I ___ 
 
 *D ^^^- 
 
384 ELECTRO-STATICS. PART 
 
 This is a result which, from its nature, cannot be 
 verified directly by experiment. By gradually increas- 
 ing the ratio of the greater to the lesser sphere, Coulomb 
 conjectured that the limit of the ratio of the depths might 
 be about two to one, being about one fifth more than 
 the value given by theory. 
 
 The result of the experiment with two spheres whose 
 diameters are as eight to one, shows that beyond that 
 proportion the relative depths on the smaller sphere will 
 acquire no sensible increase. 
 
 PROPOSITION III. 
 
 (l6'9) Two electrified conducting spheres are main- 
 tained in contact : it is required to determine the depth of 
 the electric fluid at the points diametrically opposite the 
 point of contact. 
 
 Let X and X' be those depths, and let the former 
 symbols retain their significations. ByPoisson's formula 
 we have 
 
 rrfh ft* - t T 1 
 
 rV _ -log. . 
 
 ,(l- m y-h ft 2 -t 2 1 
 
 77 / . log . . dt . 
 
 4r *J 1 / / 
 
 PROPOSITION IV. 
 
 (170.) Two equal electrified spheres are maintained 
 in contact; to find the depth of the electricity at the points 
 diametrically opposite the point of contact. 
 
 In this case r=r'; .*. m==, which renders X=X X . 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 385 
 
 And we have 
 
 To prepare this for integration let t=6 4 , and we find 
 
 ... . . 
 
 r * 1-0 4 6 r * 1+^ 
 
 Expanding - this becomes. 
 
 X= . Al-2+9 4 -8 6 + ...Jlog.-J-.rf9. 
 
 P / 
 
 But between the limits 
 
 By taking a sufficient number of terms for the neces- 
 sary approximation, we find 
 
 To find in the same case the mean depth of the fluid 
 , we have, by Proposition I., 
 
 ...... 
 
 4>nr 2 r J lt 
 
 But in this case m=~. Hence 
 
 Hence we shall have D=0'693 . for the mean 
 depth. r 
 
 VOL. i. c e 
 
386 ELECTRO- STATICS. PART I. 
 
 Hence the ratio of the depth at the point most remote 
 from the point of contact to the mean depth is 
 
 X _Q-9i6 
 D ~0-693~ 
 
 (171.) Coulomb brought several pairs of unequal 
 spheres into contact, and ascertained by the proof plane 
 and the electrometer the ratio of the depth of the elec- 
 tricity on the smaller sphere at the point most remote 
 from the point of contact with the mean density on the 
 greater sphere. In order to compare his observations 
 with the theoretical values of this ratio, we shall inves- 
 tigate the particular case of the general formula,, which 
 will include these experiments. 
 
 PROPOSITION V. 
 
 (172.) To calculate the ratio of the depth of the elec- 
 tric fluid on the point of the lesser of two electrified 
 spheres in contact most remote from the point of contact 
 to the mean depth on the greater sphere^ in the cases 
 in which the radii of the spheres are in the ratios 
 1 ! 2, 1 .* 4, and 1 : 8 respectively. 
 
 By [7.] and [8.] we find 
 
 f,- 
 
 2 2 
 
 r r- 1 -! 
 J i-* ' 
 
 dt 
 
 First. Let r=2r', /. m= . Let t=fr\ and the 
 
 o 
 
 integrals in the numerator and denominator will become 
 respectively 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 387 
 
 Each of which may be obtained by series ; and taken be- 
 tween the limits 0=0 and 0=1, we shall find 
 
 = 1-884. 
 
 Secondly. Let r=4*r' } .'. m=- . In this case let 
 
 o 
 
 i=0 f) ,, and the integrals in the numerator and denomi- 
 nator become respectively 
 
 02 
 
 /0 02 1 
 
 .log. T .*; 
 
 /1-0 4 
 !=*'> 
 
 which being integrated by series between the same 
 limits, gives 
 
 =2-477. 
 
 Thirdly. Let r=8r' . .'.m = . In this case let 
 
 y 
 
 =0 9 , and the integrals in the numerator and denomi- 
 nator will become 
 
 C C 2 
 
388 
 
 ELECTRO-STATICS. 
 
 which, as before, being integrated by series between the 
 same limits, we find 
 
 ~ =3-087. 
 
 (173.) The results of Coulomb's experiments in these 
 three cases, and of a similar experiment with equal 
 spheres, are placed in juxtaposition with the three num- 
 bers determined here by theory and the number obtained 
 for the case of equal spheres in the following table. In 
 the fourth column the ratio of the computed to the ob- 
 served numbers is given in each case. The radius of 
 the lesser sphere is taken as the unit. 
 
 Radius of greater 
 sphere. 
 
 Value of 
 
 Ratio of observed to 
 computed value. 
 
 By Theory. 
 
 By Observation. 
 
 1 
 
 1*322 
 
 1-27 
 
 0-96 
 
 2 
 
 1-834 
 
 1-55 
 
 0-845 
 
 4 
 
 , 2-477 
 
 2-35 
 
 0-95 
 
 8 
 
 3-087 
 
 3-18 
 
 1-03 
 0-946 
 
 Mean ratio of the observed to the computed 
 values - - - 
 
 It appears, therefore, that the results of theory come 
 within a twentieth part of those of observation. 
 
 (174.) Coulomb brought two equal electrified spheres 
 into contact, and ascertained with the proof plane and 
 the electrometer, that at the point of contact arid in the 
 space immediately around it, there was no sensible quan- 
 tity of electricity on either sphere. He ascertained the 
 depths at 30, 60, 90, and 180, from the point of 
 contact, as compared with what the density would be if 
 the spheres were removed from each other's influence, 
 and the electric fluid allowed to diffuse itself uniformly 
 upon them. In order to compare these results of ob- 
 
CHAP. VII. THEORY AM) EXPERIMENT COMPARED. 38Q 
 
 servation with theory, we shall investigate the formula 
 for this case. 
 
 PROPOSITION VI. 
 
 (175.) Two equal electrified spheres are maintained 
 in contact: it is required to determine the depth of 
 electricity at any points upon them. 
 
 Let ip be the angular distance of any point P on 
 either of the spheres from their point of contact, and 
 let TT be the depth of the electricity at this point. By 
 the formulae established by Poisson, we have 
 
 x= (A A t -f A a -A s + . . . .); 
 where 2+l 
 
 This series will be sufficiently convergent for calcu- 
 lation, except for those values of cos. <p which are 
 nearly = 1 ; and as these correspond to points near the 
 point of contact, this is unimportant, since for such 
 points it is known that the depth is insensible. 
 
 (176.) It is evident that for equal values of cos. $ , 
 this will give equal values of a, and therefore that the 
 depth of the electricity at equal distances from the 
 point of contact is the same. 
 In fact, if either sphere be cut 
 by a plane P M P (fig. 35.), 
 perpendicular to the line joining 
 their centres, the depth of the 
 electricity will be the same at 
 all the points of the circle in 
 which this plane intersects the 
 spherical surface. 
 
 To determine the values corresponding to the dis- 
 tances selected in Coulomb's experiments, we have 
 only to substitute these distances, viz. 30, 60, 90 _, 
 c c 3 
 
390 ELECTRO-STATICS. PART I. 
 
 and 180 for <p in the values of A n} and to make the 
 necessary computations. Let the corresponding values 
 of x be x', x" , an'", and x"" , and we find 
 
 cos./= , '= 
 cos.p"= , x" 
 
 
 p'"=90, cos. /"=<), *'"=0-803. ; 
 
 p""=180, cos./"'=-l, 4*""=0-9J6,,. 
 
 At these four points Coulomb found that the angles of 
 torsion representing the proportional depths were in the 
 ratio of the following numbers : 
 
 A"=0-640 
 A x// =0-800 
 
 Taking the depth of the electricity at 90 from the 
 point of contact to be expressed by 800, the several 
 depths observed and computed are placed in juxta- 
 position in the following table, and the ratios of the 
 observed to the computed values are expressed in the 
 fourth column. 
 
 Distances from 
 point of contact. 
 
 Depth of Electricity. 
 
 Ratio of observed to 
 computed Depth. 
 
 By Theory. 
 
 By Observation. 
 
 30 
 60 
 90 
 180 
 
 137 
 599 
 803 
 916 
 
 166 
 640 
 800 
 
 846 
 
 1*21 
 
 1-07 
 0-91 
 0-92 
 
 j 
 1-025 
 
 Mean ratio of the observed to the computed 
 values ------ 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 3$l 
 
 Thus theory in this case comes within a 40th part 
 of the results of observation. Coulomb brought two 
 spheres, whose diameters were in the ratio of 1 to 2, 
 into contact, and having electrified them observed, with 
 the proof plane and the electrometer, the depth of elec- 
 tricity on each of them,, at points whose distances were 
 60, 90, and 180, from the point of contact. To 
 compare the results of theory with those experiments 
 in this case, we shall now investigate the formulae ap- 
 plicable to it. 
 
 PROPOSITION VII. 
 
 (177.) Two conducting spheres, the diameter of one of 
 which is double that of the other, are brought into contact 
 and charged luith electricity: it is required to determine 
 the distribution of the fluid on each of them. 
 
 I. On the smaller. 
 
 2 
 In this case r=2r', and m= . Let <p preserve its 
 
 o 
 
 signification, and let 
 
 )--2(7i + 2)ncos. p-f 
 Then, by Poisson's formula, we shall have 
 
 II. On the greater. 
 
 In this case m=---; and if x be the thickness, we shall 
 \j 
 
 have 
 
 #a= {A A,-f A 2 A 3 -j- . . . .} 
 
 where A n retains its former signification. 
 
 (178.) Two such spheres were submitted to experiment 
 by Coulomb, who observed the proportional depths of 
 c c 4 
 
ELECTRO-STATICS. PART I. 
 
 the fluid on the smaller sphere, and found that through- 
 out a range of 30 round the point of contact the depth 
 was insensible. He found that at 60, 90, and 180, 
 the proportion of the depths was represented by the fol- 
 lowing numbers : 
 
 60 - - 339 
 
 90 - -.577 
 
 180 - - 770. 
 
 If the values of Wi corresponding to these values of 
 <p be computed from the preceding formula, we shall 
 find them to be as follows : 
 
 60 - - 0-321 . 4 
 
 90 - - 0-577 
 180 - - 0-781 . 
 
 r 
 h 
 
 V 
 h 
 
 These results are placed in juxtaposition in the fol- 
 lowing table, and compared as before. 
 
 Distance from 
 point of contact. 
 
 Depth of Electricity. 
 
 Ratio of the ob- 
 served to the com- 
 puted Depth. 
 
 By Theory. 
 
 By Observation. 
 
 60 
 90 
 180 
 
 321 
 
 577 
 781 
 
 339 
 
 577 
 770 
 
 1-06 
 1-00 
 0-99 
 
 1-02 
 
 Mean ratio of the observed to the computed 
 value ______ 
 
 The result of theory, therefore, comes within a 50th 
 part of the mean result of observation. 
 
