535 
 
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 of the 
 
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 Form L I 
 
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 JUN 2 
 
 Form L-9-15r-8,'24
 
 ELECTRICITY
 
 PROFESSOR JOHN TYNDALL'S WORKS. 
 
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 On Forms of Water, in Clouds, Rivers, Ice, and Gla- 
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 Faraday as a Discoverer. A Memoir. 12mo. Cloth, 
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 Contributions to Molecular Physics in the Domain of 
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 Six Lectures on Light. Delivered in America in 
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 Farewell Banquet, given to Professor Tyndall, at Del- 
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 Address delivered before the British Association, 
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 Researches on Diamagmtism and Magne-crystaUic 
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 New York : D. APPLETON & Co., 72 Fifth Avenue.
 
 LESSONS 
 
 ELECTRICITY 
 
 AT THE EOYAL INSTITUTION 
 
 JOHN TYNDALL, D.O.L., LL.1X, F.K.a 
 
 PROFESSOR OF SATTJHAL rnn^soFHT 
 
 DJ THE ROYAL DtSTTnrnON OF CKKAT BTUTATO 
 
 NEW YORK: 
 D. APPLETON AND COMPANY, 
 
 72 FIFTH AVENUE. 
 1895.
 
 . A Price-List of the Apparatus suitable for the 
 experiments described in these Lessons will be found at the end 
 of the volume. The teacher or learner may materially reduce 
 the cost by becoming his own instrument-maker.
 
 Q C 
 
 I wish to inscribe this look to Five Young Fri&nds, ivhose 
 names, in the approximate order of their ages,* ire here 
 set doivn : 
 
 HUGH SPOTTISWOODE. 
 
 HENRY HUXLEY, 
 
 ROLFE LUBBOCK, 
 
 JOHN CLAUSIUS, 
 
 REGINALD HOOKER. 
 
 J. T. 
 
 I write in Switzerland, and have to rely upon my memory, 
 hence my uncertainty
 
 PBEFACE. 
 
 MORE than fifty years ago the Board of Managers of 
 the Koyal Institution resolved to extend its usefulness, 
 as a centre of scientific instruction, by giving, during 
 the Christmas and Easter holidays of each year, two 
 courses of Lectures suited to the intelligence of boys 
 and girls. 
 
 On December 12, 1825, a Committee appointed by 
 the Managers reported 'that they had consulted Mr. 
 Faraday on the subject of engaging him to take a part 
 in the juvenile lectures proposed to be given during 
 the Christmas and Easter recesses, and they found his 
 occupations were such that it would be exceedingly in- 
 convenient for him to engage in such lectures.' 
 
 Faraday's holding aloof was, however, but tempo- 
 rary, for at Christmas 1827 we find him giving a 
 * Course of Six Elementary Lectures on Chemistry, 
 adapted to a Juvenile Auditory.' 
 
 The Easter lectures were soon abandoned, but from 
 the date mentioned to the present time the Christmas
 
 viii Preface. 
 
 lectures have been a marked feature of the Koyal 
 Institution. 1 
 
 Last Christmas it fell to my lot to give one of these 
 courses. I had heard doubts expressed as to the value 
 of Science-teaching in schools, and I had heard objec- 
 tions urged on the score of the expensiveness of appa- 
 ratus. Both doubts and objections would, I considered, 
 be most practically met by showing what could be done, 
 in the way of discipline and instruction, by experi- 
 mental lessons involving the use of apparatus so simple 
 and inexpensive as to be within everybody's reach. 
 
 With some amplification, the substance of our 
 Christmas Lessons is given in the present little volume. 
 
 1 These brief historic references have already appeared in the Pre- 
 face to the ' Forms of Water.'
 
 CONTENTS. 
 
 WCT. PAGB 
 
 1. INTRODUCTION . . . . .1 
 
 2. HISTORIC NOTES . . . . . . 1 
 
 3. THE ART op EXPERIMENT . . . .4 
 
 4. MATERIALS FOR EXPERIMENT . . . . 5 
 6. ELECTRIC ATTRACTIONS . . . . .8 
 
 6. DISCOVERT OF CONDUCTION AND INSULATION . .13 
 
 7. THE ELECTROSCOPE. FURTHER ENQUIRIES ON CONDUC- 
 
 TION AND INSULATION . .- . .15 
 
 8. ELECTRICS AND NON-ELECTRICS . . . .19 
 
 9. ELECTRIC REPULSIONS. DISCOVERT OF Two ELEC- 
 
 TRICITIES . . . . . . 22 
 
 10. FUNDAMENTAL LAW OF ELECTRIC ACTION . . 23 
 
 11. ELECTRICITT OF THE RUBBER. DOUBLE OR 'POLAR' 
 
 CHARACTER OF THE ELECTRIC FORCE . . . 29 
 
 12. WHAT is ELECTRICITY? . . . .33 
 
 13. ELECTRIC INDUCTION. DEFINITION OF THE TERM . 3G 
 
 14. EXPERIMENTAL RESEARCHES ON ELECTRIC INDUCTION . 38
 
 x Contents. 
 
 SECT. PAGB 
 
 16. THE ELECTROPHORUS . . . . . 48 
 
 16. ACTION OF POINTS AND FLAMES . . .51 
 
 17, THE ELECTRICAL MACHINE . . . . 65 
 
 18 FURTHER EXPERIMENTS ox THE ACTION OP POINTS. 
 THE ELECTRIC MILL. THE GOLDEN FISH. LIGHT- 
 NING CONDUCTORS . . . . .58 
 
 19. HISTORY OF THE LEYDEN JAR. THE LEYDEX BATTERY 64 
 
 20. EXPLANATION OF THE LEYDEN JAR . . .09 
 
 21. FRANKLIN'S CASCADE BATTERY . . ... 72 
 
 22. NOVEL LEYDEN JARS OF THE SIMPLEST FORM . 73 
 
 23. SEAT OF CHARGE IN THE LEYDEN JAR . . 77 
 
 24. IGNITION BY THE ELECTRIC SPARK. COTTRELL'S 
 
 RUBBER. THE TUBE -MACHINE . . . . 80 
 
 25. DURATION OF THE ELECTRIC SPARK . . .85 
 20. ELECTRIC LIGHT IN VACUO . . . . 88 
 
 27. LICHTENBERG'S FIGURES . . . .94 
 
 28. SURFACE COMPARED WITH MASS. DISTRIBUTION OF 
 
 ELECTRICITY IN HOLLOW CONDUCTORS . . . 95 
 
 29. PHYSIOLOGICAL EFFECTS OF THE ELECTRIC DISCHARGE 97 
 
 30. ATMOSPHERIC ELECTRICITY . . . .99 
 81. THE RETURNING STROKE . . . . . 102 
 
 32. THE LEYIEN BATTERY, ITS CURRENTS, AND SOME OF 
 
 THEIR EFFECTS . . . . . . 108 
 
 CONCLUSION .111
 
 LESSONS IN ELECTEICITY, 
 
 1. Introduction. 
 
 MANY centuries before Christ, it had been observed 
 that yellow amber (elektron\ when rubbed, pos- 
 sessed the power of attracting light bodies. 
 
 Thales, the founder of the Ionic philosophy (B.C. 580), 
 imagined the amber to be endowed with a kind of life. 
 
 This is the germ out of which has grown the science 
 of electricity, a name derived from the substance in 
 which this power of attraction was first observed. 
 
 It will be my aim, during six hours of these Christ- 
 mas holidays, to make you, to some extent, acquainted 
 with the history, facts, and principles, of this science, 
 and to teach you how to work at it. 
 
 The science has two great divisions ; the one called 
 ' Frictional Electricity,' the other ' Voltaic Electricity.' 
 For the present, our studies will be confined to the 
 first, or older portion of the science, which is called 
 ' Frictional Electricity,' because in it the electrical power 
 is obtained from the rubbing of bodies together. 
 
 2. Historic Notes. 
 
 The attraction of light bodies by rubbed amber 
 was the sum of the world's knowledge of electricity
 
 2 Lessons in Electricity. 
 
 for more than 2,000 years. In 1600 Dr. Gilbert, 
 physician to Queen Elizabeth, whose attention had 
 been previously directed with great success to mag- 
 netism, vastly expanded the domain of electricity. He 
 showed that not only amber, but various spars, gems, 
 fossils, stones, glasses and resins, exhibited, when rubbed, 
 the same power as amber. 
 
 Eobert Boyle (1675) proved that a suspended piece 
 of rubbed amber, which attracted other bodies to itself, 
 was in turn attracted by a body brought near it. He 
 also observed the light of electricity, a diamond, with 
 which he experimented, being found to emit light when 
 rubbed in the dark. 
 
 Boyle imagined that the electrified body threw out 
 an invisible, glutinous substance, which laid hold of 
 light bodies and, returning to the source from which 
 it emanated, carried them along with it. 
 
 Otto von Guericke. Burgomaster of Magdeburg, 
 contemporary of Boyle, and inventor of the air-pump, 
 intensified the electric power previously obtained. He 
 devised what may be called the first electrical machine, 
 which was a ball of sulphur, about the size of a child's 
 head. Turned by a handle, and rubbed by the dry 
 hand, the sulphur sphere emitted light in the dark. 
 
 Von Guericke also noticed, and this is important, 
 that a feather, having been first attracted to his 
 sulphur globe, was afterwards repelled, and kept at 
 a distance from it, until, having touched another 
 body, it was again attracted. He heard the hissing 
 of the ' electric fire,' and also observed that an un- 
 electrified body, when brought near his excited sphere, 
 became electrical and capable of being attracted. 
 
 The members of the Academy del Oimento examined
 
 Historic Notes. 3 
 
 various substances electrically. They proved smoke 
 to be attracted, but not flame, which, they found, 
 deprived an electrified body of its power 
 
 They also proved liquids to be sensible to the 
 electric attraction, showing that when rubbed amber 
 was held over the surface of a liquid, a little eminence 
 was formed, from which the liquid was finally dis- 
 charged against the amber. 
 
 Sir Isaac Newton, by rubbing a flat glass, caused 
 light bodies to jump between it and a table. He also 
 noticed the influence of the rubber in electric excita- 
 tion. His gown, for example, was found to be much 
 more effective than a napkin. 
 
 Newton imagined that the excited body emitted an 
 elastic fluid which penetrated glass. 
 
 In the efforts of Thales, Boyle, and Newton to form 
 a mental picture of electricity we have an illustration 
 of the tendency of the human mind, not to rest satis- 
 fied with the facts of observation, but to pass beyond 
 the facts to their invisible causes. 
 
 Dr. Wall (1708) experimented with large, elongated 
 pieces of amber. He found wool to be the best rubber 
 of amber. * A prodigious number of little cracklings ' 
 was produced by the friction, every one of them being 
 accompanied by a flash of light. ' This light and 
 crackling,' says Dr. Wall, ' seem in some degree to re- 
 present thunder and lightning.' l This is the first 
 published allusion to thunder and lightning in connec- 
 tion with electricity. 
 
 Stephen Gray (1729) also observed the electric 
 brush, snappings, and sparks. He made the prophetic 
 
 1 'Phil. Trans.' ) 708, p. 69.
 
 4 Lessons in Electricity. 
 
 reuaaik that 'though these effects are at present only 
 minute, it is probable that in time there may be found 
 out a way to collect a greater quantity of the electric 
 fire, and, consequently, to increase the force of that 
 power which by several of those experiments, if we are 
 permitted to compare great things with small, seems 
 to be of the same nature with that of thunder and 
 lightning.' l This, you will observe, is far more definite 
 than the remark of Dr. Wall. 
 
 3. The Art of Experiment. 
 
 We have thus broken ground with a few historic 
 notes, intended to show the gradual growth of electrical 
 science. Our next step must be to get some knowledge 
 of the facts referred to, and to learn how they may be 
 produced and extended. The art of producing and ex- 
 tending such facts, and of enquiring into them by 
 proper instruments, is the art of experiment. It is an 
 art of extreme importance, for by its means we can, as 
 it were, converse with Nature, asking her questions 
 and receiving from her replies. 
 
 It was the neglect of experiment, and of the reason- 
 ing based upon it, which kept the knowledge of the 
 ancient world confined to the single fact of attraction 
 by amber for more than 2,000 years. 
 
 Skill in the art of experimenting does not come of 
 itself ; it is only to be acquired by labour. When you 
 first take a billiard cue in your hand, your strokes are 
 awkward and ill-directed. When you learn to dance, 
 your first movements are neither graceful nor pleasant. 
 By practice alone, you learn to dance and to play. 
 
 1 Phil. Trans.' Vol. 39, p. 24.
 
 The Art of Experiment. 
 
 This also is the only way of learning the art of experi- 
 ment. You must not, therefore, be daunted by your 
 clumsiness at first ; you must overcome it, and acquire 
 ekill in the art by repetition. 
 
 In this way you will come into direct contact with 
 natural truth you will think and reason not on what 
 has been said to you in books, but on what has been 
 said to you by Nature. Thought springing from this 
 source has a vitality not derivable from mere book- 
 knowledge. 
 
 4. Materials for Experiment. 
 
 At this stage of our labours we are to provide ourselves 
 with the following materials : 
 
 a. Some sticks of sealing-wax; 
 
 6. Two pieces of gutta-percha 
 tubing, about 18 inches long and 
 | of an inch outside diameter ; 
 
 c. Two or three glass tubes, 
 about 18 inches long and f of an 
 inch wide, closed at one end, and 
 not too thin, lest they should 
 break in your hand and cut it ; 
 
 d. Two or three pieces of 
 clean flannel, capable of being 
 folded into pads of two or three 
 layers, about eight or ten inches 
 square ; 
 
 e. A couple of pads, composed 
 of three or four layers of silk, 
 about eight or ten inches square ; 
 
 /. A board about 18 inches square, and a piece 
 of india-rubber ;
 
 6 Lessons in Electricity. 
 
 g. Some very narrow silk ribbon, E, and a wire loop, 
 W, like that shown in fig. 1, in which sticks of sealing- 
 wax, tubes of gutta-percha, rods of glass, or a walking- 
 stick, may be suspended. I choose a narrow ribbon 
 because it is convenient to have a suspending cord that 
 will neither twist nor untwist of itself. 
 
 (I usually employ a loop with the two ends, which 
 are here shown free, soldered together. The loop 
 would thus be unbroken. But you may not be skilled 
 in the art of soldering, and I therefore choose the free 
 loop, which is very easily constructed. For the purpose 
 of suspension an arrangement resembling a towel- 
 horse, with a single horizontal rail, will be found con- 
 venient.) 
 
 Fro. 2. 
 
 h. A straw, I i', fig. 2, delicately supported on the 
 point of a sewing needle N. This is inserted in a 
 stick of sealing-wax A, attached below to a little 
 circular plate of tin, the whole forming a stand. In 
 fig. 3 the straw is shown on a larger scale, and separate
 
 Materials for Experiment. 7 
 
 from its needle The short bit of straw in the middle, 
 which serves as a cap, is stuck on by sealing-wax. 
 
 Fro. 3. 
 
 ar- 9 
 
 i. The name ' amalgam ' is given to a mixture of 
 mercury with other metals. Experience has shown 
 that the efficacy of a silk rubber is vastly increased 
 when it is smeared over with an amalgam formed of 
 1 part by weight of tin, 2 of zinc, and 6 of mercury. 
 A little lard is to be first smeared on the silk, and the 
 amalgam is to be applied to the lard. The amalgam, 
 if hard, must be pounded or bruised with a pestle or a 
 hammer until it is soft. You can purchase sixpenny- 
 worth of it at a philosophical instrument maker's. It 
 is to be added to your materials. 
 
 k. I should like to make these pages suitable for 
 boys without much pocket-money, and, therefore, aim 
 at economy in my list of materials. But provide by all 
 means, if you can, a fox's brush, such as those usually 
 employed in dusting furniture.
 