 (179-) Coulomb compared the depth on the two 
 spheres at 90 from the point of contact, and found 
 the depth on the lesser to exceed the depth on the 
 greater in the ratio of 125 to 100. 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 3Q3 
 
 If the value of # 2 corresponding to = 90 be com- 
 puted from the preceding formula, we shall find it to 
 
 be 0'928. = 0-464 . Hence the ratio of this depth 
 r r 
 
 to the depth at the corresponding points on the smaller 
 sphere is that of 0'46'4 to 0'577, or 124 to 100, very 
 nearly. The accordance of theory with observation is 
 here also perfect. 
 
 PROPOSITION VIII. 
 
 (180.) Two conducting spheres, whose diameters are 
 in the ratio of one to four, are brought into contact and 
 charged with electricity : to determine the distribution of 
 the fluid on the smaller sphere. 
 
 Preserving the signification of the symbols, let 
 8(* + 2) 
 
 and if oc l be the depth corresponding to <p, we shall 
 have 
 
 4* 
 
 *,== {Co-C t +C s -C 8 J- } 
 
 Coulomb submitted such spheres to experiment, and 
 observed the relative depths of the electricity on the 
 smaller sphere corresponding to (f> = QO and 0=180, 
 and found the ratio of the latter to the former to be 
 that of 143 to 100. 
 
 If these values be substituted for <j> in the above 
 expression, and the computations be made, we shall 
 find the corresponding values of x i to be for = 90 
 
 h h 
 
 ^ = 0-349. , and for = 180 a^O'584. . The 
 
 ratio of the latter to the former is that of 16? to 100 
 nearly. 
 
3$4< ELECTRO-STATICS. PART I. 
 
 (181,) It appears from all the cases, therefore., in 
 which electrified spheres have been observed in actual 
 contact, that the electrical state in which they are actu- 
 ally found to be is exactly the state in which the theory 
 of two fluids would enable us to predict that they should 
 be. When they are separated and removed from the 
 influence of each other, their electrical state is found 
 also to be in exact accordance with this theory. 
 
 It now remains to examine and to compare observ- 
 ation, so far as it has been carried, with theory, in the 
 cases in which two spheres without being in contact 
 are nevertheless placed so near each other that the elec- 
 tricity upon each of them shall exercise such an influ- 
 ence on the electricity upon the other as to cause it to 
 change its manner of distribution over the surface, ren- 
 dering its depth at various parts of the surface different 
 from what it would be if each sphere were uninfluenced 
 by the position of the other. We shall limit ourselves 
 here to particular cases in which the mathematical re- 
 sults are least complex, and which may be verified most 
 easily by observation. 
 
 PROPOSITION IX. 
 
 (182.) Two conducting spheres of given diameters are 
 placed at a given distance from each other, and charged 
 with given quantities of electricity : it is required to 
 determine the manner in which the fluid will be distri- 
 buted on each of them by means of their mutual in- 
 fluence. 
 
 Let the symbols in the former propositions retain 
 their signification, and let the distance between their 
 centres be a, and let the radius of the smaller sphere r' 
 be inconsiderable compared with a r. The angle <p is 
 understood to be measured from the line joining the 
 centres. Also let D and D' be the mean densities of 
 the fluids on the spheres respectively. The whole quan- 
 tities with which the spheres are charged and their 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 39^ 
 
 magnitudes being known, the depths D and D' must be 
 known. 
 
 By Poisson's formula we shall then have 
 
 j? 1= D-f V -& ' 
 
 j-^D'-SD . ~ cos. /+ (1 -3 cos.y) . ^- . D; 
 
 where (f> refers to the sphere whose radius is r, and $' to 
 that whose radius is r'. 
 
 It is necessary to observe, that D and D' are to be 
 taken with a positive or negative sign, according as 
 the electricity with which the spheres are respectively 
 charged is positive or negative. Also the values of 
 or y' which render x or x' positive correspond to 
 those parts of the spheres where the electricity is posi- 
 tive, and those which render x or of negative corre- 
 spond to points where the electricity is negative. 
 
 (183.) To investigate the particular case in which 
 the lesser sphere in its natural state is placed at the 
 distance a, under the influence of the greater sphere 
 charged with electricity, it is only necessary to sup- 
 pose D' in the above formula for o? 2 . This gives 
 
 ,zv>= -3D . cos. 0'H -- (1 -3 cos.V) D. 
 a- 2 a* 
 
 Let S and S x (fy. 36.) be the two spheres. 
 Fig. 36. 
 
ELECTRO-STATICS. 
 
 To determine the state of the sphere S' at the 
 point V, where the line CC X joining the centres meets 
 the surface, let $'=(), and we find 
 
 3 a 
 
 If the electricity of the sphere S be positive, this value 
 of #2 is negative ; and if the former be negative, this 
 value is positive. Hence the electricity at V is of a 
 different kind from that with which the sphere S is 
 charged. 
 
 (184.) To determine the state of the sphere S' at 
 the point V" most distant from the sphere S, let 
 (b'=l80 : /. cos6= 1. Hence 
 
 3 
 
 Since we have proceeded on the supposition that r is 
 inconsiderable in magnitude compared with a r, it 
 
 r' 
 
 is still more so compared with a. Hence must be 
 
 a 
 
 3 5 T' 
 less than , and therefore 1 > . Hence this value 
 
 D O Ck 
 
 of #2 must have the same sign as D, and therefore the 
 electricity at the point V" is of the same kind as that 
 with which the sphere S is charged. It is conse- 
 quently of a different kind from the electricity at the 
 point V. 
 
 The ratio of the depth of electricity at the point V 
 to the depth of the contrary electricity at the opposite 
 
 5 r e > r' 
 
 point V" is that of H to 1 - -. The 
 
 3 a 3 a 
 
 depth at V therefore is greater than at V". 
 
 By means of the proof plane and the electrometer, 
 this inference might be verified by experiment without 
 
 5 r' 1 
 difficulty. Thus, if a = 10r', = -^-. Hence 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 397 
 
 the depth at V will, in this case, be to the depth at 
 
 V" as 1 + -7T- is to 1 --, or as 7 to 5. In general, 
 O o 
 
 Y' 
 however, when is very small, these depths are nearly 
 
 equal. 
 
 (185.) Since the electricities at the opposite points 
 V and V" are of different kinds, it is evident that 
 some intermediate part of the sphere S' must be in its 
 natural state. To determine at what distance from V 
 this takes place, it is only necessary to find the value of 
 fy', which renders # 2 =0. Thus we have the condition 
 
 5 ' 
 
 3 cos. /H (1 -3cos.y ) . =0. 
 
 d 
 
 This equation will in general give two values of cos.0', 
 but it will be found that one of them is < 1, and 
 would therefore correspond to an imaginary value of fy'. 
 The other only is admissible. This value is 
 
 1 -f ' ) 2 be, developed in a series, and the 
 
 r' 
 terms involving the third and higher powers of 
 
 be neglected, we shall find 
 
 Cos. 0'=-^- 
 O a 
 
 A circle on the surface of the sphere S', at the dis- 
 tance thus determined from the point V, passes through 
 all the points of the sphere which are in their natural 
 state. 
 
 This circle can be easily found by geometrical con- 
 struction. The distance of its plane from the centre C' 
 will be r' cos. <'. But 
 
398 ELECTRO-STATICS. PART I. 
 
 5 r'2 
 
 T COS. (L =- F - . . 
 
 6 a 
 
 From the centre C of the sphere S let a tangent 
 C P' (fig. 36.) be drawn to the sphere S x , and from the 
 point of contact P' let a perpendicular P / M / be drawn 
 
 ? /2 
 
 to the line C' V. The distance C'M'= ; and there- 
 
 a 
 
 fore if C'M be taken equal to five sixths of C'M', 
 and M P be drawn perpendicular to C C', the circle P P 
 will be that which corresponds to the points of the sur- 
 face of the sphere which are in their natural state. 
 
 (186.) By the common principles of analysis, it ap- 
 pears that all values of cos. ' greater than that which 
 renders # 2 =0 will render it >0, and all less values 
 will render it < 0. Hence it is easily inferred that on 
 all parts of the surface of the spherical segment, whose 
 base is the neutral circle P P, and whose vertex is V, 
 the electricity is of a different kind from that with 
 which the sphere S is charged, and on all parts of the 
 surface of the spherical segment having the same base, 
 and whose vertex is V", the electricity is of the same 
 kind as thfct with which the sphere S is charged. 
 
 It can also be inferred that the depth of the electri- 
 city continually increases from the neutral circle to the 
 points V and V", at each of which it is a maximum. 
 
 As the sphere S' is gradually removed from S, the 
 point of contact P' of the tangent continually approaches 
 the point Z, which is the extremity of the diameter 
 perpendicular to C C' ; and since the neutral circle P 
 always lies between P' and Z, this circle must approach 
 to the great circle of the sphere perpendicular to the 
 line C C' as the distance of S' from S is increased. Tt 
 appears, therefore, that when a small sphere, such as a 
 pith ball, is placed under the influence of an electrified 
 sphere, but at a distance from it many times greater 
 than the radius of the small sphere, then that hemi- 
 sphere of the small sphere which is turned towards the 
 electrified sphere will be charged with electricity differ- 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 3^9 
 
 ent from that of the electrified sphere, and the other 
 hemisphere will be oppositely electrified. 
 
 We shall now examine a case of the mutual influence 
 of two electrified spheres, to which we have formerly 
 referred, and which was presented in the experimental 
 researches of Coulomb. 
 
 PROPOSITION X. 
 
 (187.) Two unequal spheres in contact are electrified, 
 and then separated: it is required to determine the 
 distribution of electricity on them, when placed under 
 the influence of each other at a distance. 
 
 In order to avoid very complex mathematical formulae, 
 we must here limit the question to the case in which 
 
 r . 
 
 is a small fraction ; that is, to the case in which the 
 r 
 
 two spheres are very unequal in magnitude. 
 
 It appears by what was proved in (168.) that under 
 these circumstances 
 
 D 6 
 But by Proposition IX. we have 
 
 ^=D X -3D.-^ cos. 0'+ (1-3 cos. V).-^-.D 
 a- 2 a 3 
 
 r 
 
 Omitting the terms of which is a multiplier, and eli- 
 
 a 
 
 minating D x _, we obtain 
 
 / W 2 r 2\ 
 
 a7 2 =D ( - 3 cos. <p' . -- ) . 
 
 To determine the state of S' at the point V, let 
 cos. p'=l , and we have 
 
 ' 
 
400 ELECTRO-STATICS. PART I. 
 
 This will have the same sign with D if -^->3 ;, or 
 
 o a- 1 
 
 > . It will be =0 if = , and will have 
 
 a sign contrary to that of D if < . But 
 
 3 V 2 a 3^2 
 
 3 
 = - very nearly. Hence we infer that the electricity 
 
 at V is similar to or different from that with which S 
 is charged, according as the distance of the centre C' of 
 the lesser sphere from the surface of the greater is 
 greater or less than one third of the radius of the greater ; 
 and that if its distance from that surface be equal to one 
 third of the radius of the greater, then the point V will 
 be in its natural state. 
 
 (188.) Let us briefly recapitulate these remarkable 
 consequences. If a small sphere (S') be placed in 
 contact with a great electrified sphere (S), the elec- 
 tricity at the point of contact (V x ) will be nothing., 
 that point being in its natural state. When they are 
 separated (that point (V) being still kept in the line 
 of centres), the point V will be electrified by an elec- 
 tricity contrary to that of the sphere S. This elec- 
 trified state of the point V' will first gradually increase, 
 and having attained a maximum, then gradually de- 
 crease in intensity as the sphere S' is moved from the 
 sphere S, until the distance of the centre (C') of the 
 smaller sphere from the surface of the greater sphere 
 becomes equal to one third of the radius of the greater 
 sphere (very nearly), and at that distance the point V 
 will return to its natural state. If the smaller sphere 
 be moved to a greater distance from the greater sphere, 
 the point V will be electrified with electricity similar 
 to that with which the greater sphere is charged. 
 
 (189-) To simplify the formulae, we here suppose 
 the radius of the lesser sphere to be so small that 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 401 
 
 terms of which is a multiplier may be neglected ; 
 r 
 
 but if the spheres be unequal, whatever be the propor- 
 tion of their radii, a series of phenomena similar to 
 that explained above will be produced, and the for- 
 mulae given by Poisson will always be sufficient to 
 determine by calculation the distance at which the point 
 V of the lesser sphere is reduced to its natural state. 
 These results of theory have been confirmed by ex- 
 periment, but it is desirable that such experiments 
 should be multiplied and varied. Such experiments 
 may be made by providing several spheres with me- 
 tallic surfaces, mounted on glass pillars, and attended 
 with all the usual precautions, to secure perfect insula- 
 tion, and to preclude, as far as possible, all error arising 
 from the effect of atmospheric dissipation ; the experi- 
 ments should be made in cold and dry weather, when 
 the air holds but little vapour in suspension. The 
 supports should be so regulated that the centres C C' 
 (fig. 37.) should be at the same height. After having 
 
 Fig. 37. 
 
 brought two such spheres into 
 contact, and electrified them, 
 they should be separated to a 
 very small distance. The proof 
 plane should be then applied 
 at V, the point of contact on 
 the greater sphere. On test- 
 ing it by an electroscope, it 
 will be found to indicate elec- 
 tricity of the same kind as that 
 with which the spheres were 
 charged. After having dis- 
 charged the electricity of the 
 proof plane, let it be next applied to the point V' on the 
 lesser sphere. On testing it in the same manner, it will 
 be found to indicate electricity of a different kind from 
 that of the greater sphere. Let the sphere S' be moved 
 a little further from S, and let the same process be re- 
 peated, and the same results will be obtained ; and this 
 
 VOL. I. D D 
 
402 ELECTRO- STATICS. PART I. 
 
 will continue until S' is removed to a certain distance,, 
 at which the point V will be found in its natural 
 state. 
 
 (190.) Before these results of theory were discovered. 
 Coulomb obtained,, in several of his experiments, a series 
 of phenomena similar to the preceding. In one of his 
 experiments,, the globe S was eleven inches and S' was 
 eight inches in diameter. So long as the distance V V 
 was less than an inch, the electricity at V was contrary 
 to that of the globe S. When the distance was equal to 
 an inch, the sphere S' at V was in its natural state, and 
 beyond this distance the electricity at V' was similar to 
 that of the globe S. 
 
 When a globe, S', of four inches in diameter was 
 brought into contact with one of eleven inches, the point 
 V was not reduced to its natural state until the distance 
 V V was two inches ; and when the smaller globe was 
 two inches diameter, this state was produced at the dis- 
 tance of two and one third inches. 
 
 It appears from these experiments that the distance 
 V V, at which all signs of electricity disappear at the 
 point V, diminishes as the spheres approach to equality; 
 and when they are actually equal this distance vanishes, 
 the neutral state of V being then produced by contact. 
 On the other hand, however small the sphere S' may be, 
 this distance, VV, never can exceed the limit deter- 
 mined in Prop. X. In all these remarkable circum- 
 stances, theory and observation are in the most complete 
 accordance. 
 
 (191.) In the beautiful Memoirs* of Poisson on the 
 theory of electricity, he has given formulae for the solution 
 of the general problem of the distribution of electricity 
 on the surfaces of any two spheres, each charged with 
 any given quantity of electricity of either kind, and 
 placed under the influence of each other at any given 
 
 * The mathematical student will be well rewarded for the labour of studying 
 these fine pieces of mathematical physics, and the experimental electrician 
 who may not be able to comprehend their details will be at least directed 
 by their results to the best course of experiment. See Memoir -es de Fln~ 
 liitut, 1811. 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 403 
 
 distance. In the great generality of this problem he 
 has far outstripped the limits to which experimental 
 research has yet attained, and has placed the theory in 
 a condition to supply the experimental inquirer with 
 information as to a variety of circumstances affecting the 
 electrical state of bodies, which will serve to guide his 
 proceedings in the course which will lead most surely to 
 the extension of knowledge in this part of physics. 
 
 It appears by these formulae that the phenomena 
 evolved by the mutual influence of two electrified spheres 
 will have an important dependence of the relation be- 
 tween the quantities of electricity with which they are 
 charged, and the quantities with which they would be 
 charged if they were brought into contact. 
 
 (192.) Let a certain quantity of electricity, which we 
 shall call Z, be shared between two spheres, so that E 
 being the portion given to one, E' will be the portion 
 given to the other. We shall then have 
 
 Now if the spheres, being charged with the total quan 
 tity of electricity expressed by Z, be brought into con 
 tact, the fluid will distribute itself between them in a 
 certain proportion determined by the ratio of their radii. 
 Let the portion which each would receive under these 
 circumstances be e and e\ We shall therefore have, 
 also, 
 
 Z=e + e'. 
 
 If the quantities E and E' are different from e and e' 
 (which generally happens when the spheres are electrified 
 without contact), and the spheres be placed as in fig. 37 , 
 and moved towards each other, the depth of electricity 
 at the points V and V must undergo a continual in- 
 crease as the distance V V is diminished. Now, since 
 the pressure of the electricity at any point of a con- 
 ductor against the air which confines it upon that sur- 
 face increases as the square of the depth, this continual 
 augmentation of pressure will at length enable the fluid 
 to overcome the resistance of the air at V or V; and 
 D D 2 
 
404 ELECTRO-STATICS. PART I. 
 
 when it does so it will escape, and will pass from the 
 one surface to the other ; and such an effect will usually 
 be accompanied by a visible spark, and such a noise as 
 is produced by the impact of one hard substance against 
 another. The state of the points V and V before the 
 spark is oppositely electrical, the one being positive and 
 the other negative. If the electricities E and E' are 
 of the same kind positive, for example and if E be 
 greater than E', then, as the spheres approach to con- 
 tact, the electricity at V x will become negative, while the 
 electricity at V will continue positive, and will increase 
 in depth. But if E and E' are of opposite kinds, then 
 the electricities of the points V and V, as they ap- 
 proach, will be of the same kind as that of the elec- 
 tricities with which the spheres respectively are charged. 
 
 In the case in which E and E x are of the same kind, 
 E being greater than e, and E' less than e', the influence 
 of the E upon E' is to repel it towards the more remote 
 parts of the sphere S', and at the same time E by its 
 attraction and repulsion decomposes a portion of the 
 natural electricities of the sphere S', drawing towards the 
 point V that part of the decomposed electricity which 
 is of a contrary kind from itself, and driving the electri- 
 city of the same kind towards the opposite side. 
 
 When the spark is produced and contact takes place, 
 the distribution of the electricity Z between the spheres is 
 suddenly changed, and the quantities with which the 
 spheres are respectively charged become e and e'. 
 
 These phenomena would not be produced if the con- 
 dition 
 
 E e 
 
 were observed by the electricities E and E'; that is to 
 say, if the electricities were imparted to the spheres in 
 the same proportion as they would be if electrified in 
 contact. This easily appears from the mathematical for- 
 mulae. For the first term in the expression of the value 
 of the depth of the electricity at V and V has the form 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 405 
 
 where D and D' are the mean depths of E and E', and 
 d and d' the mean depths of e and e ', and Y is a quan- 
 tity which involves log. {a (r + r')}. Now if the 
 condition 
 
 be not fulfilled,, the numerator of the above expression 
 will be finite. Hence as a is diminished, and approaches 
 to equality with (r -f- r'},a (r + O will be diminished; 
 and since {a (r -j- r') } log. {a - (r -f r') } becomes = 
 when a (r + r') becomes =0, it follows that the above 
 quantity must become infinite when the spheres come 
 into contact; and consequently before that takes place, 
 the atmospheric pressure must be overcome, and the in- 
 terchange of electricity take place, which is usually at- 
 tained by the spark and explosion. 
 But if the condition 
 
 5 JL 
 
 V~~ d' 
 
 be observed, then the numerator of the above will be 
 and the value of the depth at V and V will be = 0, and 
 no discharge will take place., since no electricity will pass 
 from either sphere to the other. 
 
 (193.) Among the various consequences of the theo- 
 retical investigations of Poisson, one is entitled to more 
 particular notice, as it suggests an experiment which 
 affords a striking verification of the theory. He has 
 shown that a relation may be adopted between the 
 radii of the two spheres and the quantities of electri- 
 city with which they are charged, so that there shall be 
 
 certain distance of the centre of the lesser sphere S' 
 from that of the greater S, at which the electricity shall 
 oe distributed uniformly over the lesser sphere, exactly 
 as it would be if that sphere were removed altogether 
 
406 ELECTRO-STATICS. PART I. 
 
 from the influence of the greater. This state, however, 
 is not owing to the sphere S exercising no action on S', 
 but from a peculiar condition of equilibrium arising 
 from the reciprocal action of the two spheres. 
 
 (194.) The difficulty which is presented in reducing 
 this and other results of the theory to the test of ex- 
 periment, is that which attends the process of giving to 
 the two spheres charges of electricity which have any 
 required ratio to each other. This may, however, be 
 effected by the following method. 
 
 The two spheres are first placed in contact, and elec- 
 trified. Being separated, the electricity will be dis- 
 tributed between them in the ratio e to e' ', which is 
 due to contact, and which is known by theory. This, 
 however, may be confirmed by the proof plane and 
 the electrometer. Let the ratio of the required charges 
 
 |7 in 
 
 be , and let us suppose that -? > . In that case 
 & e & 
 
 it will be necessary to deprive the sphere S of a part of 
 its electricity, to reduce the ratio of the charges to 
 
 TT V P 
 
 ;. Now, let -=7M, =m, and let E'=e'. 
 Jii Jit e 
 
 Hence E-e = (M-m}E'=^-^ . E. 
 Hence it appears that of the whole charge E a 
 fractional part is to be removed, expressed by ; 
 
 and as M and m are both given, the question is reduced 
 to the discovery of a method of abstracting from the 
 electrical charge of a sphere a given part. 
 