 Lessons in Electricity. 
 
 5. Electric Attractions. 
 
 Place your sealing-wax, gutta-percha tubing, and 
 flannel and silk rubbers before a fire, to ensure their 
 dryness. Be specially careful to make your glass tubes 
 and silk rubbers not only warm, but hot. Pass the 
 dried flannel briskly once or twice over a stick of seal- 
 ing-wax or over a gutta-percha tube. A very small 
 amount of friction will excite the power of attract- 
 ing the suspended straw, as shown in fig. 2. Repeat 
 the experiment several times and cause the straw to 
 follow the attracting body round and round. Do the 
 same with a glass tube rubbed with silk. 
 
 I lay particular stress on the heating of the glass 
 tube, because glass has the power, which it exercises, 
 of condensing upon its surface into a liquid film, the 
 aqueous vapour of the surrounding air. This film must 
 be removed. 
 
 I would also insist on practice, in order to render 
 you expert. You will therefore attract bran, scraps of 
 paper, gold leaf, soap bubbles, and other light bodies 
 by rubbed glass, sealing-wax, and gutta-percha. Fara- 
 day was fond of making empty egg-shells, hoops of 
 paper, and other light objects roll after his excited 
 tubes. 
 
 It is only when the electric power is very weak, 
 that you require your delicately suspended straw. With 
 the sticks of wax, tubes, and rubbers here mentioned, 
 even heavy bodies, when properly suspended, may be 
 attracted. Place, for instance, a common walking-stick 
 in the wire loop attached to the narrow ribbon, fig. 1, 
 and let it swing horizontally. The glass, rubbed with
 
 Electric Attractions. 
 
 9 
 
 its silk, or the sealing-wax, or gutta-percha, rubbed 
 with its flannel, will pull the stick quite round. 
 
 Abandon the wire loop ; place an egg in an egg-cup, 
 and balance a long lath upon the egg, as shown in fig. 4. 
 The lath, though it may be almost a plank, will ol)e- 
 
 Fio. 4. 
 
 iliently follow the rubbed glass, gutta-percha or sealing- 
 wax. 
 
 Nothing can be simpler than this lath and egg 
 arrangement, and hardly anything could be more im- 
 pressive. The more you work with it, the better you 
 will like it. 
 
 Pass an ebonite comb through the hair. In dry 
 weather it produces a crackling noise ; but its action 
 upon the lath may be made plain in any weather. It 
 is rendered electrical by friction against the hair, and 
 with it you can pull the lath quite round. 
 
 If you moisten the hair with oil, the comb will still 
 be excited and exert attraction ; but if you moisten it 
 with water, the excitement ceases; a comb passed 
 through wetted hair, has no power over the lath. You 
 will understand the meaning of this subsequently.
 
 10 Lessons in Electricity. 
 
 After its passage through dry or oiled hair, balance 
 the comb itself upon the egg : it is attracted by the 
 lath. You thus prove the attraction to be mutual: the 
 comb attracts the lath, and the lath attracts the comb. 
 Suspend your rubbed glass, rubbed gutta-percha, and 
 rubbed sealing-wax in your wire loop. They are all 
 just as much attracted by the lath as the lath was at- 
 tracted by them. This is an extension of Boyle's ex- 
 periment with the suspended amber ( 2). 
 
 How it is that any unelectri6ed body attracts, and 
 is attracted by the excited glass, sealing-wax and gutta- 
 percha, we shall learn by and by. 
 
 A very striking illustration of electric attraction may 
 be obtained with the board and india-rubber mentioned 
 in our list of materials ( 4). Place the board before 
 the fire and make it hot ; heat also a sheet of foolscap 
 paper and place it on the board. There is no attraction 
 between them. Pass the india-rubber briskly over the 
 paper. It now clings firmly to the board. Tear it 
 away, and hold it at arm's length, for it will move to 
 your body if it can. Bring it near a door or wall, it 
 will cling tenaciously to either. The electrified paper 
 also powerfully attracts the balanced lath from a great 
 distance. 
 
 The friction of the hand, of a cambric handkerchief, 
 or of wash-leather fails to electrify the paper in any 
 high degree. It requires friction by a special substance 
 to make the excitement strong. This we learn by ex- 
 perience. It is also experience that has taught us that 
 resinous bodies are best excited by flannel, and vitreous 
 bodies by silk. 
 
 Take nothing for granted in this enquiry, and 
 neglect no effort to render your knowledge complete
 
 Electric Attractions. 11 
 
 and sure. Try various rubbers, and satisfy yourself 
 that differences like that first observed by Newton exist 
 between them. 
 
 Vary also the body rubbed. Excite by friction 
 paraffin and composite candles, resin, sulphur, bees'- 
 wax, ebonite, and shellac. Also rock-crystal and other 
 vitreous substances, and attract with all of them the 
 balanced lath. A film of collodion, a sheet of vulcan- 
 ised india-rubber, or brown paper heated before the 
 fire, rubbed briskly with the dry hand, attracts and is 
 attracted by the lath. 
 
 Lay bare also the true influence of heat in the case 
 of our rubbed paper. Spread a cold sheet of foolscap 
 on a cold board on a table, for example. If the air be 
 not very dry, rubbing, even with the india-rubber, will 
 not make them cling together. But is it because they 
 were hot that they attracted each other in the first in- 
 stance ? No, for you may heat your board by plunging it 
 into boiling water, and your paper by holding it in a 
 cloud of steam. Thus heated they cannot be made to 
 cling together. The heat really acts by expelling the 
 moisture. Cold weather, if it be only dry, is highly 
 favourable to electric excitation. During frost the 
 whisking of the hand over silk or flannel, or over a cat's 
 back, renders it electrical. 
 
 The experiment of the Florentine academicians, 
 whereby they proved the electric attraction of a liquid, 
 is pretty, and worthy of repetition. P"ill a very small 
 watch-glass with oil, until the liquid forms a round 
 curved surface, rising a little over the rim of the glass. 
 A strongly excited glass tube, held over the oil, raises 
 not one eminence only, but several, each of which 
 finally discharges a shower of drops against the attract-
 
 12 Lessons in Electricity. 
 
 ing glass. The effect is shown in fig. 5, where G is the 
 
 watch-glass on the stand T, and u the excited glass tube. 1 
 
 Cause the excited glass tube to pass close by your 
 
 face, without touching it. You feel, like Hauksbee, as if 
 
 FIG. 5. 
 
 a cobweb were drawn over the face. You also sometimes 
 smell a peculiar odour, due to a substance developed 
 by the electricity, and called ozone. 
 
 Long ere this, while rubbing your tubes, you will 
 have heard the ' hissing' and ' crackling' so often re- 
 ferred to by the earlier electricians ; and if you have 
 rubbed your glass tube briskly in the dark, you will have 
 seen what they called the ' electric fire.' Using, instead 
 of a tube, a tall glass jar, rendered hot, a good warm 
 rubber, and vigorous friction, the streams of electric 
 fire are very surprising in the dark. 
 
 1 As a practical measure the watch-glass ought to rest upon a small 
 stand, and not upon a surface of large area. The eTperiment is par- 
 ticularly veil suited for projection on a screen.
 
 Discovery of Conduction and Insulation. 13 
 
 6. Discovery of Conduction and Insulation. 
 
 Here I must again refer to that most meritorious 
 philosopher Stephen Gray. In 1729, he experimented 
 with a glass tube stopped by a cork. When the tube 
 was rubbed, the cork attracted light bodies. Gray 
 states that he was ' much surprised ' at this, and he 
 * concluded that there was certainly an attractive virtue 
 communicated to the cork.' This was the starting 
 point of our knowledge of electric Conduction. 
 
 A fir stick 4 inches long, stuck into the cork, was 
 also found by Gray to attract light bodies. He made 
 his sticks longer, but still found . Fia. 6. 
 a power of attraction at their 
 ends. He then passed on to pack- 
 thread and wire. Hanging a 
 thread s, fig. 6, from the top win- 
 dow of a house, so that the lower 
 end nearly touched the ground, 
 and twisting the upper end of the 
 thread round his glass tube R, 
 on briskly rubbing the tube, light 
 bodies were attracted by the lower 
 end B of the thread. 
 
 But Gray's most remarkable 
 experiment was this: He sus- 
 pended a long hempen line hori- 
 zontally by loops of packthread, 
 but failed to transmit through it 
 the electric power. He then sus- 
 pended it by loops of silk and 
 succeeded in sending the ; attrac- 
 tive virtue' through 765 feet of thread. He at first
 
 14 Lessons in Electricity. 
 
 thought the silk was effectual because it was thin : 
 but on replacing a broken silk loop by a still thinner 
 wire, he obtained no action. Finally, he came to the 
 conclusion that his loops were effectual, not because 
 they were thin but because they were silk. This was 
 the starting point of our knowledge of Insulation. 
 
 It is interesting to notice the devotion of some men 
 of science to their work. Dr. Wells, who wrote a 
 beautiful essay, wherein he explained the origin of 
 dew, finished it when he was on the brink of the 
 grave. Stephen Gray was so near dying when his last 
 experiments were made, that he was unable to write 
 out an account of them. On his death-bed, and indeed 
 the very day before his death, his description of them 
 was taken from his lips by Dr. Mortimer, Secretary of 
 the Royal Society, and afterwards printed in the ' Philo- 
 sophical Transactions.' 
 
 One word of definition will be useful here. Some 
 substances, as proved by Stephen Gray, possess in a very 
 high degree the power of permitting electricity to pass 
 through them ; other substances stop the passage of 
 the electricity. Bodies of the first class are called 
 conductors : bodies of the second class are called 
 insulators. 
 
 You cannot do better than repeat here the experi- 
 ments of Gray. Push a cork into the open end of 
 your glass tube ; rub the tube, carrying the friction 
 up to the end holding the cork. The cork will attract 
 the balanced lath, shown in rig. 4, with which you have 
 already worked so much. 
 
 But the excited glass is here so near the end of 
 the cork that you may not feel certain that the observed 
 attraction is that of the cork. You can, however, prove
 
 Discovery of Conduction and Insulation. 
 
 13 
 
 that the cork attracts by its action upon light bodies 
 which cling to it. Stick a pen-holder into the cork, and 
 rub the glass tube as before. The free end of the holder 
 will attract the lath. Stick a deal rod three or four 
 feet long into the cork ; its free end will attract the 
 lath when the glass tube is excited. In this way, you 
 prove to demonstration that the electric power is con- 
 veyed along the rod. 
 
 7. The Electroscope. Further Enquiries on Con- 
 duction and Insulation. 
 
 A little addition to our apparatus will now be 
 desirable. You can buy a book of ' Dutch metal ' for 
 fourpence ; and a globular flask like that shown in fig. 7, 
 
 FIG. 7. 
 
 for sixpence, or at the most a shilling. Find a cork, 
 C, which fits the flask ; pass a wire, w, through the cork 
 and bend it near one end at a right angle. Attach
 
 1 6 Lessons in Electricity. 
 
 by means of wax to the bent arm, which ought to be 
 about three quarters of an inch long, two strips, L, of 
 the Dutch metal, about three inches long and from 
 half an inch to three-quarters of an inch wide. The 
 strips will hang down face to face, in contact with 
 each other. Stick by sealing-wax upon the other end 
 of the wire a little plate of tin or sheet-zinc, T, about 
 two inches in diameter. In all cases you muct be 
 careful so to use your wax as not to interrupt the metallic 
 connection of the various parts of your apparatus, which 
 we will name an electroscope. Gold leaf, instead of 
 Dutch metal, is usually employed for electroscopes. I 
 recommend the 'metal' because it is cheaper, and 
 will stand rougher usage. 
 
 See that your globular flask is dry and free from 
 dust. Bring your rubbed sealing-wax, E, or your rubbed 
 glass, near the little plate of tin, the leaves of Dutch 
 metal open out ; withdraw the excited body, the leaves 
 fall together. We shall enquire into the cause of this 
 action immediately. Practise, the approach and with- 
 drawal for a little time. Now draw your rubbed sealing- 
 wax or glass along the edge of the tin plate, T. The leaves 
 diverge, and after the sealing-wax or glass is withdrawn 
 they remain divergent. In the first experiment you com- 
 municated no electricity to the electroscope ; in the 
 second experiment you did. At present I will only ask 
 you to take the opening out of the leaves as a proof 
 that electricity has been communicated to them. 
 
 And now we are ready for Gray's experiments in 
 a form different from his. Connect the end of a long 
 wire with the tin plate of the electroscope ; coil the other 
 end round your glass tube. Eub the tube briskly, car- 
 rying the friction close to the coiled wire. A single
 
 Further Enquiries on Conduction, &c. 17 
 
 stroke of your rubber, if skilfully given, will cause the 
 leaves to diverge. The electricity has obviously passed 
 through the wire to the electroscope. 
 
 Substitute for the wire a string of common twine, 
 rub briskly and you will cause the leaves to diverge ; 
 but there is a notable difference as regards the prompt- 
 ness of the divergence. You soon satisfy yourself that 
 the electricity passes with greater facility through the 
 wire than through the string. Substitute for the 
 twine a string of silk. Mo matter how vigorously you 
 rub you can now produce no divergence. The elec- 
 tricity cannot get through the silk at all. 
 
 This is the place to demonstrate in a manner never 
 to be forgotten the influence of moisture. Wet 
 your dry silk string throughout, and squeeze it a little 
 so that the water from it may not trickle over your 
 glass tube. Coil it round the tube as before, and 
 excite the tube. The leaves of the electroscope imme- 
 diately diverge. The water is here the conductor. The 
 influence of moisture was first demonstrated by Du Fay 
 (1733 to 1737), who succeeded in sending electricity 
 through 1,256 feet of moist packthread. 
 
 It is hardly necessary to point out the meaning of 
 Gray's experiment where he found that, with loops of 
 wire or of packthread, he could not send the electricity 
 from end to end of his suspended string. Obviously 
 the electricity escaped in each of these cases through 
 the conducting support to the earth. 
 
 My assistant, Mr. Cottrell, who has been working very 
 hard for you and me, has devised an electroscope which 
 we shall frequently employ in our lessons. M, fig. 8, is 
 a little plate of metal, or of wood covered with tin-foil, 
 supported on a rod G of glass or of sealing-wax. N is
 
 18 Lessons in Electricity. 
 
 another plate of Dutch metal paper, separated about an 
 inch from M. and attached by sealing-wax to the long 
 
 FIG, 8. 
 I ait y~\ 
 
 straw 1 1' (broken off in the figure) ; A A' is a horizontal 
 pivot formed by a sewing needle, and supported on a bent 
 strip of metal, as shown in the figure. By weighting the 
 straw with a little wire near i', you so balance it that the 
 plate N shall be just lifted away from M. The wire w, 
 which may be 100 feet long, proceeds from M to your 
 glass tube, round which it is coiled. A single vigorous 
 stroke of the tube by the rubber sends electricity along 
 w to M ; N is attracted downwards, the other end of the 
 long straw being lifted through a considerable distance. 
 In subsequent figures you will see the complete straw- 
 index, and its modes of application. 
 
 A few experiments with either of these instruments 
 will enable you to classify bodies as conductors, semi- 
 conductors, and insulators. Here is a list of a few of 
 each, which, however, differ much among themselves. 
 
 Conductors. 
 
 The common metals 
 Well burned charcoal 
 Concentrated acids 
 Solutions of salts
 
 Further Enquiries on Conduction, &c. 19 
 
 Eain water 
 
 Linen 
 
 Living vegetables and animals. 
 
 Semi- conductors. 
 
 Alcohol and efcher Paper 
 
 Dry wood Straw. 
 
 Marble 
 
 Insulators. 
 
 Fatty oils Silk 
 
 Chalk Glass 
 
 India-rubber Wax 
 
 Dry paper Sulphur 
 
 Hair Shellac. 
 
 A little reflection will enable you to vary these 
 experiments indefinitely. Kub your excited sealing- 
 wax or glass against the tin plate of your electroscope, 
 and cause the leaves to diverge. Touch the plate with 
 any one of the conductors mentioned in the list ; the 
 electroscope is immediately discharged. Touch it witli 
 a semi -conductor ; the leaves fall as before, but less 
 promptly. Touch the plate finally with an insulator, the 
 electricity cannot pass, and the leaves remain unchanged. 
 
 8. Electrics and Non-Electrics. 
 
 For a long period, bodies were divided into electrics 
 and non-electrics, the former deemed capable of being 
 electrified, the latter not. Thus the amber of the an- 
 cients, and the spars, gems, fossils, stones, glasses, and 
 resins, operated on by Dr. Gilbert, were called electrics,
 
 20 Lessons ii Electricity. 
 
 while all tire metals were called non-electrics. We must 
 now determine the true meaning of this distinction. 
 
 Take in succession a piece of brass, of wood coated 
 with tin-foil, a lead bullet, apples, pears, turnips, car- 
 rots, cucumbers un coated wood not very dry will also 
 answer in the hand, arid strike them briskly with 
 flannel, or the fox's brush ; none of them will attract 
 the balanced lath, fig. 4, or show any other symptom 
 of electric excitement. All of them therefore would 
 have been once called non-electrics. 
 
 But suspend them in succession by a string of silk 
 held in the hand, and strike them again ; every one 
 of them will now attract the lath. 
 
 Eeflect upon the meaning of this experiment. We 
 have introduced an insulator the silk string be- 
 tween the hand and the body struck, and we find that 
 by its introduction the non-electric has been converted 
 into an electric. 
 
 The meaning is obvious. When held in the hand, 
 though electricity was developed in each case by the 
 friction, it passed Immediately through the hand and 
 body to the earth. This transfer being prevented by 
 the silk, the electricity, once excited, is retained, and 
 the attraction of the lath is the consequence. 
 
 In like manner, a brass tube, held in the hand and 
 struck with a fox's brush, shows no attractive power ; 
 but when a stick of sealing-wax, ebonite, or gutta- 
 percha is thrust into the tube as a handle, the striking 
 of the tube at once develops the power of attraction. 
 
 And now you see more clearly than you did at first 
 the meaning of the experiment with the heated 
 foolscap and india-rubber. Paper and wood always 
 imbibe a certain amount of moisture from the air.
 
 Electrics and Non-Electrics. 21 
 
 When the rubber was passed over the cold paper elec- 
 tricity was excited, but the paper, being rendered a 
 conductor by its moisture, allowed the electricity to 
 pass away. 
 
 Prove all things. Lay your cold foolscap on a cold 
 board supported by dry tumblers : pass your india- 
 rubber over the paper ; lift it by a loop of silk, which 
 has been previously attached to it, for if you touch 
 it it will discharge itself. You will find it electric ; 
 and with it you can charge your electroscope, or attr ict 
 from a distance your balanced lath. 
 
 The human body was ranked among the non-elec- 
 trics. Make plain to yourself the reason. Stand upon 
 the floor and permit a friend to strike you briskly with 
 the fox's brush. Present your knuckle to the balanced 
 lath, you will find no attraction. Here, however, you 
 stand upon the earth, so that even if electricity had 
 been developed, there is nothing to hinder it from 
 passing away. 
 
 But, place upon the ground four warm glass 
 tumblers, and upon the tumblers a board. 1 Stand upon 
 the board, and present your knuckle to the lath. A 
 single stroke of the fox's fur, if skilfully given, will 
 produce attraction. If you stand upon a cake of resin, 
 of ebonite, or upon a sheet of good india-rubber, the 
 effect will be the same. You can also charge your 
 electroscope with this electricity. 
 
 Throw a mackintosh over your shoulders and let a 
 friend strike it with the fox's brush, the attractive force 
 is greatly augmented. 
 
 1 Some caution is necessary here. A large class of cheap glass 
 tumblers conduct so freely that they are unfit for this and similar ex- 
 periments See 19.
 
 22 Lessons in Electricity. 
 
 After brisk striking, present your knuckle to the 
 knuckle of your friend. A spark will pass between you. 
 
 This experiment with the mackintosh further illus- 
 trates what you nave already frequently observed,namely, 
 that it is not friction alone, but the friction of special 
 substances against each other, that produces electricity. 
 
 Thus we prove that non-electrics, like electrics, can 
 be excited, the condition of success being, that an insu- 
 lator shall be interposed between the non-electric and 
 the earth. It is obvious that the old division into 
 electrics and non-electrics, really meant a division into 
 insulators and conductors. 
 
 9. Electric Repulsions. Discovery of two Electricities 
 
 We have hitherto dealt almost exclusively with 
 electric attractions, but in an experiment already re- 
 ferred to ( 2), Otto von Gruericke observed the repul- 
 sion of a feather by his sulphur globe. I also antici- 
 pated matters in the use of our Dutch metal electroscope 
 ( 7), where the repulsion of the leaves informed us 
 of the arrival of the electricity. 
 
 Du Fay, who was the real discoverer here, found a 
 gold-leaf floating in the air to be fir.-t attracted and 
 then repelled by the same excited body. He afterwards 
 proved that when thefloating leaf was repelled by rubbed 
 glass, it was attracted by rubbed resin, and that when 
 it was repelled by rubbed resin, it was attracted by 
 rubbed glass. Hence the important announcement, by 
 Du Fay, that there are two kinds of electricity. 
 
 The electricity excited on glass was for a time 
 called vitreous electricity, while that excited on 
 sealing-wax was called resinous electricity. These
 
 Discovery of two Electricities. 23 
 
 terms are however improper ; because, by changing the 
 rubber, we can obtain the electricity of sealing-wax 
 upon glass, and the electricity of glass upon sealing- 
 wax. 
 
 Eoughen, for example, the surface of your glass tube 
 with a grindstone, and rub it with flannel, the electricity 
 of sealing-wax will be found upon the vitreous surface. 
 Rub your sealing-wax with vulcanised india-rubber, the 
 electricity of glass will be found upon the resinous 
 surface. You will be able to prove this immediately. 
 
 "We now use the term positive or plus electricity to 
 denote that developed on glass bythe friction of silk; and 
 negative or minus electricity to denote that developed 
 on sealing-wax by the friction of flannel. These terms 
 are adopted purely for the sake of convenience. There is 
 no reason in nature why the resinous electricity should 
 not be called positive, and the vitreous electricity nega- 
 tive. Once agreed, however, to apply the terms as here 
 fixed, we must adhere to this agreement throughout. 
 
 10. Fundamental Law of Electric Action. 
 
 In all the experiments which we have hitherto made, 
 one of the substances operated on has been electrified 
 by friction, and the other not. But once engaged in 
 enquiries of this description, questions incessantly occur 
 to the mind, the answering of which extends our know- 
 ledge, and suggests other questions. Suppose, instead of 
 exciting only one of the bodies presented to each other, 
 we were to excite both of them, what would occur ? This 
 is the question which was asked and answered by Du 
 Fay, and which we must now answer for ourselves. 
 
 Here your wire loop, fig. 1, comes again into play
 
 24 Lessons in Electricity. 
 
 Place an un-rubbed gutta-percha tube, or a stick of 
 sealing-wax, in the loop, and be sure that it is un-rubbed 
 that no electricity adheres to it from former experi- 
 ments. If it fail to attract light bodies, it is unexcited ; 
 if it attract them, pass your hand over it several times, 
 or, better still, pass it over or through the flame 
 of a spirit lamp. This will remove every trace of 
 electricity. Satisfy yourself that the un-rubbed gutta- 
 percha tube is attracted by a rubbed one. 
 
 Eemove the un-rubbed tube from the loop, and ex- 
 cite it with its flannel rubber. One end of the tube is 
 held in your hand and is therefore unexcited. Eeturn 
 the tube to the loop, keeping your eye upon the excited 
 end. Bring a second rubbed tube near the excited end 
 of the suspended one : strong repulsion is the conse- 
 quence. Drive the suspended tube round and round 
 by this force of repulsion. 
 
 Bring a rubbed glass tube near the excited end of 
 the gutta-percha tube : strong attraction is the result. 
 
 Repeat this experiment step by step with two 
 glass tubes. Prove that the rubbed glass tube attracts 
 the un-rubbed one. Remove the un-rubbed tube from 
 the loop, excite it by its rubber, return it to the loop, 
 and establish the repulsion of glass by glass. Bring 
 rubbed gutta-percha or sealing-wax near the rubbed 
 glass : strong attraction is the consequence. 
 
 These experiments lead you directly to the funda- 
 mental law of electric action, which is this : Bodies 
 charged with the same electricity repel each other, 
 while bodies charged with opposite electricities attract 
 each other. Positive repels positive, and attracts nega~ 
 tive. Negative repels negative, and attracts positive. 
 
 Devise experiments which shall still further illus-
 
 Fundamental Law of Electric Action. 25 
 
 trate this law. Repeat, for example, Otto von Gruericke's 
 experiment. Hang a feather by a silk thread and, 
 bring your rubbed glass tube near it : the feather is 
 attracted, touches the tube, charges itself with the elec- 
 tricity of the tube, and is then repelled. Cause it to 
 retreat from the tube in various directions. Du Fay's 
 experiment with he gold-leaf will be repeated and 
 explained further on. See 18. 
 
 Hang your feather by a common thread : if no insu- 
 lating substance intervenes between the feather and the, 
 earth, you can get no repulsion. Why ? Obviously 
 because the charge of positive electricity communicated 
 by the rod, is not retained by the feather, but passes 
 away to the earth. Hence, you have not positive acting 
 against positive at all. Why the neutral body is at- 
 tracted by the electrified one, will, as already stated, 
 appear by and \>y. 
 
 Attract your straw needle by your rubbed glass 
 tube. Let the straw strike the tube, so that the one 
 
 FIG. 9. 
 
 1 1 
 
 ball rub against the other. The straw accepts
 
 2(5 Lessons in Electricity. 
 
 electricity of the tube and repulsion immediately follows 
 attraction, as shown in fig. 9. 
 
 Mr. Cottrell has devised the simple electroscope 
 represented in fig. 10 to show repulsion. A is a stem of 
 sealing-wax with a small circle of tin T at the top. w 
 is a bent wire proceeding from T, with a small disk 
 attached to it by wax. i i' is a little straw index, 
 supported by the needle N, as shown in fig. 10. The 
 stem A', also of sealing-wax, is not quite vertical, the 
 ooject being to cause the bit of paper V, to rest close 
 
 FIG. 10. 
 
 to w when the apparatus is not electrified. When 
 electricity is imparted to T, it flows through the wires 
 w and w, over both disk and index : immediate repul- 
 sion of the straw is the consequence. 
 
 No better experiment can be made to illustrate the 
 self-repulsive character of electricity than the follow- 
 ing one. Heat your square board ( 5), and warm, 
 as nefore, your sheet of foolscap. Spread the paper
 
 Fundamental Law of Electric Action. 27 
 
 apon the board, and excite it by the friction of india- 
 rubber. Cut from the sheet two long strips with your 
 penknife. Hold the strips together at one end. Separate 
 them from the board, and lift them into the air : they 
 forcibly drive each other apart, producing a wide diver- 
 gence. 
 
 Cut several strips, so as to form a kind of tassel. 
 Hold them together at one end. Separate them from 
 
 Fia. 11. 
 
 the board, and lift them into the air : they are driven 
 asunder by the self-repellent electricity, presenting an 
 appearance which may remind you of the hair of 
 Medusa. The effect is represented in fig. II. 1 
 
 1 In one of my earliest lectures at the Royal Institution, having 
 rubbed a sheet of foolscap, I was about to lift it bodily from the hot 
 board, and to place it against the wall, when the thought of cutting it 
 into strips, and allowing them to act upon each other, occurred to me. 
 The result, of course, was that above described. Simple and obvious aa 
 it was, it pave Faraday, who was present at the time, the most lively 
 pleasure. The simplest experiment, if only suited to its object, de- 
 lighted him.
 
 28 
 
 Lessons in Electricity. 
 
 Another very beautiful experiment fits in here, 
 Let fine silver sand, s, fig. 12, issue in a stream from a 
 glass funnel, through an aperture one-eighth of an inch 
 in diameter. Connect the sand in the funnel by a fine 
 wire w, fig. 13, with your warm glass tube. Uneiectrified, 
 
 Fio. 12. 
 
 FIG 13. 
 
 the sand particles descend as a continuous stream, s s', 
 fig. 1 2, but at every stroke of the rubber they fly asunder, 
 as in fig. 13, through self- repulsion. 1 
 
 Or let three or four fine fillets of water issue from 
 three or four pin-holes in the bottom of a vessel close 
 
 1 For these, and also for experiments "with the electroscope, the 
 teacher of a large class will find the lime-light shadows upon a white 
 screen (or better still, those of the electric light) exceedingly useful. 
 The effects are thus rendered risible to all at once.
 
 Fundamental Law of Electric Action. 29 
 
 to each other. Connect the water of the vessel with 
 your glass tube, and rub as before. The liquid veins 
 are scattered into spray by every stroke of the rubber. 
 
 These experiments are best made with 'Cottrell's 
 rubber,' described in 24. 
 
 And now you must learn to determine with cer- 
 tainty the quality of the electricity with which any body 
 presented to you may be charged. You see immediately 
 that attraction is no sure test, because un-electrified 
 bodies are attracted. Further on ( 14) you will be 
 able to grapple with another possible source of error in 
 the employment of attraction. 
 
 In determining quality, you must ascertain, by 
 trial, the kind of electricity by which the charged body 
 is repelled ; if, for example, any electrified body repel, 
 or is repelled by, sealing-wax rubbed with flannel, the 
 electricity of the body is negative ; if it repel, or is 
 repelled by, glass, rubbed with silk, its electricity is 
 positive. Du Fay had the sagacity to propose this 
 mode of testing quality. 
 
 Apply this test to the strips of foolscap paper ex- 
 cited by the india- rubber. Bring a rubbed gutta-percha 
 tube near the electrified strips, you have strong attrac- 
 tion. Bring a rubbed glass tube between the strips, 
 you have strong repulsion and augmented divergence. 
 Hence, the electricity, being repelled by the positive 
 glass, is itself positive. 
 
 11. Electricity of the Rubber. Double or ' Polar' 
 Character of the Electric Force. 
 
 "We have examined the action of each kind of elec- 
 tricity upon itself, and upon the other kind; but hitherto
 
 50 Lessons in Electricity. 
 
 we have kept the rubber out of view. One of the ques- 
 tions which inevitably occur to the enquiring scientific 
 mind would oe : how is the rubber affected by the act of 
 friction ? Here, as elsewhere, you must examine the 
 subject for yourself, and base your conclusions on the 
 facts you establish. 
 
 Test your rubber then, by your balanced lath. The 
 lath is attracted by the flannel which has rubbed against 
 gutta-percha ; and it is attracted by the silk, which has 
 rubbed against glass. 
 
 Kegarding the quality of the electricity of the 
 flannel or of the silk rubber, the attraction of the lath 
 teaches you nothing. But, suspend your rubbed glass 
 tube, and bring the flannel rubber near it : repulsion 
 follows. The silk rubber, on the contrary, attracts the 
 glass tube. Suspend your rubbed gutta-percha tube, 
 and bring the silk rubber near it : repulsion follows. 
 The flannel, on the contrary, attracts the tube. 
 
 The conclusion is obvious : the electricity of the 
 flannel is positive, that of the silk is negative. 
 