 Now, it has been shown (101.) that if a circular 
 disc of metal be brought into contact with a sphere 
 charged with electricity, the fluid will distribute itself 
 between the sphere and disc in proportion to the mag- 
 nitude of their surfaces. If, therefore, the magnitude 
 of the disc be a certain fractional part of the magnitude 
 of the spherical surface, the disc by contact will receive 
 a proportional part of the electricity with which the 
 
CHAP. VII. THEORY AND EXPERIMENT COMPARED. 407 
 
 sphere is charged ; and being removed from the sphere, 
 the latter will be deprived of that proportion of its 
 charge. 
 
 To apply this principle, let A be the total superficial 
 magnitude of the disc, including both surfaces ; and let 
 S be that of the sphere. Then, if E be the charge of 
 
 the sphere, the disc will receive the quantity - . E. 
 
 V^e shall then have 
 
 M m A 
 
 /. A=S . 
 
 By which the magnitude of the disc, which will take 
 the necessary portion of electricity from the sphere, 
 may be determined. 
 
 To reduce these expressions to terms involving only 
 the radii of the disc and sphere, let r= the radius of 
 the sphere, and R=the radius of the disc. Hence 
 we have 
 
 ...R=r 
 
 y 
 
 (195.) It appears, from the theoretical expressions 
 obtained for the depth of electricity at various parts of 
 two electrified spheres brought near each other, that 
 these depths are, even when the spheres are nearest 
 each other, materially different from what they are 
 after contact. This difference is not confined to those 
 parts of the spheres which are near the point of con- 
 tact, but prevails throughout their entire surfaces. It 
 follows, therefore, that at the moment of contact, or, 
 more strictly, at the moment of the spark passing be- 
 tween them, a sudden change is produced in the depth 
 of the electricity on every part of each of the spheres. 
 This is equally true with respect to conductors of every 
 form, and may be easily verified by experiment. For 
 
 D D 4 
 
408 ELECTRO-STATICS. PART I. 
 
 this purpose let a conductor, such as is represented in 
 %. 38 ., having several pairs of pith balls placed upon 
 
 Fig. 38. 
 
 { 1 jf Jf j' # 
 
 A 1 A A\ A 
 
 it, suspended by linen thread, be electrified. The di- 
 vergence of the balls will be greater or less, according 
 to the depth of electricity, at the parts where they are 
 suspended. If a metallic sphere be brought into con- 
 tact with this conductor, and a spark passes between 
 them, all the pith balls will undergo a sudden change, 
 each pair changing instantaneously their angle of di- 
 vergence - 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED. 409 
 
 CHAP. VIII. 
 
 ELECTRICAL ATTRACTIONS AND REPULSIONS EXPLAINED. 
 
 (196.) HAVING explained the theory by which the 
 phenomena of electricity have been brought under the 
 dominion of mathematics, and by which the effects of 
 bodies electrified under given circumstances may be 
 predicted, it now only remains to complete this branch 
 of our subject by showing how, by the agency of the 
 two electric fluids, the movements which are imparted 
 to electrified bodies brought near each other can be ac- 
 counted for. 
 
 It appears, in the first place, to be satisfactorily esta- 
 blished by direct experiment, that the electric fluids exert 
 no force of attraction or repulsion on the material par- 
 ticles of the bodies upon which those fluids are dif- 
 fused. The immediate escape of electricity from con- 
 ductors, when relieved from the atmospheric pressure, 
 is a sufficient evidence of this with regard to that class 
 of bodies ; and although its departure from noncon- 
 ductors under like circumstances is not quite so rapid, 
 there is nothing in the phenomena to indicate the 
 existence of any attraction general or specific between 
 the molecules of electricity and the particles com- 
 posing these bodies, the nonconducting quality being 
 simply one by which the superficial movement of the 
 electric fluid on the body is prevented or very much 
 obstructed. Whether,, then, we consider the one class 
 of bodies or the other, the conductors or the noncon- 
 ductors, the atmosphere must be regarded as a coating 
 of nonconducting matter, under and within which is 
 confined the shell of electric fluid by which the body is 
 invested. The form and thickness of that shell are de- 
 
410 ELECTRO-STATICS. PART I. 
 
 ter mined by the forces exerted by the molecules of elec- 
 tricity on each other, and on the natural electricities of 
 the body, as well as by the forces exercised on them by 
 the electricity of other bodies in the neighbourhood ; 
 but the shell itself is held together by the atmosphere 
 alone. In fact, the surrounding atmosphere confines 
 this shell of electric fluid exactly as gas is confined 
 within a bladder. 
 
 (197.) Let us first, then, consider the case of the 
 attraction or repulsion which is apparently exerted by 
 an electrified body, S, on a nonconducting body, S', also 
 electrified. 
 
 In this case the electric fluid on S attracts or repels 
 the electric fluid on S', each molecule of the one fluid 
 acting separately on each molecule of the other, accord- 
 ing to the laws already assigned. Now, since the pres- 
 sure of the surrounding air prevents any motion of the 
 fluid in a direction perpendicular to the surface of S', 
 and the nonconducting power forbids any superficial 
 motion of the particles of fluid, no change of position 
 inter se can ensue, and the shell of electric fluid will 
 preserve its form exactly as if it were solid matter 
 encrusting the body S'. The attraction or repulsion of 
 the fluid on S must therefore cause a motion of the en- 
 tire shell of fluid on S' to or from the body S ; and as 
 this preservation of the form of the electric shell neces- 
 sarily requires the continuance of the body which it in- 
 vests within it, that body must accompany it as it moves 
 to or from the body S. 
 
 (198.) This may be illustrated in the following 
 manner. Let us suppose a sphere of cork to have its 
 surface covered by iron dust, and imagine this 4ust to 
 be pressed against the surface of the cork by a surround- 
 ing atmosphere whose pressure is sufficient to prevent 
 its escape from the surface. Also suppose that the 
 roughness of the surface of the cork is sufficient to pre- 
 vent the particles of iron from moving upon it. Let 
 this sphere be placed near a powerful magnet. The 
 iron will be strongly attracted, and, if free, would leave 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED. 411 
 
 the cork and fly to the magnet. But this is prevented 
 by the causes just stated. The iron can neither leave 
 the cork nor shift its position upon it. It must there- 
 fore move towards the magnet in virtue of the attraction 
 exerted on it, carrying the sphere of cork which it in- 
 vests along with it. 
 
 (199.) If the hody S' attracted or repelled be a con- 
 ductor, the circumstances are somewhat different. In 
 that case, the electric fluid upon it has a freedom of 
 superficial motion more or less perfect in proportion to 
 the conducting power of the body. There is, however, 
 a certain distribution which it will assume, and in which 
 it will maintain itself, depending on the form of the 
 body S x , and on the influence exerted by the electricity 
 of S. So long as the position of S and S' remain un- 
 changed, the electric fluid on S', thus distributed, is 
 stationary. Let us suppose that while S' is fixed in any 
 given position it were suddenly to be deprived of its 
 conducting power, and to become a nonconductor, and 
 left at liberty to move. Then S' would move towards 
 or from S, as we have already shown that an electrified 
 nonconductor would do. Let it be supposed that this 
 motion is allowed to take place through a very small 
 space, which we shall call dx ; and after having moved 
 through this space, suppose the conducting power of S' 
 to be restored, and the body again to be fixed. The 
 influence of the electricity of S on that of S x will now be a 
 little different in consequence of the small change, dx, 
 which has taken place in the distance between them. On 
 the restoration of the conducting power, the electricity 
 of S'will undergo a change in its distribution on the 
 surface of that body, and it will assume that distribution 
 which corresponds to the new relative position which the 
 two bodies have assumed. 
 
 When the electric fluid on S' has come to rest after 
 this change, let S' be supposed a second time to be de- 
 prived of its conducting power, and to be at liberty again 
 to move. As before, it will move, as a nonconductor 
 would do, to or from S. After it has moved through a 
 
412 ELECTRO-STATICS. PART I. 
 
 small space, doc, let it be stopped as before, and let its 
 conducting power be restored. Another change in the 
 distribution of electricity will now take place, and the 
 fluid will take a new position of equilibrium on S', con- 
 formably to the change which has taken place in the 
 relative position of the two bodies. If, in like manner, 
 the conducting power be once more annihilated, and the 
 body S x liberated, another small motion, doc, will take 
 place, and so on. 
 
 Now, let it be supposed that this succession of changes 
 shall proceed without any intermission, and that the 
 spaces dx, which divide the intervals at which the elec- 
 trical state of S x undergoes a sudden change, are infi- 
 nitely small. In fact, suppose the change of the distri- 
 bution of electricity on S' not to be intermitting but 
 continuous, and the progress of S towards or from S' not 
 to be made by a succession of movements broken by in- 
 tervals of rest, but to be made by one continued motion, 
 then the same reasoning would hold good, and the actual 
 effects which take place when an electrified conductor is 
 placed near another electrified body would ensue. 
 
 (200.) In this illustration we have regarded only the 
 effects produced on one of the two electrified bodies, con- 
 sidering the other as fixed ; but the same reasoning will 
 be evidently applicable to the other, and the reciprocal 
 attractions and repulsions thus become matter of easy 
 explanation. When the bodies thus mutually acted upon 
 are conductors, it is not only the distribution of the 
 electricity with which they are originally charged that 
 undergoes a continuous change with their continuous 
 change of mutual position, but there is a constant deve- 
 lopment of natural electricity, by which their electrical 
 condition is affected. This, however, does not in any 
 way interfere with the reasoning by which we have here 
 accounted for the reciprocal movements by which such 
 bodies are affected. 
 
 One of the two bodies, as S', may be originally in its 
 natural state, and it nevertheless will exhibit the effects 
 of an attraction exerted upon it by an electrified body, 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED, 413 
 
 S, in its neighbourhood. In this case, a decomposition 
 of its natural electricities takes place before any motion 
 is produced,, and a shell of free electricity is formed at 
 its surface, a part of which is of one kind, and a part 
 of the other. This shell, being acted upon by the elec- 
 tricity of S, the effect of the attraction on the nearer 
 portion of electricity (of a contrary kind to that with 
 which S is charged) is greater than the repulsion exerted 
 on the electricity of the same kind which is on the side 
 of S' most remote from S. The whole effect is, there- 
 fore, an attraction on the shell, by which it is drawn 
 towards S, and with it, according to the reasoning 
 already explained, the body S', enveloped by this shell 
 of electricity, is carried. 
 