 But the flannel is the rubber of the gutta-percha, 
 whose electricity is negative ; and the silk is the rubber 
 of the glass, whose electricity is positive. Consequently, 
 we have not only proved the rubber to be electrified by 
 the friction, but also proved the electricity of the rubber 
 to be opposite in quality to that of the body rubbed. 
 
 All your subsequent experience will verify the 
 statement that the two electricities always go together; 
 that you cannot excite one of them without at the 
 same time exciting the other, and that the electricity 
 of the rubber, though opposite in quality, is in all 
 cases precisely equal in quantity to that of the body 
 rubbed.
 
 Electricity of the Rubber. 31 
 
 And now we will test these principles by a nev? 
 experiment. In 5 we learned that an ebonite comb is 
 electrified by its passage through dry hair. You can 
 readily prove the electricity of the comb to be negative. 
 But the hair is here the rubber, and, in accordance with 
 the principle just laid down, an equal quantity of 
 positive electricity has been excited in the hair. If 
 you stand on the ground uninsulated, the electricity of 
 the hair passes freely through your body to the earth. 
 
 But stand upon an insulating stool ' on your board, 
 for example, supported by four warm tumblers while 
 I, standing on the ground, pass the comb briskly through 
 your hair. I pass it ten, twenty, thirty times, and then 
 ask you to attract your balanced lath. You present 
 your knuckle, but there is no attraction. 
 
 Here the comb and the hair soon reach their maxi- 
 mum excitement, beyond which no further development 
 of electricity occurs. Now, though the comb, as shown 
 in 5, is competent to attract the lath, while your body 
 is here incompetent to do so, this may be because the 
 small quantity of electricity existing in a concen- 
 trated form upon the comb becomes, when diffused over 
 the body, too feeble to produce attraction. 
 
 Can we not exalt the electricity of your body? 
 Guided by the principles laid down, let us try to do so. 
 First I pass the unelectrified comb through your hair ; it 
 comes away electrified. Alter discharging the comb by 
 passing my hand closely over it, I pass it again through 
 the hair. As before, it quits the hair electrified, and I 
 
 1 A stool with glass legs which, to protect them from the moisture 
 of the air, are usually coated with a solution of shellac. Kegardir:? 
 the attraction of glass for atmospheric humidify, you will call to mmJ 
 what has been said in 5.
 
 32 Lessons in Electricity. 
 
 again discharge it. I do this ten or twenty times, 
 always depriving the comb of its electricity after it has 
 quitted the hair. Now present your knuckle to the 
 balanced lath. It is powerfully attracted. 
 
 Here, as I have said, the unelectrified comb carried 
 in each case electricity away with it ; but, in accordance 
 with the foregoing principles, it left an equal quantity 
 of the opposite electricity behind it. And though the 
 amount of electricity corresponding to a single charge 
 of the comb, when diffused over the body, proved insen- 
 sible to our tests, that amount ten or twenty times 
 multiplied became not only sensible, but strong. Indeed, 
 by discharging the comb, and passing it in each case 
 unelectrified through the hair, the insulated human body 
 can be rendered highly electrical. 
 
 Near the beginning of this section I said, in rather 
 an off-hand way, that rubbed flannel repels rubbed glass, 
 while rubbed silk repels rubbed gutta-percha. Now, 
 while it is generally easy to obtain the repulsion by the 
 flannel, it is by no means always easy to obtain the 
 repulsion by the silk. Over and over again I have 
 been foiled in my attempts to show this repulsion. I 
 wish you, therefore, to be aware of an infallible method 
 of obtaining it. 
 
 Stand on your insulating stool, and rub your glass 
 tube briskly with the amalgamated silk ; hand me the 
 tube. I pass my hand closely over its surface, removing 
 from that surface nearly the whole of its electricity. I 
 hand you the tube again, and you again excite it. You 
 hand it to me, and I again discharge it. In each case, 
 therefore, you excite an unelectrified glass tube, and in 
 each case the tube leaves behind upon the rubber an 
 amount of negative electricity equal in quantity to the
 
 Double or Polar, 1 &c. 33 
 
 positive carried away. By thus adding charge to 
 charge, the rubber is rendered highly electrical ; aiid, 
 even should its insulating power be impaired by the 
 amalgam, it can now afford to yield a portion of its 
 electricity to your hand and body, and still powerfully 
 repel rubbed gutta-percha. The principle, which 
 might be further illustrated, is obviously the same as 
 that applied in the case of the comb. 
 
 12. What is Electricity* 
 
 Thus far we have proceeded from fact to fact, ac- 
 quiring knowledge of a very valuable kind. But facts 
 alone cannot satisfy us. We seek a knowledge of the 
 principles which lie behind the facts, and which are to 
 be discerned by the mind alone. Thus, having spoken 
 as we have done, of electricity passing hither and 
 thither, and of its being prevented from passing ; 
 hardly any thoughtful boy or girl can avoid asking 
 what is it that thus passes ? what is electricity ? 
 Boyle and Newton betrayed their need of an answer to 
 this question when the one imagined his unctuous 
 threads issuing from and returning to the electrified 
 body ; and when the other imagined that an elastic 
 fluid existed which penetrated his rubbed glass. 
 
 When I say c imagined ' I do not intend to repre- 
 sent the notions of these great men as vain fancies. 
 Without imagination we can do nothing here. By 
 imagination I mean the power of picturing mentally 
 things which, though they have an existence as real as 
 that of the world around us, cannot be touched directly 
 by the organs of sense. I mean the purified scientific 
 imagination, without the exercise of which we cannot
 
 34 Lessons in Electricity. 
 
 take a single step into the region of causes and prin- 
 ciples. 
 
 It was by the exercise of the scientific imagination 
 that Franklin devised the theory of a single electric 
 fluid to explain electrical phenomena. This fluid he 
 supposed to be self-repulsive, and diffused in definite 
 quantities through all bodies. He supposed that when 
 a body has more than its proper share it is positively, 
 when less than its proper share it is negatively elec- 
 trified. It was by the exercise of the same faculty that 
 Symmer devised the theory of two electric fluids, each 
 self-repulsive, but both mutually attractive. 
 
 At first sight Franklin's theory seems by far the 
 simpler of the two. But its simplicity is only apparent. 
 For though Franklin assumed only one fluid, he was 
 obliged to assume three distinct actions. Firstly, he 
 had the self-repulsion of the electric particles. Secondly, 
 the mutual attraction of the electric particles and the 
 ponderable particles of the body through which the 
 electricity was diffused. Thirdly, these two assump- 
 tions when strictly followed out lead to the unavoidable 
 conclusion that the material particles also mutually 
 repel each other. Thus the theory is by no means so 
 simple as it appears. 
 
 The theory of Symmer, though at first sight the 
 most complicated, is in reality by far the simpler of the 
 two. According to it electrical actions are produced 
 by two fluids, each self-repulsive, but both mutually 
 attractive. These fluids cling to the atoms of matter, 
 and carry the matter to which they cling along with 
 them. Every body, in its natural condition, pos- 
 sesses both fluids in equal quantities. As long as 
 the fluids are mixed together they neutralise each other,
 
 What is Electricity f 35 
 
 the body in which they are thus mixed being in its 
 natural or unelectrical condition. 
 
 By friction (and by various other means) these two 
 fluids may be torn asunder, the one clinging by pre- 
 ference to the rubber, the other to the body rubbed. 
 
 According to this theory there must always be at- 
 traction between the rubber and the body rubbed, be- 
 cause, as we have proved, they are oppositely electrified. 
 This is in fact the case. And mark what I now say. 
 Over and above the common friction, this electrical at- 
 traction has to be overcome whenever we rub glass with 
 silk, or sealing-wax with flannel. 
 
 You are too young to fully grasp this subject 
 yet ; and indeed it would lead us too far away to 
 enter fully into it. But I will throw out for future 
 reflection the remark, that the overcoming of the 
 ordinary friction produces heat then and there upon 
 the surfaces rubbed, while the force expended in over- 
 coming the electric attraction may be converted into 
 heat which shall appear a thousand miles away from 
 the place where it was generated. 
 
 Theoretic conceptions are incessantly checked and 
 corrected by the advance of knowledge, and this theory 
 of electric fluids is doubted by many eminent scientific 
 men. It will, at all events, have to be translated into 
 a form which shall connect it with heat and light, 
 before it can be accepted as complete. Nevertheless, 
 keeping ourselves unpledged to the theory, we shall 
 find it of exceeding service both in unravelling ard in 
 connecting together electrical phenomena.
 
 36 Lessons in Electricity. 
 
 1 3. Electric Induction. Definition of the Term. 
 
 We have now to apply the theory of electric fluida 
 to the important subject of electric induction. 
 
 It was noticed by early observers that contact was 
 not necessary to electrical excitement. Otto von 
 Guericke, as we have already seen (2), found that a 
 body brought near his sulphur globe became electrical. 
 By bringing his excited glass tube near one end of a 
 conductor, Stephen Gray attracted light bodies at the 
 other end. He also obtained attraction through the 
 human body. From the human body also Du Fay, 
 to his astonishment, obtained a spark. Canton, in 
 1753, suspended pith-balls by thread, and holding an 
 excited glass tube, at a considerable distance from them, 
 caused them to diverge. On removing the tube the 
 balls fell together, no permanent charge being im- 
 parted to them. Such phenomena were further studied 
 and developed by Wilcke and ^pinus, Coulomb anil 
 Poisson. 
 
 These and all similar results are embraced by the 
 law, that when an electrified body is brought near an 
 unelectrified conductor, the neutral fluid of the latter is 
 decomposed ; one of its constituents being attracted, the 
 other repelled. When the electrified body is with- 
 drawn, the separated electricities flow again together 
 and render the conductor unelectric. 
 
 This decomposition of the neutral fluid by the mere 
 presence of an electrified body is called induction. It 
 is also called electrification by influence. 
 
 If, while it is under the influence of the electrified 
 body, the body influenced be touched, the free electri-
 
 Electric Induction. Definition of the Term. 37 
 
 city (which is always of the same kind as that of the 
 influencing body) passes away, the opposite electricity 
 being held captive. 
 
 On removing the electrified body the captive elec- 
 tricity is set free, the conductor being charger! with 
 electricity opposite in kind to that of the body which 
 electrified it. 
 
 You cannot do better here than repeat Stephen 
 Gray's experiment. Support a small plank or lath, L if, 
 fig. 14, upon a warm tumbler, G, and bring under one of 
 
 Fio. 14. 
 
 its ends, L, and within four or five inches of that end, 
 scraps of light paper or of gold leaf. Excite your 
 glass tube, E, vigorously, and bring it over the other end 
 of the plank, without touching it. The ends may be six 
 or eight feet apart ; the light bodies will be attracted. 
 The experiment is easily made, and you are not to rest, 
 satisfied till you can make it with ease and certainty. 
 
 This is a fit place to repeat that you must keep a 
 close eye upon the tumblers you employ for insulation.
 
 58 Lessons in Electricity. 
 
 Some of them, made of common glass, are hardly to be 
 accounted insulators at all. 
 
 14. Experimental Researches on Electric Induction. 
 
 Our mastery over this subject of induction must 
 be complete; for it underlies all our subsequent enquiries. 
 Without reference to it nothing is to be explained ; 
 possessed of it you will enjoy not only a wonderful 
 power of explanation, but of prediction. We will 
 attack it, therefore, with the determination to exhaust it. 
 And here a slight addition must be made to our 
 apparatus. We must be in a condition to take samples 
 FIG 15 ^ electricity, and to convey them, with 
 the view of testing them, from place to 
 place. For this purpose the little ' car- 
 rier,' shown in fig. 15, will be found 
 convenient. T is a bit of tin-foil, two or 
 three inches square. A straw stem is 
 stuck on to it by sealing-wax, the lower 
 end of the stem being covered by seal- 
 ing-wax. To make the insulation sure, 
 the part between R and s' is wholly of 
 sealing-wax. You can have stems of 
 ebonite, which are stronger, for a few 
 pence ; but you can have this one for a 
 fraction of a penny. The end R' is to be 
 held in the hand ; the electrified body is 
 to be touched by T, and the electricity 
 conveyed to an electroscope to be tested. 
 
 Touch your rubbed glass rod with T, and then touch 
 your electroscope : the leaves diverge with positive 
 electricity. Touch your rubbed gutta-percha or seal-
 
 Experimental Researches on Electric Induction. 39 
 
 ing-wax with T, and then touch your electroscope : the 
 ieaves diverge with negative electricity. If the elec- 
 tricity of any body augment the divergence produced 
 by the glass, the electricity of that body is positive. 
 If it augment the divergence produced by the gutta- 
 percha, the electricity is negative. And now we are 
 ready for further work. 
 
 Place an egg, E, fig. 1 6, on its side upon a dry wine- 
 
 FiQ. 16. 
 
 glass ; bring your excited glass tube, G, within an inch or 
 so of the end of the egg. What is the condition of the 
 egg? Its electricity is decomposed; the negative fluid 
 covering the end a adjacent to the glass, the positive 
 covering the other end 6. Kemove the glass tube : 
 what occurs? The two electricities flow together and 
 neutrality restored. Prove this neutrality. Neither 
 a carrier touching the egg, nor the egg itself, has any 
 power to affect your electroscope, or to attract your 
 balanced lath. 
 
 Again, bring the excited tube near the egg. Touch 
 its distant part b with your carrier. The carrier now 
 4
 
 40 Lessons in Electricity. 
 
 attracts the straw (fig. 2) or the balanced lath (fig. 4). 
 It also causes the leaves of your electroscope to diverge. 
 What is the quality of the electricity ? It repels and is 
 repelled by rubbed glass ; the electricity at b is therefore 
 positive. Discharge the carrier by touching it, and 
 bring it into contact with the end a of the egg nearest 
 to the glass tube. The electricity you take away repels 
 and is repelled by gutta-percha. It is therefore 
 negative. Test the quality also by the electroscope. 
 
 While the tube G is near the egg touch the end 6 with 
 your finger ; now try to charge the carrier by touching 
 b : you cannot do so the positive electricity has dis- 
 appeared. Has the negative disappeared also ? No. 
 Kemove the glass tube, and once more touch the egg 
 at b by the carrier. It is charged, not with positive, 
 but with negative electricity. Clearly understand this 
 experiment. The neutral electricity of the egg is first 
 decomposed into negative and positive ; the former 
 attracted, the latter repelled by the excited glass. The 
 repelled electricity is free to escape, and it has escaped 
 on your touching the egg with your finger. But the 
 attracted electricity cannot escape as long as the in- 
 fluencing tube is held near. On removing the tube 
 which holds the negative fluid in bondage, that fluid 
 immediately diffuses itself over the whole egg. An 
 apple, or a turnip, will answer for these experiments at 
 least as well as an egg. 
 
 Discharge the egg by touching it. .Re-excite the 
 glass tube and bring it again near. Touch the egg 
 with a wire or with your finger at a. Is it the negative at 
 a, into which you plunge your finger, that escapes ? No 
 such thing. The free positive fluid passes through the 
 negative, and through your finger to the earth. Prove
 
 Experimental Researches on Electric Induction. 41 
 
 this by removing, first, your finger, and then the glass 
 tube. The egg is charged negatively. 
 
 Again ; place two eggs, E E, fig. 17, lengthwise on two 
 dry wine-glasses, g g, and cause two of their ends to 
 
 FIG. 17. 
 
 touch each other, as at c. Bring your rubbed glass rod 
 near the end a, and while it is there separate the eggs by 
 moving one glass away from the other. Withdraw the 
 rod and test both eggs, a repels rubbed sealing-wax, and 
 6 repels rubbed glass ; a is therefore negative, b is posi- 
 tive. The two charges, moreover, exactly neutralise each 
 other in the electroscope. Again bring the eggs toge- 
 ther and restore the rubbed tube to its place near a. 
 Touch a and then separate the eggs. Eemove the glass 
 rod and test the eggs, a is negative, 6 is neutral. Its 
 electricity has escaped through the finger, though placed 
 at a. 
 