 Such appears, according to my views, to be the manner 
 in which the apparent attractions and repulsions of elec- 
 trified bodies may be accounted for, on the principles of 
 the theory of two fluids. As this, however, is not the 
 manner in which these effects are usually explained, and as 
 the question may be important, involving, as it does, the 
 application of the theory to the most familiar effects of 
 electricity, we shall here briefly state the method of 
 accounting for these phenomena, proposed by M. Biot, 
 and adopted and quoted by most other elementary writers 
 on this part of physics. 
 
 Let S' (fig. 39.) be a sphere of conducting matter 
 
 Fig. 39. 
 
 charged with either kind of electricity, positive for exam- 
 ple, and let it be placed near another sphere also positively 
 electrified. By the influence of the electricity of the sphere 
 
414 ELECTRO-STATICS. PART I. 
 
 S, the electricity of S' will not be distributed uniformly on 
 '"t, as would otherwise be the case, but will accumulate in 
 greater quantity on the side most remote from S. In 
 fact, the inferior surface of the electric fluid on S' may, 
 in a general sense, be imagined to be represented by the 
 dotted circle. It is proved by theoretical principles 
 that the electricity confined upon a body by the sur- 
 rounding air presses against the air with a force which 
 is proportional to the square of the depth of the electric 
 fluid. In the present case, the depth of the fluid on the 
 hemisphere S' N s is greater than on the hemisphere 
 S' M s; and if the sum of the squares of the depths at 
 every point of the one be compared with the sum of the 
 squares of the depths at every point of the other, the ex- 
 cess of the sum on the hemisphere S' N s would be much 
 greater than on the hemisphere S' M s. The electric 
 fluid on S' N s, therefore, presses against the atmosphere 
 in the direction N with greater force than that with 
 which the fluid on S' M s presses in the contrary direc- 
 tion ; and therefore the fluid and the body invested by 
 it must move in the direction of the greater pressure ; 
 that is, must recede from the body S, which is similarly 
 electrified. Hence an apparent repulsion will take 
 place. 
 
 If the sphere S' be negatively electrified, the fluid 
 upon it will accumulate in greater quantity on the side 
 S' M s, and its inferior surface will be represented 
 by the dotted circle in fig. 40. In this case, for the 
 
 Fig. 40. 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED. 415 
 
 reasons explained above, the fluid on the hemisphere 
 S'M s will press against the air with greater force thai? 
 the fluid on the hemisphere S'Ns, and a movement of 
 the sphere S' in the direction N M will ensue. Hence 
 an apparent attraction of the sphere S electrified posi- 
 tively will be exerted on the sphere & electrified nega- 
 tively. 
 
 Such is the ordinary method of explaining these 
 effects, and the following illustration of it is given by 
 M. Biot: "Although the proposition which declares 
 that similarly electrified bodies attract, and oppositely 
 electrified bodies repel each other/' says M. Biot, ' c ap- 
 pears to be a mere statement of the phenomena, we 
 must beware not to attach to these terms (attraction and 
 repulsion) any notion of absolute reality, since mo- 
 tions of the same kind may be produced without any 
 real attraction or repulsion existing between the parti- 
 cles of the two bodies. As an example of this, let us 
 imagine a glass vessel AB (fig. 41.) filled with a heavy 
 
 Fig. 41. 
 
 fluid, such as water or mercury, and suspended vertically 
 by a cord from a fixed point D. So long as this vessel 
 is untouched, it will remain at rest, according to the 
 common laws of statics, and the fluid which it contains 
 will suffer no motion in the horizontal direction, since 
 it presses with equal force on opposite points of the 
 inner surface of the vessel. But suppose that, by means 
 of a burning glass, a cone of rays is directed upon a 
 point A. in the side of the vessel, and a hole is made 
 
41 6 ELECTRO-STATICS. PART I. 
 
 by melting the glass. The fluid will issue from this 
 hole, and the pressure at A, in the direction B A, 
 will be removed ; while the pressure at the point B, 
 corresponding to A on the other side of the vessel, will 
 continue. The vessel and its contents will accordingly 
 yield to the excess of pressure in the direction A B, and 
 will deviate from the vertical position, in the same 
 manner as if it were repelled by the lens E, 
 
 " If, on the other hand, the cone of rays be directed 
 on the point B, as represented in fig. 42., and the hole 
 be made there, the contrary 
 effect will be produced, and 
 the vessel with its contents 
 will deviate from the verti- 
 cal in the direction B A, as 
 if it were attracted by the 
 lens. Nevertheless, there is 
 not in either case any true 
 attraction or repulsion, and 
 the effects only arise from 
 the common principles of 
 hydrostatic pressure. Now, 
 not only does this example put us on our guard against 
 entertaining the notion of any real attraction or repul- 
 sion exercised by the particles of the electrified body ; 
 but these very motions produced in this example result 
 from a mechanism similar to that which produces the 
 apparent attractions and repulsions of electrified bodies. 
 The attraction and repulsion take place not between 
 the bodies, but between the positive or negative fluids 
 which envelop them, and of which the mutual action 
 causes the pressure to be increased on some parts of their 
 surfaces, and diminished on others ; which pressure is 
 exerted against the surrounding air, by which the fluids 
 are retained on the bodies, or in general against what- 
 ever obstacles are opposed to its displacement/' 
 
 (201.) The explanation founded on these principles 
 has always appeared to me to be inconclusive and un- 
 satisfactory, and the illustration by which it is accom- 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED. 41? 
 
 panied to be inapplicable. The motion of the sphere 
 S' is here not ascribed directly to the attraction or re- 
 pulsion of the electricity of S acting on the electricity of 
 S'. It is ascribed to the unequal diffusion of the elec- 
 tricity on S', which unequal diffusion is produced by 
 the proximity of S. But if the unequal diffusion of the 
 fluid on S' by producing an excess of pressure on the 
 one side against the air above that on the other is really 
 the cause of the motion of S', then that motion would 
 ensue by any other cause which, without the proximity 
 of another electrified body, would produce such unequal 
 distribution of the fluid. But such would not be the 
 case ; for if a nonconducting sphere, a sphere of gum lac 
 for example, be strongly electrified on one side, and 
 either feebly or not at all electrified on the other, then 
 such sphere ought, according to the preceding explana- . 
 tion of the phenomena, to move spontaneously towards 
 that side on which it is most strongly electrified. It is 
 scarcely necessary to observe that no such motion would 
 take place. 
 
 But, independently of this practical instance of the 
 inconclusiveness of this explanation, it is evident that 
 by a general principle of mechanics no motion can be 
 imparted to the centre of gravity of any system by the 
 reciprocal actions of the parts, or any of them, composing 
 that system ; and whenever such a motion takes place, 
 it must be due to the action of some cause external to 
 and separate from the system. In the preceding ex- 
 planation, nevertheless, it is assumed that by the mutual 
 repulsions of the unequally distributed fluid, and by 
 those alone, a motion of translation is given to the 
 whole body S'. 
 
 In fact, whatever be the pressure which the electric 
 fluid at any part of an electrified body exerts against 
 the air, an equal pressure in the contrary direction 
 must be exerted on the body itself by the inferior 
 surface of the electric fluid. This is a necessary con- 
 sequence of the perfect fluidity of that fluid. Thus, in 
 fig. 40., if the fluid presses against the air at N with 
 VOL. i. E E 
 
418 ELECTRO-STATICS. PART I. 
 
 a certain force,, there must be an equal pressure exerted 
 against the matter of the sphere at n, and in the con- 
 trary direction. These two pressures are then in equi- 
 librium ; and the same may be said of the pressure 
 exerted by the electricity at any other point. However 
 unequal, therefore,, may be the pressures exerted by the 
 electricity against the air, these pressures will be ba- 
 lanced by equal and contrary pressures exerted against 
 the sphere S' itself, and no motion could consequently 
 ensue. 
 
 The illustration of the vessel of liquid having a hole 
 in its side, from which a stream of the liquid issues, is 
 quite inapplicable. That example is altogether ana- 
 logous to the cases of motion produced in electrified 
 bodies by the rapid escape of electricity at points, ex- 
 amples of which have been given in (117.) et seq. But 
 in the present case no escape of the fluid from the 
 electrified body takes place, and therefore the motion of 
 recoil or reaction cannot ensue. 
 
 In fine, it appears that the motion of an electrified 
 body from a body similarly electrified, and towards a 
 body oppositely electrified, is due not to the unequal dis- 
 tribution of the electric fluid upon it, which in this 
 respect must be considered as a merely accidental cir- 
 cumstance, but to the mutual attraction or repulsion of 
 the masses of electric fluid on the two bodies. 
 
 (201.) In the reciprocal motion exhibited by elec- 
 trified bodies, some cases occur which, at the first view, 
 would appear to be contrary to those which might be 
 inferred from theory ; but, as happens with all hypotheses 
 having a foundation in truth, such apparently excep- 
 tional cases, when they come to be strictly examined, 
 serve only as further verifications of the theory. 
 
 When two electrified bodies, one of which is very small 
 compared with the other, but feebly electrified, approach 
 each other, effects ensue contrary to what are usually pro- 
 duced. Suppose that S, the greater, is strongly charged 
 with positive electricity, and S' is feebly electrified and 
 also positively. At a certain distance S' is, as usual, 
 
CHAP. VIII. ATTRACTION AND REPULSION EXPLAINED. 41Q 
 
 repelled. But if it be gradually brought nearer to S, 
 the repulsion is by degrees diminished, and it is at a 
 certain distance attracted. This attraction continues to 
 increase until contact takes place, after which S' is re- 
 pelled, as bodies in the natural state usually are after 
 contact with an electrified body. 
 
 These effects are easily accounted for. The body S', 
 being first invested with a very thin coat of positive 
 electricity, and placed at such a distance from S that 
 an inconsiderable decomposition of the natural elec- 
 tricities of S is produced, the usual repulsion exerted 
 on the shell of electric fluid is manifested. But when 
 the distance is diminished, the increased influence of 
 the electricity of S repels the small charge of positive 
 electricity previously given to S', and causes it to ac- 
 cumulate at the side of S' most remote from S. At the 
 same time, the influence of S x decomposes a portion of 
 the natural electricities of S', causing the negative por- 
 tion to accumulate at the side of S' nearest to S, and 
 the positive at the side most remote from it, where it is 
 added to the charge of positive fluid collected there. 
 There will be thus an accumulation of positive fluid on 
 the side of S' most remote from S, which we shall call 
 P, and of negative fluid on the nearest side, which we 
 shall call N. Now, the quantity of P is greater than 
 that of N, and on that account the repulsion exercised 
 by the electricity of S on P would be greater than its 
 attraction on N. But, on the other hand, the distance 
 of P being greater than the distance of N from S, the 
 repulsion on that account exerted on P would be less 
 than the attraction exerted on N. When the distance 
 of S' from S is so small that the excess of the attraction 
 above the repulsion due to the difference of distance is 
 greater than the excess of the repulsion above the at- 
 traction due to difference of quantity, the body S' will 
 be attracted, and will, a fortiori, be attracted at all less 
 distances. 
 