 Equally good, if not indeed more handy, for these 
 experiments are two apples A A, fig. 18, supported on 
 stems of sealing-wax. A needle is heated and sunk in 
 each case into the stick of wax at the top, and on to the 
 needle the apple is pushed. The sealing-wax stems are
 
 42 
 
 Lessons in Electricity. 
 
 ptuck on by melting to little foot-boards. By arrange- 
 ments of this kind you make experiments which are 
 
 Fm. 18. 
 
 more instructive than those usually made with instiu- 
 ments twenty times more expensive. 
 
 Fio. 19. 
 
 r i 
 
 Push your researches still farther, and instead of 
 -'n^ing the eggs or apples together, place them six feet
 
 Experimental Researches on Electric Induction. 43 
 
 or so apart, and let a light chain, c, fig. 1 9, or a wire, 
 stretch from one to the other. Two brass balls, or wooden 
 balls covered with tin-foil, supported by tall drinking 
 glasses, G G', will be better than the eggs for this experi- 
 ment, for they will bear better the strain of the chain ; 
 but you can make the experiment with the eggs, or very 
 readily with the two apples or two turnips. For the pre- 
 sent we will suppose the straw-index 1 i' not to be ther^. 
 Eub your glass tube R, and bring it near one of the 
 balls ; test both: the near one, T 7 , is negative, the distant 
 one, T, positive. Touch the near one, the positive elec- 
 tricity, which had been driven along the chain to the 
 remotest part of the system, returns along the chain, 
 passes through the negative, which is held captive by 
 the tube, and escapes to the earth. When the tube R 
 is removed, negative electricity overspreads both chain 
 and balls. 
 
 In fig. 8 you made the acquaintance of the plate N, 
 and the straw-index 1 i', shown on a smaller scale in 
 fig. 19. By their means you immediately see both the 
 effect of the first induction, and the consequence of 
 touching any part of the system with the finger. The 
 plate N rests over the ball or turnip T, the position of 
 the straw-index being that shown by the dots. Bring 
 the rubbed tube near T? : the end N of the index im- 
 mediately descends and the other end rises along the 
 graduated scale. Remove the glass rod ; the index I i' 
 immediately falls. Practise this approach and with- 
 drawal, and observe how promptly the index declares 
 the separation and recomposition of the fluids. 
 
 While the tube is near T', and the end N of the index 
 is attracted, let if be touched by the finger. The end N 
 is immediately liberated, for the electricity which pulled
 
 44 
 
 Lessons in Electricity. 
 
 ft down escapes along the chain and through the finger 
 to the earth. Now remove your excited tube. The 
 captive negative electricity diffuses itself over both balls, 
 and the index is again attracted. 
 
 Instead of the chain you may interpose between the 
 balls 100 feet of wire supported by silk loops. This is 
 done in fig. 20, which shows the wire w supported by 
 
 FIG. 20. 
 
 the silk strings s s s. For the ball or turnip i 7 , fig. 
 1 9, the cylinder c, on a glass support G, is substituted, 
 the little table M taking the place of the ball T. Every 
 approach and withdrawal of the rubbed glass tube R is 
 followed obediently by the attraction and liberation of 
 N, and the corresponding motion of the index N I. 
 
 Kepeat here an experiment, first made by a great elec- 
 trician named ^Epinus. I wish you to make these 
 historic experiments. Insulate an elongated metal con- 
 ductor, c c', fig. 21, or one formed of wood coated with 
 tin-foil even a carrot, cucumber, or parsnip, so that
 
 Experimental Researches on Electric Induction. 45 
 
 it be insulated, will answer. Let a small weight, w, 
 suspended from a silk string, s, rest on one end of the 
 conductor, and hold your rubbed glass tube, K, over the 
 
 FIG. 21. 
 
 other end. You can predict beforehand what will occur 
 when you remove the weight. It carries away with it 
 electricity, which repels rubbed glass, and attracts your 
 balanced lath. 
 
 Stand on an insulating stool ; or make one by 
 placing a board on four warm tumblers. Present 
 the knuckles of your right hand to the end of the 
 balanced lath, and stretch forth your left arm. There
 
 It) 
 
 Lessons in Electricity. 
 
 is no attraction. But let a friend or an assistant 
 bring the rubbed glass tube over the left arm ; the 
 lath immediately follows the right hand. 
 
 Touch the lath, or any other uninsulated body ; the 
 ' attractive virtue,' as it was called by Gray, disappears. 
 After this, as long as the excited tube is held over the 
 arm there is no attraction. But when the tube is 
 removed the attractive power of the hand is restored. 
 Here the first attraction was by positive electricity 
 driven to the right hand from the left, and the second 
 attraction by negative electricity, liberated by the re- 
 moval of the glass rod. Experiment proves the logic 
 of theory to be without a flaw. 
 
 Stand on an insulating stool, and place your right 
 hand on the electroscope : there is no action. Stretch 
 forth the left arm and permit an assistant alternately 
 to bring near, and to withdraw, an excited glass tube. 
 The gold leaves open and collapse in similar alternation. 
 At every approach, positive electricity is driven over the 
 gold leaves ; at every withdrawal, the equilibrium is 
 restored. 
 
 We are now in a condition to repeat, with ease, the 
 FIG. 22. 
 
 experiment of Du Fay mentioned in 13. A board is 
 supported by four silk ropes, and on the board i?
 
 Experimental Researches on Electric Induction. 47 
 
 stretched a boy. Bring his forehead, or better still his 
 nose, under the end of your straw index 1 i 7 , fig. 22. 
 Then bring down over his legs your rubbed glass tube ; 
 instantly the end i' is attracted and the end I rises 
 along the graduated scale. Before the end i' comes 
 into contact witli the nose or forehead a spark passes 
 between it and the boy. 
 
 I will now ask you to charge your Dutch metal elec- 
 troscope (fig. 7) positively by rubbed gutta-percha, and 
 to charge it negatively by rubbed glass. A moment's 
 reflection will enable you to do it. You bring your 
 excited body near : the same electricity as that of the 
 excited body is driven over the leaves, and they diverge 
 by repulsion. Touch the electroscope, the leaves col- 
 lapse. Withdraw your finger, and withdraw afterwards 
 the excited body : the leaves then diverge with the 
 opposite electricity. 
 
 The simplest way of testing the quality of electri- 
 city is to charge the electroscope with electricity of a 
 known kind. If, on the approach of the body to be 
 tested, the leaves diverge still wider, the leaves and the 
 body are similarly electrified. The reason is obvious. 
 
 Omitting the last experiment, the wealth of know- 
 ledge which these researches involve might be placed 
 within any intelligent boy's reach by the wise expen- 
 diture of half-a~crown. 
 
 Once firmly possessed of the principle of induction 
 and versed in its application, the difficulties of our subject 
 will melt away before us. In fact our subsequent work 
 will consist mainly in unravelling phenomena by the 
 aid of this principle. 
 
 Without a knowledge of this principle we could
 
 48 Lessons in Electricity. 
 
 render no account of the attraction of neutral bodies 
 by our excited tubes. In reality the attracted bodies 
 are not neutral : they are first electrified by influence, 
 and it is because they are thus electrified that they 
 are attracted. 
 
 This is the place to refer more fully to a point 
 already alluded to. Neutral bodies, as just stated, 
 are attracted, because they are really converted into 
 electrified bodies by induction. Suppose a body to be 
 feebly electrified positively, and that you bring your 
 rubbed glass tube to bear upon the body. You clearly 
 see that the induced negative electricity may be strong 
 enough to mask and overcome the weak positive charge 
 possessed by the body. We should thus have two bodies 
 electrified alike, attracting each other. This is the 
 danger against which I promised to warn you in 10, 
 where the test of attraction was rejected. 
 
 We will now apply the principle of induction to 
 explain a very beautiful invention, made known by 
 the celebrated Volta in 1775. 
 
 15. The Electrophorus. 
 
 Cut a circle, T, fig. 23, 6 inches in diameter out of 
 sheet zinc, or out of common tin. Heat it at its centre 
 by the flame of a spirit-lamp or of a candle. Attach to 
 it there a stick of sealing-wax, H, which, when the metal 
 cools, is to serve as an insulating handle. You have now 
 the lid of the electrophorus. A resinous surface, or what 
 is simpler a sheet of vulcanised india-rubber, p, or even 
 of hot brown paper, will answer fo : the plate of the 
 electrophorus. 
 
 Rub your 'plate ' with flannel, or whisk it briskly
 
 The Electrophorus. 
 
 49 
 
 with a fox's brush. It is thereby negatively electrified. 
 Place the lid ' of your ' electrophorus on the excited 
 
 FIG. 23. 
 
 surface : it touches it at a few points only. For the 
 most part lid and plate are separated by a film of 
 air. 
 
 The excited surface acts by induction across this 
 film upon the lid, attracting its positive and repelling 
 its negative electricity. You have in fact in the lid 
 two layers of electricity, the lower one, which is 'bound,' 
 positive ; the upper one, which is ' free,' negative. Lift 
 the lid : the electricities flow again together ; neutrality 
 is restored, and your lid fails to attract your balanced 
 lath. 
 
 Once more place the lid upon the excited surface : 
 touch it with the finger. What occurs ? You ought 
 to know. The free electricity, which is negative, will 
 escape through your body to the earth, leaving the 
 chained positive behind. 
 
 Now lift the lid by the handle : what is its condi- 
 tion ? Again I say you ought to know. It is covered
 
 50 Lessons in Electricity. 
 
 with free positive electricity. If it be presented to the 
 lath it will strongly attract it: if it be presented to the 
 knuckle it will yield a spark. 
 
 A smooth half-crown, or a penny, will answer for this 
 experiment. Stick to the coin an inch of sealing-wax 
 as an insulating handle : bring it down upon the excited 
 india-rubber : touch it, lift it, and present it to your 
 lath. The lath may be six or eight feet long, three inches 
 wide and half an inch thick ; the little electrophorus lid, 
 formed by the half crown, will pull it round and round. 
 The experiment is a very impressive one. 
 
 Scrutinise your instrument still further. Let the 
 end of a thin wire rest upon the lid of your electro- 
 phorus, under a little weight if necessary ; and connect 
 the other end of the wire with the electroscope. As 
 you lower the lid down towards the excited plate of the 
 electrophorus, what must occur ? The power of previ- 
 sion now belongs to you and you must exercise it. The 
 repelled electricity will flow over the leaves of the 
 electroscope, causing them to diverge. Lift the lid, 
 they collapse. Lower and raise the lid several times, 
 and observe the corresponding rhythmic action of the 
 electroscope leaves. 
 
 A little knob of sealing-wax, B, coated with tin-foil, 
 or indeed any knob with a conducting surface, stuck to 
 the lid of the electrophorus, will enable you to obtain a 
 better spark. The reason of this will Immediately 
 appear. 
 
 More than half the value of your present labour 
 consists in arranging each experiment in thought before 
 it is realised in fact ; and more than half the delight of 
 your work will consist in observing the verification of 
 what you have foreseen and predicted.
 
 Action of Points and Flames. 61 
 
 16. Action of Points and Flames. 
 
 The course of exposition proceeds naturally from 
 the electrophorus to the electrical machine. But be- 
 fore we take up the machine we must make our minds 
 clear regarding the manner in which electricity diffuses 
 itself over conductors, and more especially over elongated 
 and pointed conductors. 
 
 Kub your glass tube and draw it over an insulated 
 sphere of metal of wood covered with tin-foil, or indeed 
 any other insulated spherical conductor. Eepeat the 
 process several times, so as to impart a good charge to 
 the sphere. Touch the charged sphere with your 
 carrier, and transfer the charge to the electroscope. 
 Note the divergence of the leaves. Discharge the 
 electroscope, and repeat the experiment, touching, how- 
 ever, some other point of the sphere. The electroscope 
 shows sensibly the same amount of divergence. Even 
 when the greatest exactness of the most practised ex- 
 perimenter is brought into play, the spherical conductor 
 is found to be equally charged at all points of its 
 surface. You may figure the electric fluid as a little 
 ocean encompassing the sphere, and of the same depth 
 everywhere. 
 
 But supposing the conductor, instead of being a 
 sphere, to be a cube, an elongated cylinder, a .cone, or 
 a disk. The depth, or as it is sometimes called the 
 density of the electricity, will not be everywhere the 
 game. The corners of the cube will impart a stronger 
 charge to your carrier than the sides. The end of the 
 cylinder will impart a stronger charge than its middle. 
 The edge of the disk will impart a stronger charge than
 
 52 
 
 Lessons in Electricity. 
 
 its flat surface. The apex or point of the cone will 
 impart a stronger charge than its curved surface or its 
 base. 
 
 You can satisfy yourself of the truth of all this in a 
 rough, but certain way, by charging, after the sphere, a 
 turnip cut into the form of a cube; or a cigar-box 
 coated with tin-foil ; a metal cylinder, or a wooden one 
 coated with tin-foil ; a disk of tin or of sheet zinc ; 
 a carrot or parsnip with its natural shape improved 
 so as to make it a sharp cone. You will find the 
 charge imparted to the carrier by the sharp corners 
 and points of such bodies, when electrified, to be greater 
 than that communicated by the gently rounded or flat 
 surfaces. The difference may not be great, but it will 
 be distinct. Indeed an egg laid on its side, as we have 
 
 FIG. 24. 
 
 already used it in our experiments on induction (fig. 
 16), yields a stronger charge from its ends than from 
 its middle. 
 
 Let me place before you an example of this distribu- 
 tion, taken from the excellent work on ' Frictional Elec- 
 tricity ' by Professor Eiess of Berlin. Two cones, fig. 
 24, are placed together base to base. Calling the strength 
 of the charge along the circular edge where the two 
 bases join each other 100, the charge at the apex of the 
 blunter cone is 133; and at the apex of the sharper one
 
 Action of Points and Flames. 53 
 
 202. The other numbers give the charges taken from 
 the points where they are placed. Fig. 25, moreover, 
 
 represents a cube with a cone placed upon it. The 
 charge on the face of the cube being 1, the charges at 
 the corners of the cube and at the apex of the cone 
 are given by the other numbers ; they are all far in ex- 
 cess of the electricity on the flat surface. 
 
 Eiess found that he could deduce with great 
 accuracy the sharpness of a point, from the charge 
 whicli it imparted. He compared in this way the 
 sharpness of various thorns, with that of a fine English 
 sewing needle. The following is the result : Euphor- 
 bia thorn was sharper than the needle ; gooseberry 
 thorn of the same sharpness as the needle ; while cactus, 
 blackthorn, and rose, fell more and more behind the 
 needle in sharpness. Calling, for example, the charge 
 obtained from euphorbia 90 ; that obtained from the 
 needle was 80, and from the rose only 53. 
 
 Considering that each electricity is self-repulsive, and 
 that it heaps itself up upon a point in the manner here 
 shown, you will have little difficulty in conceiving that
 
 54 Lessons in Electricity. 
 
 when the charge of a conductor carrying a point is 
 sufficiently strong, the electricity will finally disperse 
 itself by streaming from the point. 
 
 The following experiments are theoretically impor- 
 tant : Attach a stick of sealing-wax to a small plate 
 of tin or of wood, so that the stick may stand upright. 
 Heat a needle and insert it into the top of the stick 
 of wax; on this needle mount horizontally a carrot. 
 You have thus an insulated conductor. Stick into your 
 carrot at one of its ends a sewing needle; and hold for 
 an instant your rubbed glass tube in front of this needle 
 without touching it. What occurs ? The negative 
 electricity of the carrot is immediately discharged from 
 the point against the glass tube. Eemove the tube, test 
 the carrot : it is positively electrified. 
 