 When contact takes place, S' receives from S a charge 
 of positive electricity so great that the excess of attrac- 
 
420 
 
 ELECTRO-STATICS. 
 
 PART I. 
 
 tion and repulsion due to difference of distance is in- 
 sufficient to neutralise the contrary excess due to differ- 
 ence of quantity, and repulsion will take place at all 
 distances. 
 
 (202.) These effects may be verified experimentally 
 in the following manner. Let A A' (fig. 43.) be an 
 insulated conductor, and let B 
 be a pith ball suspended near 
 it by a thread of silk, and at- 
 tached to the conductor by ano- 
 ther horizontal thread, A B, by 
 which it shall be prevented from 
 receding further from the con- 
 ductor. Let A A' be first feebly 
 electrified : the ball B will be 
 attracted, will touch the con- 
 ductor, and will then be repelled as far as the thread AB 
 will permit. Let the conductor now receive a stronger 
 charge : the ball B will be again attracted, and again 
 repelled, and this may be continued until the ball B be- 
 comes so strongly charged with electricity that it can be 
 no longer repelled. 
 
CHAP. IX. ELECTRICAL MACHINES. 421 
 
 CHAP. IX, 
 
 ELECTRICAL MACHINES. 
 
 (203.) IN the commencement of this work, the 
 structure and the forms of the machines by which 
 the electric fluid is developed and accumulated for 
 the purposes of experiment, and the details of some of 
 the principal instruments by which the presence of 
 that fluid is ascertained, its quality detected, and its 
 quantity measured, were explained, so far as was ne- 
 cessary to render intelligible the development of the 
 electrical phenomena, and the theory founded upon 
 them, which are the subject-matter of the preceding 
 chapters. A full and accurate explanation of the prin- 
 ciple and structure of electrical machines, and other 
 apparatus commonly used either in imparting a know- 
 ledge of the science, so far as it is known, or in experi- 
 mental investigations made with an immediate view to 
 extend its limits, required a previous acquaintance with 
 those facts and principles which have now, it is hoped, 
 been explained with sufficient clearness and precision. 
 We shall, therefore, proceed with the description of 
 the form, structure, and operation of electrical ma- 
 chines, and such other experimental apparatus as admit 
 of being explained on those general principles of the 
 science 'which have been already established, reserv- 
 ing for a subsequent part of this work such other in- 
 struments and apparatus as rest upon principles and 
 facts to be developed hereafter. 
 
 All machines, whatever be their form, for the evolu- 
 tion and accumulation of free electricity, must consist 
 of three principal parts ; firstly, the rubber ; secondly, 
 the substance on which electricity is disengaged by 
 E E 3 
 
422 ELECTRO-STATICS. PART I. 
 
 friction ; and, thirdly, the conductor on which the free 
 electricity thus evolved is collected and accumulated for 
 the purposes of experiment. 
 
 In all ordinary electrical machines, the rubber is a 
 cushion stuffed with hair, hearing upon its surface 
 some substance which, by friction with the second body 
 above mentioned, will freely, and abundantly evolve 
 electricity. The substance against which the friction 
 of this rubber takes place is invariably glass, that 
 having been found in all respects most convenient and 
 eligible for the purpose; and the form given to it is 
 one which will admit of every part being brought in 
 rapid and uninterrupted succession under the action of 
 the rubber. The form best adapted for this purpose is 
 either that of a cylinder revolving on its geometrical 
 axis, the cushion being pressed against its side, or a flat 
 circular plate of sufficient thickness to give it the re- 
 quisite strength kept revolving in its own plane on an 
 axis at right angles to that plane passing through 
 its centre, the cushion being pressed against its faces. 
 Electrical machines are accordingly divided into two 
 classes, cylindrical machines and plate machines. 
 
 The conductor is a body having a metallic surface, 
 and supported on insulating pillars, or suspended by 
 insulating cords. 
 
 The Common Cylindrical Machine. 
 
 (204.) A hollow cylinder of glass, A B (fig. 44.), 
 is supported in bearings at C, the extremities of its 
 geometrical axis, on which it is capable of receiving a 
 motion of continued rotation. In order to impart by 
 a moderate velocity of the hand a sufficiently rapid 
 motion to the cylinder, a small grooved wheel is at- 
 tached to the end of the cylinder C, which is driven by 
 a much larger grooved wheel, D, the two wheels being 
 connected by a band in the usual manner, as repre- 
 sented in the figure. The bearings in which the 
 
CHAP. IX. COMMON CYLINDRICAL MACHINE. 
 
 423 
 
 cylinder turns are formed in upright pieces of wood, 
 E F, attached to a rectangular base, E G, of sufficient 
 magnitude and solidity to give the necessary firmness 
 and stability to the apparatus. The cushion is repre- 
 sented at H, formed of soft leather stuffed with hair, 
 and supported on a glass pillar. This cushion is more 
 clearly represented in fig. 45. Between the cushion 
 and the wooden back which supports it is interposed a 
 steel spring, by means of which the face of the cushion 
 is urged with the necessary pressure against the cylin- 
 der. By the elasticity of this spring the rubber ac- 
 commodates itself to the inequalities of the cylinder. The 
 foot of the glass pillar supporting the rubber is inserted 
 in a groove formed in the base or table of the machine, 
 in which groove the pillar carrying the rubber with it 
 may, within certain small limits, be varied in its dis- 
 tance from the cylinder. It is fixed in any desired po- 
 sition by the adjusting screw I. A screw having a 
 large spherical head, K (fig. 44. and fig. 45.), passes 
 
 Fig. 45. 
 
 H 
 
 through the wooden back of the rubber. By means of 
 this screw the tension of the spring behind the rubber 
 is adjusted. This screw and its metallic knob, being 
 conductors, form a free communication with the elec- 
 tricity evolved upon the rubber ; and by suspending a 
 metallic chain, K L, from the knob K, a communica- 
 tion may be made at pleasure between the rubber and 
 
 E E 4 
 
424 ELECTRO-STATICS. PART I. 
 
 the ground, or with any other conductor. When the 
 chain K L is removed, the rubber is insulated by the 
 glass pillar on which it stands, and whatever electricity 
 may be evolved upon it will be retained there. 
 Attached to the superior edge of the cushion is a flap 
 of silk, usually varnished, which extends over the 
 cylinder, and terminates within about an inch of the 
 place where the prime conductor is presented to it. 
 
 (205.) Although by the friction of a leather or silk 
 cushion against the glass electricity would be evolved, 
 its production would not be so rapid or abundant as 
 other expedients, the effects of which depend on prin- 
 ciples which will be explained hereafter, render it. It 
 is found that the evolution of the electric fluid is stimu- 
 lated by smearing the face of the cushion by certain 
 amalgams of metals, which in this case, therefore, be- 
 come the real rubber. An amalgam consisting of 
 mercury and tin, in the proportion of two parts of the 
 former to one of the latter, with the addition of a little 
 chalk, wa.s used by Canton. Singer proposed a com- 
 pound of two parts by weight of zinc and one of tin, 
 with which, in a fluid state, six parts by weight of 
 mercury are mixed. The whole is then shaken in an 
 iron or thick wooden box until it cools. It is then 
 reduced to fine powder in a mortar, and mixed with 
 lard in sufficient quantity to reduce it to the consistency 
 of paste. This preparation should be spread cleanly 
 over the surface of the cushion up to the line formed 
 by the junction of the silk flap with the cushion ; but 
 care should be taken that the amalgam should not be 
 extended to the flap. It is necessary, occasionally, to 
 wipe the cushion, flap, and cylinder, to cleanse them 
 from the dust which the electricity evolved upon the 
 cylinder always attracts in a greater or lesser quantity. 
 It is found that, from this cause, a very rapid accumu- 
 lation of dirt takes place on the cylinder^ which appears 
 in black spots and lines upon its surface. As this 
 obstructs the action of the machine, it should be con- 
 stantly removed, which may be done by applying to the 
 
CHAP. IX. COMMON CYLINDRICAL MACHINE. 425 
 
 cylinder as it revolves a rag wetted with spirits of wine. 
 The production of electricity is greatly promoted by 
 applying with the hand to the cylinder a piece of soft 
 leather, five or six inches square, covered with amalgam. 
 This is, in fact, equivalent to giving a temporary en- 
 largement to the cushion. 
 
 The purpose of the flap of silk coated with a varnish 
 of gum is to protect the electricity evolved by the 
 rubber on the glass from the contact of the air, by 
 which it would be more or less dissipated. The gum 
 varnish of the silk possessing in a very high degree the 
 nonconducting quality, the electricity does not pass 
 from the glass to it ; and the flap covering the surface 
 of the cylinder nearly to the points where it is in com- 
 munication with the prime conductor, the electricity is 
 protected until it passes to the latter. 
 
 The quality of glass best adapted for the cylinders 
 has not been certainly ascertained ; it is evident that it 
 should be rendered as little hygrometrical as possible ; 
 which condition will be attained by rendering its com- 
 position as free from alkaline matter as is practicable. 
 The use of minium in a greater quantity than is neces- 
 sary should also be avoided, since it injures the insu- 
 lating power of the glass. Cylinders are generally 
 made of flint glass, and as thin as is consistent with 
 the necessary strength. They should be free from 
 knots and veins, and should be as true in their cylin- 
 drical form as practicable, that the pressure of the 
 cushion may be uniform. 
 
 The practice of coating the inner surface of the cy- 
 linder with a resinous cement or varnish is now gene- 
 rally discontinued. It may, however, contribute to the 
 improved action of a bad cylinder. The process con- 
 sists in melting together four parts of Venice turpentine, 
 one part of resin, and one of bees' wax, and boiling the 
 whole together for about two hours in an earthen pip- 
 kin on a slow fire. The cylinder is to be carefully 
 heated, and a portion of the liquid cement poured in. 
 Then, by causing the cylinder to revolve while the ce- 
 
426 ELECTRO-STATICS. PART I. 
 
 ment is fluid, the latter will gradually harden, and form 
 a permanent coating on the inner surface. 
 
 (206.) The prime conductor, M N, is a cylinder 
 made of thin brass, the extremities being segments of a 
 sphere of greater radius than the cylinder, and greater 
 than hemispheres. This cylinder is supported on a 
 strong glass pillar, O P, resting on a wooden stand of 
 sufficient magnitude to give stability to the conductor. 
 To the end of the conductor which is presented to 
 the cylinder is attached a row of points, the bar from 
 which these points project terminating in two varnished 
 Fig. 46. wooden balls. This appendage to the con- 
 ductor is represented separately in fig. 46. 
 A hole is made in the end of the conductor, 
 
 in which the rod Q may be inserted, so that 
 the points may be attached to it or removed 
 at pleasure. When the conductor is properly 
 
 placed, these points should stand within 
 about half an inch of the surface of the cy- 
 
 * linder. 
 