 And now (or another experiment, not so easily made, 
 but still certain to succeed if you are careful. Excite 
 your glass rod, turn your needle away from it, and bring 
 the rod near the other end of the carrot. What occurs? 
 The positive electricity is now repelled to the point, 
 from which it will stream into the air. Eemove the 
 rod and test the carrot : it is negatively electrified. 
 
 Again turn the point towards you, and place in 
 front of it a plate of dry glass, wax, resin, shellac, 
 paraffin, gutta-percha, or any ether insulator. Pass your 
 rubbed glass tube once downwards or upwards, the in- 
 sulating plate being between the excited tube and the 
 point. The point will discharge its electricity against 
 tbe insulating plate, vrhich on trial will be found nega- 
 tively electrified.
 
 The Electrical Machine. 
 
 55 
 
 17. The Electrical Machine. 
 
 An electrical machine consists of two principal parts; 
 the insulator which is excited by friction, and the 
 * prime conductor.' 
 
 The sulphur sphere of Otto von Guericke was, as 
 already stated, the first electrical machine. The hand 
 was the rubber, and indeed it long continued to be so. 
 For the sulphur sphere, Hauksbee and Winckler substi- 
 tuted globes of glass. Boze of Wittenberg (1741) 
 added the prime conductor, which was at first a tin 
 tube supported by resin, or suspended by silk. Soon 
 afterwards Gordon substituted a glass cylinder for 
 the globe. It was sometimes mounted vertically, 
 sometimes horizontally. Gordon so intensified his 
 discharges as to be able to kill small birds with 
 them. In 1760 Planta introduced the plate machine 
 
 now commonly in use. 
 
 FIG. 26. 
 
 Mr. Cottrell has constructed for these Lessons the 
 small cylinder machine shown in fig. 26. The glass 
 5
 
 56 Lessons in Electricity. 
 
 cylinder is about 7 inches long and 4 inches in dia- 
 meter: its cost is eighteen pence. Through the 
 cylinder passes tightly, as an axis, a piece of lath, 
 rendered secure by sealing-wax where it enters and where 
 it quits the cylinder. G is a glass rod supporting the 
 conductor c, which is a piece of lath coated with tin- 
 foil. Into the lath is driven the series of pin points, 
 p, P. The rubber K, is seen at the further side of the 
 cylinder, supported by the upright lath R', and caused 
 to press against the glass, s' is a flap of silk attached 
 to the rubber. When the handle is turned sparks may 
 be taken, or a Leyden jar * charged at the knob c. 
 
 A plate machine is shown in fig. 27. p is the plate, 
 which turns on an axis passing through its centre : 
 
 FIG. 27. 
 
 B and a' are two rubbers which clasp the plate, with the 
 flaps of silk s s* attached to them. A and A' are rows of 
 
 1 To be subsequently explained.
 
 . The Electrical Machine. 57 
 
 points forming part of the prime conductor, c. G G' is an 
 insulating rod of glass, which cuts off the connection 
 between the conductor and the handle of the machine. 
 
 The prime conductor is charged in the following 
 manner. When the glass plate is turned, as it passes 
 each rubber it is positively electrified. Facing the 
 electrified glass is the row of points, placed midway 
 between the two rubbers. On these points the glass acts 
 by induction, attracting the negative and repelling 
 the positive. In accordance with the principles 
 already explained in 16, the negative electricity 
 streams from the points against the excited glass, which 
 then passes on neutralised to the next rubber, where it 
 is again excited. 
 
 Thus the prime conductor is charged, not by the 
 direct communication to it of positive electricity, but 
 by depriving it of its negative. 
 
 If when the conductor is charged you bring the 
 knuckle near it, the electricity passes from the con- 
 ductor to the knuckle in the form of a spark. 
 
 Take this spark with the blunt knuckle while the 
 machine is being turned; and then try the effect of 
 presenting the finger ends, instead of the knuckle, to 
 the conductor. The spark falls exceedingly in bril- 
 liancy. Substitute for the finger ends a needle point : 
 you fail to get a spark at all. To obtain a good spark 
 the electricity upon the prime conductor must reach 
 a sufficient density (or tension as it is sometimes 
 called) ; and to secure this no points from which the 
 electricity can stream out must exist on the conductor, 
 or be presented to it. All parts of the conductor are 
 therefore carefully rounded off. sharp points and edges 
 being avoided.
 
 58 Lessons in Electricity. 
 
 It is usual to attach to the conductor an electro- 
 scope consisting of an upright metal stern, A c, fig, 
 FIG. 28. 28, to which a straw with a pitli 
 
 ball, B, at its free end, is attached. 
 The straw turns loosely upon a 
 pivot at c. The electricity pass- 
 ing from the conductor is diffused 
 over the whole electroscope, and 
 the straw and stem being both 
 positively electrified, repel each 
 other. The straw, being the 
 movable body, flies away. The 
 amount of the divergence is 
 measured upon a graduated arc. 
 
 18. Further Experiments on the Action of Points. 
 The Electric Mill. The Gulden Fish. Lightning 
 Conductors. 
 
 If no point exist on the conductor, a single turn of 
 the handle of the machine usually suffices to cause the 
 straw to stand out at a large angle to the stem. If, on 
 the contrary, a point be attached to the conductor, 
 you cannot produce a large divergence, because the 
 electricity, as fast as it is generated, is dispersed by 
 the point. The same effect is observed when you pre- 
 sent a point to the conductor. The conductor acts by 
 induction upon the point, causing the negative electri- 
 city to stream from it against the conductor, which is 
 thus neutralised almost as fast as it is charged. Flames 
 and glowing embers act like points ; they also rapidly 
 discharge electricity. 
 
 The electricity escaping from a point or flame into 
 the air renders the air self-repulsive. The consequence
 
 The Electric Mill 
 
 59 
 
 is that when the hand is placed over a point mounted 
 on the prime conductor of a machine in good action, a 
 cold blast is distinctly felt. Dr. Watson noticed this 
 blast from a flame placed on an electrified conductor ; 
 while Wilson noticed the blast from a point. Jallabert 
 and the Abbe Nollet also observed and described the 
 influence of points and flames. The blast is called the 
 ' electric wind.' Wilson moved bodies by its action : 
 Faraday caused it to depress the surface of a liquid : 
 Hamilton employed the reaction of the electric wind 
 to make pointed wires rotate. The ' wind ' was also 
 found to promote evaporation. 
 
 Hamilton's apparatus is called the c electric mill.' 
 Make one for yourself thus : Place two straws s s, s' /, 
 
 FIG. 29. 
 
 fig. 29, about eight inches long, across each other at a 
 right angle. Stick them together at their centres by a 
 bit of sealing-wax. Pass a fine wire through each straw 
 and bend it where it issues from the straw, so as to
 
 60 Lessons in Electricity. 
 
 form a little pointed arm perpendicular to the straw, and 
 from half an inch to three-quarters of an inch long. It 
 is easy, by means of a bit of cork or sealing-wax, to 
 fix the wire so that the little bent arms shall point 
 not upwards or downwards, but sideways, when the cross 
 is horizontal. The points of sewing needles may also 
 be employed for the bent arms. A little bit of straw 
 stuck into the cross at the centre, forms a cap. This 
 slips over a sewing needle, N, supported by a stick of seal- 
 ing-wax, A. Connect the sewing needle with the electric 
 machine, and turn. A wind of a certain force is dis- 
 charged from every point, and the cross is urged round 
 with the same force in the opposite direction. 
 
 You might easily, of course, so arrange the points 
 that the wiad from some of them would neutralise the 
 wind from others. But the little pointed arms are to be 
 so bent that the reaction in every case shall not oppose 
 but add itself to the others. 
 
 The following experiments will yield you important 
 information regarding the action of points. Stand, as 
 you have so often done before, upon a board supported 
 by four warm tumblers. Hold a small sewing needle, 
 with its point defended by the fore finger of your right 
 hand, towards your Dutch metal electroscope. Place 
 your left hand on the prime conductor of your machine. 
 Let the handle be turned by a friend or an assistant : 
 the leaves of the electroscope open out a little. Un- 
 cover the needle point by the removal of your finger ; 
 the leaves at once fly violently apart. 
 
 Mount a stout wire upright on the conductor, c, fig. 
 30, of your machine ; or support the wire by sealing- 
 wax, gutta-percha or glass, at a distance from the
 
 Experiments on Action of Points. 
 
 61 
 
 conductor, and connect both by a fine wire. Bend your 
 stout wire into a hook, and hang from it a tassel, T, 
 
 Fio. 30. 
 
 composed of many strips of light tissue paper. Work 
 the machine. Electricity from the conductor flows 
 over the tassel, and the strips diverge. 1 Hold your 
 closed fist towards the tassel, the strips of paper 
 stretch towards it. Hold the needle, defended by the 
 finger, towards the tassel : attraction also ensues. 
 Uncover the needle without moving the hand; the 
 strips retreat as if blown away by a wind. Holding the 
 needle N, fig. 31, upright underneath the tassel, its 
 strips discharge themselves and collapse utterly. 
 
 And now repeat Du Fay's experiment which led to 
 
 1 This is always the case in London. Still even here some daya 
 wo so dry as to render it difficult to electrify the tassel.
 
 6S 
 
 Lessons in Electricity. 
 
 FIG. 31. 
 
 the discovery of two electricities. Excite your glass 
 tube, and hold it in readiness while a friend, or an 
 assistant, liberates a real 
 gold or silver leaf in the 
 air. Bring the tube near 
 the leaf: it plunges to- 
 . wards the tube, stops 
 suddenly, and then flies 
 away. You may chase 
 it round the room for 
 hours without permitting 
 it to reach the ground. 
 The leaf is first acted 
 upon inductively by the 
 tube. It is powerfully 
 attracted for a moment, 
 and rushes towards the 
 tube. But from its thin
 
 Experiments on Action of Points. 
 
 68 
 
 edges and corners the negative electricity streams forth, 
 leaving the leaf positively electrified. Repulsion then 
 sets in, because tube and leaf are electrified alike, as 
 shown in fig. 32. The retreat of the tassel in the last 
 experiment is due to a similar cause. 
 
 There is also a discharge of positive electricity 
 into the air from the more distant portions of the 
 
 FIG. 33. 
 
 gold-leaf, to which that electricity is repelled. Both 
 discharges are accompanied by an electric wind. It ia 
 possible to give the gold-leaf a shape which shall enable 
 it to float securely in the air, by the reaction of the two 
 winds issuing from its opposite ends. This is Franklin's 
 experiment of the Golden Fish. It was first made with 
 the charged conductor of an electrical machine. M.
 
 64 Lessons in Electricity. 
 
 Srtsczek revived it in a more convenient form, using 
 instead of the conductor the knob of a charged Leyden 
 jar. You may walk round a room with the jar in your 
 hand ; the ' fish ' will obediently follow in the air an 
 inch or two, or even three inches, from the knob. See 
 A B, fig. 33. Even a hasty motion of the jar will not 
 shake it away. 
 
 Well-pointed lightning conductors, when acted on 
 by a thunder cloud, discharge their induced electricity 
 against the cloud. Franklin saw this with great clear- 
 ness, and illustrated it with great ingenuity. The 
 under side of a thunder cloud, when viewed horizon- 
 tally, he observed to be ragged, composed, in fact, of 
 fragments one below the other, sometimes reaching 
 near the earth. These he regarded as so many 
 stepping-stones which assist in conducting the stroke 
 of the cloud. To represent these by experiment he 
 took two or three locks of fine loose cotton, tied them 
 in a row, and hung them from his prime conductor. 
 When this was excited the locks stretched downwards 
 towards the earth ; but by presenting a sharp point 
 erect under the lowest bunch of cotton, it shrunk up- 
 wards to that above it, nor did the shrinking cease till 
 all the locks had retreated to the prime conductor 
 itself. ' May not,' says Franklin, ' the small electrified 
 clouds, whose equilibrium with the earth Is so soon 
 restored by the point, rise up to the main body, and 
 by that means occasion so large a vacancy, that the 
 grand cloud cannot strike in that place ? ' 
 
 19. Histoi^y of the Leyden Jar. The Ley den Battery. 
 
 The next discovery which we have to master throws 
 
 all former ones into the shade. It was first announced in
 
 Uiatory of the Leyden Jar. 65 
 
 a letter addressed on the 4th of November, 1745, to Dr. 
 Lieberkiihn, of Berlin, by Kleist, a clergyman of 
 Cammin, in Pomerania. By means of a cork, c, fig. 34, 
 
 FIG. 34. 
 
 he fixed a nail, N, in a phial, G, into which he had poured 
 a little mercury, spirits, or water, w. On electrifying the 
 nail he was able to pass from one room into another with 
 the phial in his hand and to ignite spirits of wine with it. 
 'If,' said he, ' while it is electrifying I put my finger, or a 
 piece of gold which I hold in my hand, to the nail, I 
 receive a shock which stuns my arms and shoulders.' 
 
 In the following year Cunseus of Leyden made sub- 
 stantially the same discovery. It caused great wonder 
 and dread, which arose chiefly from the excited imagi- 
 nation. Musschenbroek felt the shock, and declared 
 in a letter to a friend that he would iiot take a second 
 one for the crown of France. Bleeding at the nose, 
 ardent fever, a heaviness of head which endured for 
 days, were all ascribed to the shock. Boze wished that 
 he might die of it, so that he might enjoy the honour 
 of having his death chronicled in the Paris ' Academy
 
 66 Lessons in Electricity. 
 
 of Sciences.' Kleist missed the explanation of the 
 phenomenon ; while the Leyden philosophers correctly 
 stated the conditions necessary to the success of the 
 experiment. Hence the phial received the name of the 
 Leyden phial, or Leyden jar. 
 
 The discovery of Kleist and Cunaeus excited the 
 most profound interest, and the subject was explored in 
 all directions. Wilson in 1746 filled a phial partially 
 with water, and plunged it into water, so as to bring 
 the water surfaces, within and without the phial, to the 
 same level. On charging such a phial the strength 
 of the shock was found greater than had been observed 
 before. 
 
 Two years subsequently Dr. Watson and Dr. Bevis 
 noticed how the charge grew stronger as the area of the 
 conductor in contact with the outer surface of the phial 
 increased. They substituted shot for water inside 
 the jar, and obtained substantially the same effect. 
 Dr. Bevis then coated a plate of glass on both sides 
 with silver foil, to within about an inch of the edge, and 
 obtained from it discharges as strong as those obtained 
 from a phial containing half a pint of water. Finally 
 Dr. Watson coated his phial inside and out with silver 
 foil. By these steps the Leyden jar reached the form 
 which it possesses to-day. 
 
 It is easy to repeat the experiment of Dr. Bevis. 
 Procure a glass plate nine inches square ; cover it on 
 both sides, as he did, with tin-foil seven inches square, 
 leaving the rim uncovered. Connect one side with the 
 earth and the other with the machine. Charge and dis- 
 charge : you obtain a brilliant spark. 
 
 In our experiment with the Golden Fish (fig. 33), we 
 employed a common form of the Leyden jar, only with
 
 History of the Ley den Jar. 67 
 
 the difference that to get to a sufficient distance from 
 the glass, so as to avoid the attraction of the fish by 
 the jar itself, the knob was placed higher than usual. 
 But with a good flint-glass tumbler, a piece of tin-foil, 
 and a bit of stout wire, you can make a jar for yourself. 
 Bad glass, remember, is not rare. 1 In fig. 35 you have 
 
 FIG. 35. 
 
 such a jar. T is the outer, Tf the inner coating, 
 reaching to within an inch of the edge of the tumbler 
 G. w is the wire fastened below by wax, and sur- 
 mounted by a knob, which may be of metal, or of 
 wax or wood, coated with tin-foil. In charging the 
 jar you connect the outer coating with the earth say 
 with a gas-pipe or a water-pipe and present the knob 
 to the conductor of your machine. A few turns will 
 charge the jar. It is discharged by laying one knob of 
 a ' discharger ' against the outer coating, and causing 
 the other knob to approach the knob of the jar. Before 
 
 1 In preparing these Lessons we have made several jars which re- 
 fused to be charged, through the badness of their glass, and which 
 ihowed their imperfect insulation by discharging our electroscope.
 