 (207.) The bearings on which the cylinder rests at 
 each end are fastened into brass caps, which close two 
 large orifices, one at each end of the cylinder. By 
 means of these openings, the inner surface of the glass 
 is made perfectly clean and dry before the cylinder is 
 mounted ; and a sufficient bed of some resinous cement 
 is melted into each cap on the inside, so that no com- 
 munications can take place between the external air 
 and the air within the cylinder. 
 
 Since the electric fluid is evolved only on that part 
 of the cylinder to which the rubber is applied, the glass 
 beyond the ends of the rubber is covered with a varnish 
 of gum lac, to prevent more effectually the electricity 
 under the flap from passing along the cylinder and 
 escaping by the metallic caps at its extremities. 
 
 As the object of the row of points is to act on, or be 
 acted on, by the electricity on the cylinder, in a manner 
 which will be presently explained, their extent, should 
 
CHAP. -IX. COMMON CYLINDRICAL MACHINE. 427 
 
 be equal to, and should not exceed, that of the cushion 
 and the flap. 
 
 (208.) The efficiency of electrical machines must 
 mainly depend on each of their several points discharging 
 the functions assigned to them with the highest practica- 
 ble degree of perfection; and among these functions, none 
 is more important than perfect insulation. Now glass, 
 which for many reasons sufficiently obvious is the most 
 convenient substance which can be used as an insulator, 
 offers a peculiar facility to the condensation of moisture 
 upon its surface, and the moment such a film of water 
 is formed upon it, its insulating virtue ceases. To 
 remove or diminish the liability to this injurious effect, 
 it is usual to cover the insulating pillar, and a portion 
 of the cylinder, with a coating of some resinous varnish, 
 which, while its nonconducting power is even greater 
 than that of glass, offers less attraction to the moisture 
 suspended in the air. A varnish for this purpose may 
 be made by dissolving common sealing-wax in alcohol ; 
 but it is better, if possible, to apply the wax directly, 
 by heating the surface which is to be coated by it. / 
 varnish of gum lac is still better, that substance being 
 a more perfect insulator than sealing-wax, and equall) 
 repellant of moisture. 
 
 (209.) With all the precautions which can be used, 
 and with the very best experimental apparatus, electri- 
 cal experiments will be very sensibly affected by the 
 hygrometrical state of the air. They should be always 
 performed in a warm and dry room, and eiery part of 
 the apparatus intended to be used should be kept for 
 some hours at a moderate distance before a fire, in order 
 that all moisture deposited on them may be dissipated. 
 This precaution must be understood as applying not 
 only to electrical machines of every form, but also to 
 all electrical apparatus whatever in which the property 
 of insulation is required. 
 
 (210.) In fixing the insulating pillars or handles 
 into their sockets, a cement is required which admits of 
 being easily softened by heat. One of the best for this 
 
428 ELECTRO-STATICS. PART I. 
 
 purpose is composed of five pounds of resin melted 
 with one pound of bees' wax, one pound of red ochre, 
 and two table-spoonfuls of plaster of Paris. The two 
 last substances should be well dried, and gradually 
 mixed with the wax and resin while in a state of 
 fusion. 
 
 (211.) The only essential condition to be fulfilled 
 by the material of the prime conductor is, that its ex- 
 ternal surface should be metallic ; and the only essential 
 condition to be fulfilled by its form is, that it should 
 be such as not to permit the spontaneous dispersion of 
 electricity. It has been shown that the free electricity 
 of a conductor may be practically considered as super- 
 ficial ; for although, strictly speaking, it penetrates to a 
 depth generally variable below its surface, yet that depth 
 being incomparably more minute than the thinnest me- 
 tallic foil with which any body can be coated, a con- 
 ductor of a given form and magnitude of solid metal 
 cannot contain more free electricity than one of the 
 same superficial extent and form, however thin it may 
 be. A globe of solid gold cannot contain more free 
 electricity than a globe of equal magnitude formed of 
 gilt paper. 
 
 For the sake, therefore, of lightness, metallic con- 
 ductors are usually hollow. If they be cylindrical, 
 which they generally are, their extremities are formed 
 of segments of a sphere, not less than a hemisphere, to 
 avoid corners or edges, which would facilitate the spon- 
 taneous dispersion of electricity. They are generally 
 formed of thin copper or brass, which have the advan- 
 tage of cheapness, are little liable to injury, and have 
 surfaces easily polished. 
 
 Conductors formed of wood or pasteboard, coated 
 with tin foil, or gold leaf, are sometimes convenient, 
 and, if kept clean and in good order, answer very well. 
 Whatever be the matter or form of the prime conductor, 
 small round holes should be made in it at convenient 
 places having a diameter of about the eighth of an inch 
 
CHAP. IX. COMMON CYLINDRICAL MACHINE. 429 
 
 into which the stems of electroscopes and other appa- 
 ratus may be occasionally inserted. 
 
 (212.) To explain the operation of the electrical 
 machine above described, let us suppose the handle R, 
 fig. 44., turned, so as to make the wheel D revolve in the 
 direction of the arrows. The wheel C and the cylinder 
 will therefore turn in the opposite direction, so that the 
 glass, after moving in contact with the cushion, and 
 having positive electricity developed upon it, passes 
 under the flap, and afterwards comes opposite the points 
 projecting from the conductor. Negative electricity is 
 at the same time developed in the cushion, and passes 
 by the metallic screw and knob K and the chain K L 
 to the ground. The positive electricity accumulated on 
 the surface of the cylinder acts by induction on the 
 natural electricities of the conductor NM ; and according 
 to what was explained in Chapter VI. the positive 
 electricity of the conductor is driven towards the ex- 
 tremity N, while the negative electricity is attracted 
 and accumulated round the extremity M. In this state 
 of things let us suppose for a moment the conductor not 
 to be armed with the system of points already described, 
 but to be terminated by a spherical surface. The at- 
 traction of the positive electricity of the glass would, 
 according to the principles already explained, accumulate 
 negative electricity on the spherical surface at M, and this 
 negative electricity would re-act on the positive electricity 
 of the glass, and would have a tendency to collect it in 
 increased quantity at the part nearest the conductor ; 
 but this tendency would be resisted by the noncon- 
 ducting quality of the glass, on which the stratum of 
 free electricity would maintain a depth, little, if at all, 
 augmented. Under such circumstances no electricity 
 could pass either from the cylinder to the conductor, or 
 from the conductor to the cylinder, unless the depth of 
 the electric fluid on the one or the other surface were 
 so great as to overcome by its force the pressure of the 
 surrounding air. Now it is clear that the nonconducting 
 property of the glass conspires with its cylindrical form 
 
430 ELECTRO-STATICS. PART I. 
 
 to prevent this on the one hand, while the facility to 
 accumulation offered by the conducting power of the 
 spherical surface M is counteracted by the property of 
 that surface in virtue of which it favours the uniform 
 distribution of electricity on the other. That the con- 
 ductor may become charged with free electricity, either 
 of two effects must be produced. The depth of elec- 
 tricity on the cylinder must be so increased as to over- 
 come the restraining power of the air, so that it may 
 force its way to the spherical surface M, and thus the 
 conductor may become charged with the positive elec- 
 tricity which passes to it from the cylinder ; or the 
 depth of the negative fluid on the surface M must be 
 so increased as to surpass the restraining power of the 
 air, so that a portion of the negative fluid shall pass 
 from the conductor to the cylinder. In this latter case 
 the quantity of positive fluid remaining on the con- 
 ductor would exceed the quantity of negative fluid, and 
 if the conductor were removed from the cylinder it 
 would be found, charged with positive electricity. The 
 negative fluid, which would in this case pass from the 
 extremity M of the conductor to the cylinder, would 
 there combine with an equal portion of the positive fluid 
 diffused upon the cylinder, and would, to a proportionate 
 extent, restore the cylinder to its natural state. If the 
 quantity of negative fluid thus supplied by the conductor 
 to the cylinder were exactly equal to the quantity of 
 negative fluid which escaped through the rubber and the 
 chain K L to the earth, then it would necessarily be equal 
 to the quantity of positive fluid developed on the cylin- 
 der, and the cylinder would be obviously restored to its 
 natural state. The electricities which would thus be 
 combined as the cylinder passes before the conductor 
 would be again decomposed as it passes under the rubber, 
 and the negative fluid which was just received from the 
 conductor would be dismissed through the rubber and 
 the chain KL to the earth. 
 
 Now it is evident that, consistently with the principles 
 of this theory, either of these two methods of charging 
 
CHAP. IX. COMMON CYLINDRICAL MACHINE. 431 
 
 the conductor with positive electricity may be adopted. 
 Either a positive charge of electricity may be supplied 
 from the cylinder to the conductor, or a quantity of 
 negative electricity may be drawn from the conductor 
 to the cylinder, and dismissed through the rubber to the 
 earth, leaving an equal portion of free and uncombined 
 positive electricity on the conductor. 
 
 But there are practical circumstances which decide 
 the choice between these two methods. To produce 
 such an increased depth of fluid as would overcome 
 the resistance of the air, the adoption of an angular or 
 pointed form would be necessary, and this would be 
 incompatible with the continued action of the cylinder ; 
 but, independently of this, supposing that points could 
 be attached to the cylinder to facilitate the escape of 
 electricity from it, the nonconducting power of the 
 cylinder would obstruct the flow of the electric fluid 
 towards these points, and would therefore render the 
 expedient inefficacious. 
 
 Neither of these difficulties, however, attends the 
 adoption of" the other method. The conductor, being 
 stationary, may have a system of points or any other 
 appendages easily attached to it ; and when the points 
 are thus attached to it, the perfect freedom of motion 
 of the electric fluid upon it enables it to collect at the 
 points with the depth which the general condition of 
 electrical equilibrium requires ; and as the depth greatly 
 exceeds what would give a force equal to that of the 
 atmosphere, a rapid escape of electricity will take place. 
 
 After these explanations, there can be little difficulty 
 in perceiving the process which is effected by the opera- 
 tion of this electrical machine. By the friction of the 
 amalgam on the cushion with the glass, the natural 
 electricities of one or both of these substances are de- 
 composed. The negative electricity escapes by the chain 
 to the earth; and the positive electricity remaining upon 
 the glass, over a width of the cylinder determined by 
 the length of the cushion, is carried by the motion of 
 the cylinder under the flap towards the end of the 
 
432 ELECTRO-STATICS. PART I. 
 
 conductor. Coming opposite the conductor, its attrac- 
 tion draws the negative portion of the natural electricity 
 of the conductor towards M, and drives the positive 
 portion towards N. The negative portion accumulates 
 in great depth at the row of points which are close to 
 that part of the cylinder electrified positively. This 
 great accumulation at the points is favoured partly by the 
 property of points formerly explained (11 .5.), and partly 
 by the proximity of the point to the attracting fluid. The 
 negative fluid, therefore, issues in copious jets from the 
 several points, and, diffusing itself on the cylinder, com- 
 bines with and neutralises the positive fluid ; and ac- 
 cordingly the lower part of the cylinder between the 
 point and the rubber is always in its natural state. 
 