 68 
 
 Lessons in Electricity. 
 
 FIG. 36. 
 
 contact, the electricity flies from knob to knob in the 
 
 form of a spark. 
 
 A ' discharger ' suited to our means and purposes 
 is shown in fig. 36. H is a stick 
 of sealing-wax, or, better still, of 
 ebonite : w w a stout wire bent as 
 in the figure, and ending in the 
 knobs B B'. These may be of wax 
 coated with tin-foil. Any other 
 light conducting knobs would of 
 course answer. The insulating 
 handle H protects you effectually 
 from the shock. 
 
 You must render yourself expert 
 in the use of the discharger. The 
 mode of using it is shown in fig. 37. 
 FIG. 37. 
 
 By augmenting the size of a Leyden jar we render
 
 Explanation of the Leyden Jar. 
 
 69 
 
 it capable of accepting a larger charge of electricity. 
 But there is a limit to the size of a jar. When, there- 
 fore, larger charges are required than a single jar can 
 furnish, we make use of a number of jars. In fig. 38 
 
 FIG. 38. 
 
 nine of them are shown. All their interior coatings 
 are united together by brass rods, while all the outer 
 coatings rest upon a metal surface in free communica- 
 tion with the earth. 
 
 This combination of Leyden jars constitutes the 
 Leyden Battery, the effect of which is equal to that of 
 a single jar of nine times the size of one of the jars. 
 
 20. Explanation of the Leyden Jar. 
 
 The principles of electrical induction with which 
 you are now so familiar will enable you to thoroughly
 
 70 Lessons in Electricity. 
 
 analyse and understand the action of the Leyden jar. 
 In charging the jar the outer coating is connected 
 with the earth, and the inner coating with the electrical 
 machine. Let the machine, as usual, be of glass yielding 
 positive electricity. When it is worked the electricity 
 poured into the jar acts inductively across the glass 
 upon the outer coating ; attracting its negative and 
 repelling its positive to the earth. Two mutually at- 
 tractive electric layers are thus in presence of each 
 other, being separated merely by the glass. When the 
 machine is in good order and the glass of the jar is thin, 
 the attraction may be rendered strong enough to per- 
 forate the jar. By means of the discharger the oppo- 
 site electricities are enabled to unite in the form of a 
 spark. 
 
 Franklin saw and announced with clearness the 
 escape of the electricity from the outer coating of the 
 jar. His statement is that whatever be the quantity of 
 the 'electric fire' thrown into the jar, an equal quantity 
 was dislodged from the outside. We have now to prove 
 by actual experiment that this explanation is correct. 
 
 Place your Leyden jar upon a table, and connect 
 the outer coating with your electroscope. There is no 
 divergence of the leaves when electricity is poured into 
 the jar. 
 
 But here the outer coating is connected through the 
 table with the earth. Let us cut off this communica- 
 tion by an insulator. Place the jar upon a board sup- 
 ported by warm tumblers, or upon a piece of vulcanised 
 india-rubber cloth, and again connect the outer coating 
 with the electroscope. The moment electricity is com- 
 municated to the knob of the jar the leaves of Dutch 
 metal diverge. Detach the wire by your discharger
 
 Explanation of the Leyden Jar. 
 
 71 
 
 and test the quality of the electricity it is positive, 
 us theory declares it must be. 
 
 Consider now the experiment of Kleist and Cungeus 
 (fig. 34). You will, I doubt not, penetrate its meaning. 
 You will see that in their case the hand formed the 
 outer coating of the jar. When electricity was com- 
 municated through the nail to the water within, that 
 electricity acted across the glass inductively upon the 
 hand, attracting the one fluid and repelling the other 
 to the earth. 
 
 Again I say, prove ail things ; and what is here 
 affirmed may be proved by the following beautiful and 
 
 FIG. 39. 
 
 conclusive experiment : Stand on your board, I r ,hg. 39,
 
 72 Lesson? in Electricity. 
 
 insulated by its four tumblers ; or upon a sheet of 
 gutta-percha, or vulcanised india-rubber. Seize the old 
 Leyden phial, J, with your left hand, and present the 
 knuckle of your right hand to your balanced lath, I/ L. 
 When electricity is communicated to the nail, the lath 
 is immediately attracted by the knuckle. Or touch your 
 electroscope with your right hand : when the phial is 
 charged theleaves immediately diverge, by the electricity 
 driven from your left hand to the electroscope. 
 
 Here the nail may be electrified either by connect- 
 ing it with the prime conductor of the machine, or by 
 rubbing it with an excited glass rod. Indeed I should 
 prefer your resorting to the simplest and cheapest means 
 in making these experiments. 
 
 21. Franklin's Cascade. Battery. 
 
 As a thoughtful and reflective boy or girl you 
 cannot, I think, help wondering at the power which your 
 thorough mastery of the principles of induction gives 
 you over these wonderful and complicated phenomena. 
 By those principles the various facts of our science are 
 bound together into an organic whole. But we have 
 not yet exhausted the fruitfulness of this principle. 
 
 Consider the following problem. Usually we allow 
 the electricity of the outer coating to escape to the earth. 
 Suppose we try to utilise it. Place, then, your jar A B, 
 fig. 40, upon vulcanised india-rubber, and connect by a 
 wire B c its outer coating with the knob or inner coating 
 of a second jar c D. What will occur when the first jar is 
 charged ? Why, the second one will be charged also by 
 the electricity which has escaped from the outer coating 
 of the first. And suppose you connect the outer coat-
 
 Franklin's Cascade Battery. 73 
 
 ing of the second insulated jar with the inner coating of 
 a third, E F ; what occurs ? The third jar will obviously 
 be charged with the electricity repelled from the outer 
 coating of the second. Of course we need not stop here. 
 We may have a long series of insulated jars, the outer 
 coating of each being connected with the inner coating 
 of the next succeeding one. Connect the outer coat- 
 ing of the last jar I K by a wire e with the earth, and 
 charge the first jar. You charge thereby the entire 
 series of jars. In this simple way you master practi- 
 Fio. 40. 
 
 cally, and grasp the theory of Franklin's celebrated 
 ' cascade battery.' 
 
 You must see that before making this important 
 experiment you could really have predicted what would 
 occur. This power of prevision is one of the most 
 striking characteristics of science. 
 
 22. Novel Ley den Jars of the Simplest 
 
 Possessed of its principles, we can reduce the 
 
 Leyden jar to far simpler forma than any hitherto
 
 74 Le&sons in Electricity. 
 
 dealt with. Spread a sheet of tin-foil smoothly upon a 
 table, and lay upon the foil a pane of glass. Remember 
 that the glass, as usual, must be dry. Stick on to the 
 glass by sealing-wax two loops of narrow silk ribbon, 
 by which the pane may be lifted ; and then lay 
 smoothly upon the glass a second sheet of tin-foil, 
 less than the pane in size, leaving a rim of uncovered 
 glass all round. Carry a fine wire from the upper 
 sheet of tin-foil to your electroscope. A little weight 
 will keep the end of the wire attached to the tin-foil. 
 
 Rub this weight with your excited glass tube, two 
 or three times if necessary, until you see a slight diver- 
 gence of the Dutch metal leaves. Or connecting the 
 weight with the conductor of your machine, turn very 
 carefully until the slight divergence is observed. What 
 is the condition of things here? You have poured, 
 say positive electricity on to the upper sheet of metal. 
 It acts inductively across the glass upon the under 
 sheet, the positive fluid of which escapes to the 
 earth, leaving the negative behind. You see before your 
 mind's eye two layers holding each other in bondage. 
 Now take hold of your loops and lift the glass plate, so 
 as to separate the upper tin- foil from the lower. What 
 would you expect to occur ? Freed from the grasp of 
 the lower layer, the electricity of the upper one will 
 diffuse itself over the electroscope so promptly and 
 powerfully, that if you are not careful you will destroy 
 the instrument by the mutual repulsion of its leaves. 
 
 Practise this experiment, which is a very old one 
 of mine, by lowering and lifting the glass plate, and 
 observing the corresponding rhythmic action of the 
 leaves of the electroscope. 
 
 Common tin-plate may be used in this experiment
 
 Novel Ley den Jars. 75 
 
 instead of tin-foil, and a sheet of vulcanised india-rubber 
 instead of the pane of glass. Or simpler still, for the 
 tin-foil a sheet of common unwarmed foolscap may be 
 employed. Satisfy yourself of this. Spread a sheet of 
 foolscap on a table ; lay the plate of glass upon it, and 
 spread a leaf of foolscap, less than the glass in size, on the 
 plate of glass. Connect the leaf with the electroscope, 
 and charge it, exactly as you charged the tin-foil. On 
 lifting the glass with its leaf of foolscap, the leaves of 
 the electroscope instantly fly apart ; on lowering the 
 glass they again fall together. Abandon the under 
 sheet altogether, and make the table the outer coating ; 
 if it be not of very dry wood, or covered by an insu- 
 lating varnish, you will obtain with it the results 
 obtained with the tin-foil, tin, and foolscap. Thus by 
 the simplest means we illustrate great principles. 
 
 The withdrawal of the electricity from the electro- 
 scope, by lowering the plate of glass, so as to bring the 
 electricity of the upper coating within the grasp of the 
 lower one, is sometimes called ' condensation.' The elec- 
 tricity on one plate or sheet was figured as squeezed 
 together, or condensed, by the attraction of the other. 
 A special instrument called a condenser is constructed 
 by instrument makers to illustrate the action here ex- 
 plained. 
 
 You may readily make a condenser for yourself. 
 Take two circles, p p 7 , fig. 41, of tin or of sheet zinc, 
 and support the one, p 7 , by a stick of sealing-wax or 
 glass, G ; the other, p, by a metal stem, connected with 
 the earth. The insulated plate, p', is called the col- 
 lecting plate ; the uninsulated one, p, the condensing 
 plate. Connect the collecting plate with your electro-
 
 76 
 
 Lessons in Electriciiy. 
 
 scope by the wire w 9 and briDg the condensing plate 
 near it, leaving, however, a thin space of air between 
 
 FIG. 41. 
 
 them. Charge the collector, p', or the wire, w, with your 
 glass rod, until the leaves of the electroscope begin to 
 diverge. Withdraw the condensing plate, the leaves 
 fly asunder; bring the condensing plate near, the leaves 
 
 again collapse. 
 
 FIG. 42. 
 
 Or vary your construction, and make your con-
 
 Novel Leyden Jars. 77 
 
 denser tLus. Employing the table, or a sheet of foolscap 
 if the table be an insulator, as one plate of the con- 
 denser, spread upon it the sheet of india-rubber, p, fig. 
 42, and lay upon the rubber the sheet of block-tin A B. 
 Connect the tin by the wire, w, with the electroscope, 
 T. Impart electricity to the little weight, A, till the 
 leaves, L, begin to diverge ; then lift the tin-plate by 
 its two silk loops ; the leaves at once fly asunder. 
 
 Finally show your complete knowledge of the Leyden 
 jar, and your freedom from the routine of the instru- 
 ment makers, by making a 'jar' in the following novel 
 way. Stand upon a board supported by warm tumblers. 
 Hold in your right hand a sheet of vulcanised india- 
 rubber, and clasp, with it between you, the left hand 
 of a friend in connection with the earth. Place your 
 left hand on the conductor of the machine, and let it 
 be worked. You and your friend soon feel a crackling 
 and a tickling of the hands, due to the heightening at- 
 traction of the opposite electricities across the india- 
 rubber. The ' hand-jar ' is then charged. To discharge 
 it you have only to bring your other hands together : 
 the shock of the Leyden jar is then felt and its spark 
 seen and heard. 
 
 By the discharge of the hand-jar you can fire gun- 
 powder. But this will be referred to more particularly 
 further on. (See 25.) 
 
 23. Seat of Charge in the Leyden Jar* 
 
 Franklin sought to determine how the charge was 
 hidden in the Leyden jar. He charged with electricity 
 a bottle half-filled with water and coated on the outside 
 with tin-foil : dipping the finger of one hand into the
 
 78 Lessons in Electricity. 
 
 water, and touching the outside coating with the ether, 
 he received a shock. He was thus led to inquire, is the 
 electricity in the water? He poured the water into a 
 second bottle, examined it, and found that it had car- 
 ried no electricity along with it. 
 
 His conclusion was ' that the electric fire must 
 either have been lost in the decanting, or must have 
 remained in the bottle. The latter he found to be 
 true ; for, filling the charged bottle with fresh water, he 
 Dbtained the shock, and was therefore satisfied that the 
 power of giving it resided in the glass itself.' l 
 
 (An account of Franklin's discoveries was given by 
 him in a series of letters addressed to Peter Collinson, 
 Esq., F.K.S., from 1747 to 1754.) 
 
 So much for history ; but you are to verify the his- 
 tory by repeating Franklin's experiments. Place water 
 in a wide glass vessel ; place a second glass vessel within 
 the first, and fill it to the same height with water. 
 Connect the outer water by a wire with the earth, and 
 the inner water by a wire with the electric machine. 
 One or two turns furnish a sufficient charge. Re- 
 moving the inner wire, and dipping one finger into 
 the outside and the other into the inside water, a 
 smart shock is felt. This was Franklin's first ex- 
 periment. 
 
 Pass on to the second. Coat a glass jar with tin-foil 
 (not too high) ; fill it to the same height with water, and 
 place it on india-rubber cloth. Charge it by con- 
 necting the outside ccating with the earth, and the 
 water inside (by means of a stem cemented to the bottom 
 of the jar and ending above in a knob) with an electric 
 
 1 Priestley's 'History of Electricity, 3rd edition, p. 149.
 
 Seat of Charge in the Leyden Jar. 
 
 '9 
 
 machine. You obtain a bright spark on discharging. 
 This proves your apparatus to be in good order. 
 
 Ee-charge. Take hold of the charged jar with the 
 india-rubber, and pour the water into a second similar 
 jar. No sensible charge is imparted to the latter. Pour 
 fresh unelectrified water into the first jar, and discharge 
 it. The retention of the charge is shown by a brilliant 
 spark. Be careful in these experiments, or you will 
 fail as I did at first. The edge of the jar out of which 
 the water is poured has to be surrounded by a band of 
 bibulous paper to catch the final drop, which, trickling 
 down, would discharge the jar. 
 
 Experiments like those of Franklin are now made by 
 rendering the coatings of the Leyden jar movable. Such 
 a jar being charged, the interior coating F IG 43, 
 may be lifted out and proved unelectric. 
 The glass may then be removed from 
 the outer coating and the latter proved un- 
 electric. Eestoring the jar and coatings, 
 on connecting the two latter, the dis- 
 charge passes in a brilliant spark. 
 
 Make a jar with movable coatings 
 thus : Eoll cartridge paper round a good 
 flint-glass tumbler, G, fig. 43, to within 
 about an inch of the top. Paste down the 
 lower edge of the paper, and put a paper 
 bottom to it corresponding to the bottom 
 of the glass. Coat the paper, T, inside 
 and out with tin-foil. Make a similar 
 coating, T', for the inside of the tum- 
 bler, attaching to it an upright wire, w, 
 ending in a hook. You have then to all intents and 
 purposes a Leyden jar.
 