 (213.) It is apparent, therefore, that the effect pro- 
 duced by the operation of this machine is a continued 
 decomposition of the natural electricities of the prime 
 conductor, and an abstraction of the negative fluid. 
 This process would continue until the quantity of 
 positive fluid disengaged upon the conductor would be- 
 come so great that the repulsive force of the positive fluid 
 on the cylinder would be insufficient to expel it from 
 the extremity of the conductor next the cylinder, where 
 its presence would be incompatible with the accumu- 
 lation of the negative fluid, and, consequently, all trans- 
 mission of negative electricity from the conductor to 
 the cylinder would cease, and no further accumula- 
 tion of positive electricity on the conductor would take 
 place. 
 
 Nairnes Cylindrical Machine. 
 
 (214.) The electrical machine represented in^. 47- 
 was constructed by Mr. Nairne with a view to the de- 
 velopment of either positive or negative electricity. 
 The chief parts of this machine are so similar to those 
 of the last, that it will not be necessary to give any de- 
 tailed description of them. The corresponding parts 
 
CHAP. IX. NAIRN E*S CYLINDRICAL MACHINE. 433 
 
 are marked in the two figures by the same letters. The 
 conductor M is in this case placed with its length parallel 
 
 Fig. 47. 
 
 to the cylinder ; and the points project from its side, 
 and not from its end as in the former case. The nega- 
 tive conductor M' supports the rubber, and receives from 
 it the negative electricity ; not by induction, as is the 
 case with the positive conductor, but by communication. 
 If it be required to accumulate positive electricity, a 
 chain must be carried from the negative conductor to 
 the ground, otherwise an inconsiderable quantity only of 
 positive electricity could be collected on the conductor 
 M. If, on the other hand, negative electricity be re- 
 quired, then the conductor M must be put in commu- 
 nication with the ground, and the conductor M' insu- 
 lated. By this arrangement the repulsion of the positive 
 electricity on the cylinder will constantly drive a portion 
 of the positive element of the natural electricities of the 
 conductor M to the earth, and its attraction will act 
 with undiminished effect on the negative element col 
 lected about the points. The accumulation of negative 
 electricity on the conductor M' will continue until its 
 repulsive power becomes so great as to stop the further 
 decomposition of electricity by the cushion. 
 
434 
 
 ELECT BO-STATICS. 
 
 PART I. 
 
 The Common Plate Machine. 
 
 (215.) This machine is represented in fig. 48., and 
 consists of a circular disc 
 or plate of glass, A B, 
 fixed in a vertical posi- 
 tion on a horizontal axis, 
 C, supported in a wooden 
 framing. It is capable 
 of being made to revolve 
 in its own plane by a 
 handle or winch, D, at- 
 tached to the end of its 
 axis. At the lowest part, 
 E, the plate is embraced 
 between two cushions, to 
 which a regulated pressure is given, and a casing of silk 
 covers both faces of the plate from the cushion to a point, 
 F, within about an inch of the horizontal diameter. 
 The cushions and cover together, therefore, extend over 
 something less than a quadrant, and the breadth of the 
 silk cover is equal to the length of the cushion. 
 
 The plate is ^embraced in a similar manner at its 
 highest point by two similar cushions, E', and covered by 
 a similar casing of silk extending to F x , a point a little 
 above the opposite end of the horizontal diameter. The 
 handle being turned in the direction of the arrow, 
 vitreous electricity will be developed upon the glass by 
 the friction of each of the cushions. The conductor is 
 composed of a long narrow cylinder, bent at angles so as 
 to bring it into the proper position with respect to the 
 plate, and formed at its angles into spheres to avoid edges 
 or corners. It is represented at N M, which is in the 
 direction of the axis of the plate. A branch, M O, is 
 thence carried parallel to the plate, which, being bent at 
 right angles, is terminated in a sphere at P, close to the 
 plate. A short branch, P Q, is thence carried parallel to 
 the plate contiguous to the edge of the silk flap. A simi- 
 
CHAP. IX. PLATE .MACHINES. 435 
 
 lar bent branch of the conductor extends on the other 
 side, terminating just above the edge of the lower flap. 
 The principle of this machine is so similar to that of 
 the common cylindrical machine, that it is needless here 
 to offer any further explanation of it. With the same 
 weight and bulk,, the extent of rubbing surface is much 
 greater than in the cylindrical machine, and the evo- 
 lution of electricity is proportionably more rapid. In 
 the construction and adaptation of the rubbers, and in 
 the general adaptation and management of the parts of 
 the machine, the same precautions are to be taken and 
 provisions made as in the cylindrical machine. The 
 principal objection to this, as compared to the cylin- 
 drical machine, is the difficulty of insulating the rubbers 
 so as to. obtain the negative electricity when required. 
 This end is sometimes attained by insulating the entire 
 apparatus, which is done by mounting it on glass legs. 
 The object is, however, much more elegantly and ef- 
 fectually attained in the machine which we shall now 
 describe. 
 
 The Haerlem Plate Machine. 
 
 (216.) By far the most splendid and powerful elec- 
 trical machine which has been constructed is that which 
 was made under the superintendence of Doctor Van 
 Marum for the Teylerian Museum of Haerlem. As 
 it may be considered one of the most perfect models for 
 such a piece of apparatus, we shall here give a detailed 
 account of it. 
 
 A view of this machine is given in fig. 4>9 The 
 cushions are each insulated on a pillar of glass, and are 
 applied at opposite ends of the horizontal diameter of 
 the plate. Silk covers are attached to them, as already 
 described in the common plate machine. A metallic 
 sphere, B, supported on a glass pillar, is the prime 
 conductor; and to it is attached a rod of metal bent 
 into a semicircular form, the diameter of the semicircle 
 
Pis. 49. 
 
 436 ELECTRO-STATICS. PART I. 
 
 corresponding to that of the plate, and the plane of the 
 semicircle being perpendicular to the plate. At the ex- 
 tremities of this semicir- 
 cular piece are two small 
 cylinders, C and C', pre- 
 sented with their sides to 
 the plate, and having 
 their sides armed with 
 points presented to the 
 surface of the glass. The 
 effect produced by these 
 metallic points upon the 
 positive electricity dif- 
 fused upon the glass as 
 it leaves the flap is ex- 
 actly the same as in the 
 case of the common cy- 
 lindrical machine. 
 (217.) Since it is necessary that the cushions should 
 each communicate with the ground when the conductor 
 B is to be charged with positive electricity, there is 
 another semicircular branch, similar to C B C', attached 
 to the axis of the plate at D, but placed in a horizontal 
 position, so that its extremities shall be in contact with 
 the cushions respectively; and as this semicircular piece 
 is in metallic communication with the ground, the ne- 
 gative electricity developed on the cushions escapes by 
 it to the earth. 
 
 If it be required to charge the conductor B with ne- 
 gative electricity, the semicircular piece C B C' is moved 
 from the vertical to the horizontal position, so that the 
 cylinders C and C' shall have their points presented to 
 the cushions instead of the glass. A provision for this 
 movement is made in the apparatus. The position of 
 the other semicircular piece on the opposite side of the 
 glass is in this case likewise changed, its extremities 
 being presented to the glass at the top and bottom of 
 the plate near the points where it moves from under the 
 flapa charged with positive electricity. 
 
CHAP. IX. APPENDAGES TO MACHINES. 437 
 
 Whether the prime conductor be charged with po- 
 sitive or negative electricity, the effect is produced by a 
 decomposition of its natural electricities, and the ab- 
 straction of the electricity contrary to that with which 
 it is charged. This process has been so fully explained 
 in the case of the common cylindrical machine, that it 
 need not be repeated here. 
 
 (218.) Having explained the principal varieties of 
 electrical machines, we shall now briefly notice some 
 appendages to them which are necessary or useful in 
 almost all experiments in which these machines are 
 applied. 
 
 Appendages to Electrical Machines. 
 
 (219.) When it is required to preserve the elec- 
 tricity on a conductor, and to prevent its escape to the 
 earth, it is usual to place it on a stool (fig. 50.) having 
 glass legs coated with a var- 
 ^' nish of gum lac. The top 
 
 is usually made of a piece of 
 mahogany or other strong 
 and hard wood, which 
 should be baked and sub- 
 sequently varnished. The 
 legs should be six or eight inches in length, and formed 
 of solid cylinders of glass, from an inch to an inch and 
 a half in diameter. They should be fixed into holes in 
 the under side of the stool by proper cement. 
 
 As it is frequently necessary to examine the effects 
 of points and spheres, pieces such as are represented in 
 figs. 51, 52, should be provided, to be occasionally in- 
 serted in holes in the prime conductor ; also metallic 
 balls attached to rods having glass handles (figs. 53,54.), 
 for cases in which it is desired to apply to an electrified 
 body a conductor, without allowing the electricity to 
 pass to the body of the person conducting the expe- 
 riment. 
 
438 
 
 ELECTRO-STATICS. 
 
 Figs. 51. 52. 
 
 53. 
 
 O 
 
 54. 
 
 /o 
 
 \J 
 
 Pieces of br,ass chain to make occasional commu- 
 nications between one conductor and another ; boxes of 
 small balls about the size of medical pills made of the 
 pith of rush or elder, silken threads, fine silver wires, 
 fine linen threads steeped in salt and water, for the sus- 
 pension of the balls, should be also provided. 
 
 (220.) It is frequently necessary to establish a tem- 
 porary metallic communication between two conductors, 
 so that the electricity with which one is charged may 
 be shared by the other without passing through the 
 body of the experimenter. This might be done by at- 
 taching a chain of sufficient length to one of the two 
 conductors, and supporting the end of it by a glass rod 
 held in the hand of the operator. It might thus be 
 brought into contact with the other conductor. Under 
 such circumstances, the electricity would pass between 
 the two conductors through the chain. But as this 
 process is one so constantly necessary, instruments 
 called dischargers are constructed expressly for the 
 purpose. 
 
 The jointed discharger is represented in fig. 55., 
 where A B represents a glass handle, B D and B C 
 brass wires terminated by metallic balls moving on a 
 pivot or joint at B, by which they may be opened or 
 closed at pleasure, so that the distance between D and C 
 
CHAP. IX. 
 
 APPENDAGES TO MACHINES. 
 
 Fig. 55. 
 
 439 
 
 can be varied as may be required. Previously to the 
 experiment, the opening of the legs is adjusted ac- 
 cording to the distance between the bodies between 
 which a metallic communication is to be made. 
 
 It is sometimes more convenient to be enabled to 
 adjust the distance of the balls D and C in the per- 
 formance of the experiment: for this purpose handles 
 of glass are attached to the legs of the discharger, as 
 represented in^. 56. 
 
 Fig. 56. 
 
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