 80 Lessons in Electricity. 
 
 Put the pieces together and charge the jar. By 
 means of a rod of glass, sealing-wax, or gutta-percha, 
 lift out the interior coating. It will carry a little 
 electricity away with it. Place it upon a table and 
 discharge it wholly. Then by the hand lift the glass 
 out of the outer coating. Neither of the coatings now 
 shows the slightest symptom of electricity. Restore the 
 tumbler to its outer coating, and, by means of the hook 
 and insulating rod, restore the inner coating to its place. 
 Discharge the jar : you obtain a brilliant spark. The 
 electricity which produces this spark must have been 
 resident in and on the glass. 
 
 Here, as in all other cases, you can charge your jar 
 with a rubbed glass tube, though a machine in good 
 working order will do it more rapidly. With f Cot- 
 trell's rubber,' described in the next section, you may 
 greatly exalt the performance of your glass tube. 
 
 24. Ignition by the Electric Spark. CottrelL's 
 Rubber. The Tube-machine. 
 
 Various attempts had been vainly made by Nollet 
 and others to ignite inflammable substances by the 
 electric spark. This was first effected by Ludolf, at 
 the opening of the Academy of Sciences by Frederick 
 the Great at Berlin, on the 23rd of January, 1744. 
 With a spark from the sword of one of the court 
 cavaliers present on the occasion, Ludolf ignited 
 sulphuric ether. 
 
 Dr. Watson also made numerous experiments on 
 the ignition of bodies by the electric spark. He fired 
 gunpowder and discharged guns. Causing, moreover, a 
 spoon containing ether to be held by an electrified person,
 
 Ignition by the Electric Spark. 81 
 
 he ignited the ether by the finger of an unelectrified 
 person. He also noticed that the spark varied in colour 
 when the substances between which it passed varied. 
 
 These, and numerous other experiments may be 
 made with a far simpler ' machine ' than any hitherto 
 described. It was devised for your benefit by Mr. 
 Cottrell. In the electric machine, as we have learned, 
 the prime conductor is flooded with positive electricity 
 through the discharge of the negative from the points 
 against the excited glass. Your glass tube and rubber 
 may be similarly turned to account. A strip of sheet- 
 brass or copper, p, fig. 44, is sewn on to the edge of 
 
 Fio. 44 
 
 the silk pad, R, employed as a rubber. Through aper- 
 tures in the strip about twenty pin-points are intro- 
 duced, and soldered to the metal. When the tube is 
 clasped by the rubber, the metal strip and points quite 
 encircle the tube. 
 
 When a fine wire, w, connects the strip of metal with 
 the knob of a Ley den jar, by every downward stroke 
 of the rubber the glass tube is powerfully excited, and
 
 82 
 
 Lessons in Electricity. 
 
 hotly following the exciting rubber is the circle of points. 
 From these, against the rod, negative electricity is dis- 
 charged, the free positive electricity escaping along the 
 wire to the jar, which is thus rapidly charged. 
 
 The ignition of gas is readily effected by Cottrell's 
 rubber. Connecting the strip of metal, R, fig. 45, with an 
 insulated metallic knob, B, placed within a quarter or an 
 eighth of an inch of an uninsulated argand burner 
 connected with the earth, at every downward stroke of 
 the rubber a stream of sparks passes between the knob 
 
 FIG. 45. 
 
 and burner. If gas be turned on, it is immediately 
 ignited by the stream of sparks. Blowing out the flame 
 and repeating the experiment, every stroke of the 
 rubber infallibly ignites the gas. 
 
 Sulphuric ether, in a spoon which has been previously 
 warmed, is thus ignited : but the ether soon cools by 
 evaporation ; its vapour is diminished by the cold, and 
 it is then less easy to ignite. Bisulphide of carbon 
 may be substituted for the ether, with the certainty
 
 Cotireil's Rubber. 
 
 83 
 
 that every stroke of the rubber will set it ablaze. 1 
 The spark thus obtained also fires a mixture of 
 oxygen and hydrogen. The two gases unite with ex- 
 plosion to form water, when an electric spark is passed 
 through them. 
 
 Mr. Cottrell has also mounted his glass tube so as to 
 render friction in both directions available. The tube- 
 machine is represented in fig. 46. A B is the glass 
 
 Fio. 46. 
 
 tube, clasped by the rubber, R. p p 7 are two strips of 
 metal furnished with rows of points. From p V wires 
 proceed to the knob c, which is insulated by the 
 horizontal stem, G. This insulating stem may be 
 abolished with advantage, the wires from p and P' 
 being rendered strong enough to support the ball c. 
 
 i am indebted to Dr. Debus for the suggostion of the bisulphide 
 as a substitute for the ether.
 
 84 Lessons in Electricity. 
 
 At c sparks may be taken, a Leyden jar charged, the 
 electric mill turned, while wires carried from it may be 
 employed in experiments on ignition. I however 
 strongly recommend to your attention the more simple 
 rubber shown in tig. 44. 
 
 ' Seldom,' says Riess, ' has an experiment done so 
 much to develope the science to which it belongs as 
 (his of the ignition of bodies by the electric spark.' 
 It aroused universal interest ; and was repeated in all 
 Royal houses. Money was ready for the further pro- 
 secution of electrical research. The experiment after- 
 wards spread among the people. Riess considers it 
 probable that the general interest thus excited led to 
 the discovery of the Leyden jar, which was made soon 
 afterwards. 
 
 Klingenstierna astonished King Frederick of Sweden 
 by igniting a spoon of alcohol with a piece of ice. 
 With Cottrell's rubber and bisulphide of carbon this 
 striking experiment is easily made, and you ought to 
 render your knowledge complete by repeating it. At 
 every stroke of the rubber the spark from the end of a 
 pointed rod of ice infallibly sets the bisulphide on fire. 
 
 Cadogan Morgan, in 1785, sought to produce the 
 electric spark in the interior of solid bodies. He 
 inserted two wires into wood, and caused the spark to 
 pass between them : the wood was illuminated with 
 blood-red light, or with yellow light, according as the 
 depth at which the spark was produced was greater or 
 less. The spark of the Leyden jar produced within an 
 ivory ball, an orange, an apple, or under the thumb, 
 illuminates these bodies throughout. A lemon is espe- 
 cially suited to this experiment; flashing forth at every 
 Bpark as a spheroid of brilliant golden light. The
 
 Duration of the Electric Spark. 
 
 85 
 
 manner in which the lemon is mounted on the brass 
 stem B is shown in fig. 47. The spark occurs at 8, 
 in the interval between the stems A and B. A row of 
 
 eggs in a glass cylinder is also brilliantly illuminated 
 at the passage of every spark from a Leyden jar. 
 
 25. Duration of the Electric Spark. 
 
 The duration of the electric spark is very brief: in 
 a special case, Sir Charles Wheatstone found it to be 
 ? 4 o o otb f a second. This, however, was the maximum 
 duration. In other cases it was less than the millionth 
 of a second. 
 
 When a body is illuminated for an instant, the 
 image of the body remains upon the retina of the eye 
 for about one-fifth of a second. If, then, a body in swift 
 motion be illuminated by an instantaneous flash, it 
 will be seen to stand motionless for one-fifth of a 
 second at the point where the flash falls upon it. A 
 rifle bullet passing through the air, and illuminated by
 
 86 Lessons in Electricity. 
 
 an electric flash, would be seen thus motionless ; a 
 circle like D D', fig. 48, divided into black and white 
 sectors, and rotating so quickly as to cause the sectors 
 
 FIG. 48. 
 
 to blend to a uniform grey, appears, when illuminated 
 by the spark of a Ley den jar, perfectly motionless, with 
 all its sectors revealed. A falling jet of water, which 
 appears continuous, is resolved by the electric flash into 
 its constituent drops. Lightning, as shown by Professor 
 Dove, is similarly rapid in its discharge. 
 
 For a long time it was found almost impossible to 
 ignite gunpowder by the electric spark. Its duration is 
 so brief that the powder, when the discharge occurred in 
 its midst, was simply scattered violently about. In 1 787 
 Wolff introduced into the circuit through which the 
 discharge passed a glass tube wetted on the inside. He 
 thereby rendered the ignition certain. This was owing to 
 the retardation of the spark by the imperfect conductor. 
 Gun-cotton, phosphorus, and amadou, which are torn 
 asunder by the unretarded spark, are ignited when the 
 discharge is retarded by a tube of water. A wetted 
 string is the usual means resorted to for retardation 
 when gunpowder is to be discharged.
 
 Duration of the Electric Spark. 
 
 8? 
 
 The instrument usually employed for the ignition 
 of powder is the universal discharger. We make our 
 own discharger thus : I and i' (fig. 49) are insulating 
 
 Fio. 49. 
 
 rods of glass or sealing-wax, supporting two metal arms, 
 the ends of which can be brought down upon the little 
 central table s. One of the metal arms of the discharger 
 being connected by a wire e with the earth, the sepa- 
 rated ends of the two arms are surrounded with powder 
 at s. Sending through, it the unretarded charge, the 
 powder is scattered mechanically. Introducing the wet 
 string w into the circuit, ignition infallibly occurs when 
 the spark passes. 
 
 This is the place to fulfil our promise to ignite gun- 
 powder by the ( hand-jar.' Fig. 50 explains the arrange- 
 ment. H H' are the hands of the insulated person. F
 
 88 
 
 Lessons in Electricity. 
 
 the hand of the uninsulated friend, I the india-rubber 
 
 between both hands. The lead ball B is suspended by 
 
 a wet string s. On the little stand r, connected with 
 
 Fio. 50. 
 
 n 
 
 the earth, is placed the powder. The charging of the 
 hand-jar is described in 22. When charged, it is 
 only necessary to bring the ball B down upon the powder 
 to cause it to explode. 
 
 26. Electric Light in Vacuo. 
 
 The electric light in vacuo was first observed by 
 Picard in 1675. While carrying a barometer from the 
 Observatory to the Porte St. Michel in Paris, he saw 
 light in the upper portion of the tube. Sebastien and 
 Cassini observed it afterwards in other barometers. John 
 Bernouilli devised a ' mercurial phosphorus,' by shaking 
 mercury in a tube which had been exhausted by a^
 
 Electric Light in Vacuo. 
 
 b'J 
 
 FIG. 51. 
 
 air-pump. This was Landed to the King of Prussia 
 Frederick I. who awarded for it a medal of fortv 
 ducats value. The great mathematician wrote a poem 
 in honour of the occasion. 
 
 Bernouilli failed to explain the effect. The ex- 
 planation was reserved for Haukshee, who in 1705 took 
 up the subject and experimented upon it before the 
 Koyal Society. On the plate of an air-pump he placed 
 two bell-jars, one over the other. The outer and larger 
 jar was open at the top. Into the opening Hauksbee 
 fixed, air-tight, a funnel, which he stopped with a plug of 
 wood and filled with mercury. He exhausted the space 
 between the two jars, withdrew 
 the wooden plug and allowed 
 the mercury to stream against 
 the outer surface of the inner jar. 
 He thus obtained a shower of fire. 
 This is a truly beautiful experi- 
 ment when witnessed by an ob- 
 server close at hand. 
 
 A copy of Hauksbee's own 
 figure illustrating this experiment 
 is annexed, fig. 51. M is the funnel 
 containing the mercury, p the plug 
 of wood, s the outer and s' the in- 
 ner bell-jar. Instead of the plug 
 P, an india-rubber tube, held by a 
 clip, may be employed with ad- 
 vantage to connect the funnel 
 with the exhausted jar. By 
 gradually relaxing the clip the 
 
 mercury may be made to fall at a rate corresponding 
 to the maximum luminous effect. The streams of light
 
 90 Lessons in Electricity. 
 
 pioduced are very beautiful, but they are more con- 
 tinuous than they are shown to be by Hauksbee. 
 
 In 1706 HauKsbee referred the phenomenon to its 
 true cause, namely, the friction between mercury and 
 glass in the highly rarefied air. John Bernouilli ridi- 
 culed Hauksbee's explanation. But trutli outlives 
 ridicule, and it is now universally admitted that 
 Hauksbee was right. 
 
 Hauksbee also made the following experiment, 
 which, as shown by Eiess, is explained by reference 
 to the principle of induction. A hollow glass globe 
 was mounted so as to be capable of quick rotation. 
 It was exhausted, and while it rotated the hand 
 was placed against it in the dark. It was positively 
 electrified by the hand. This positive electricity acted 
 inductively on the glass itself, attracting its negative, 
 but discharging its positive as a luminous glow through' 
 the rarefied air within. Hauksbee was able to read by 
 the light thus produced. 
 
 By such experiments it was shown that rarefied air 
 favoured the passage of electricity. Dry air is in fact 
 an insulator, which must be broken through to produce 
 the electric spark. Through an exhausted glass tube 
 six feet long a discharge freely passes which would 
 be incompetent to leap over the fiftieth part of this 
 interval in air. But whereas the spark in air is dense 
 and brilliant, the discharge in vacuo fills the exhaust< d 
 tube with a diffuse light. 
 
 (It is here worthy of remark that at a very early 
 period Grummert, a Pole, proposed the empl6yment 
 of this diffuse electric light to illuminate coal mines 
 a notion which has been revived in our day. The light
 
 Electric Light in Vacua. 91 
 
 .n this form is not competent to ignite the explosive 
 gases which produce such terrible disasters in mines.) 
 
 Priestley, in his History of Electricity,' thus de- 
 scribes the light in vacuo. ' Take a tall receiver, very 
 dry, and in the top of it insert with cement a wire not 
 very acutely pointed, then exhaust the receiver and pre- 
 sent the knob of the wire to the conductor, and every 
 spark will pass through the vacuum in a broad stream of 
 light, visible through the whole length of the receiver, 
 be it ever so tall. This stream often divides itself into 
 a variety of beautiful rivulets, which are continually 
 changing their course, uniting and dividing again in 
 the most pleasing manner. If a jar be discharged 
 through this vacuum, it gives the appearance of a very 
 dense body of fire, darting directly through the centre 
 of the vacuum without ever touching the sides.' 
 
 Cavendish employed a double barometer-tube, bent 
 into the form of a horseshoe, with its curved portion 
 empty, to show the passage of electricity through a 
 vacuum. It is really not the vacuum which con- 
 ducts the electricity, but the highly attenuated air and 
 vapour which fill the space above the barometric 
 columns. When the mercury employed is carefully 
 purged of air and moisture by previous boiling, the space 
 above the mercury, as proved by Walsh, De Luc, Morgan, 
 and Davy, is wholly incapable of conducting electricity. 
 Similar experiments have been made in the laboratory 
 of Mr. Grassiot, to whom we are indebted for so many 
 beautiful electrical experiments. Professor Dewar has 
 also brought his experimental skill to bear with success 
 upon this subject. 
 
 Electricity therefore does not pass through a true 
 : it requires ponderable matter to carry it. If
 
 92 Lessons in Electricity. 
 
 a gold-leaf electroscope be kept at a distance from ail 
 conductors, it may be kept charged for an almost in- 
 definite period in a good air-pump vacuum. 
 
 The matter rendered thus luminous by the electrical 
 discharge is attracted and repelled like other electrified 
 matter. ' A finger,' says Priestley, ' put on the outside 
 of the glass will draw it [the luminous stream] wherever 
 a person pleases. If the vessel be grasped with both 
 hands, every spark is felt like the pulsation of a great 
 artery, and all the fire makes towards the hands. This 
 pulsation is felt at some distance from the receiver ; and 
 in the dark a light is seen betwixt the hands and glass.' 
 
 ' All this,' continues the historian of Electricity, 
 ' while the pointed wire is supposed to be electrified 
 positively ; if it be electrified negatively the appearance 
 is remarkably different. Instead of streams of fire, 
 nothing is seen but one uniform luminous appearance, 
 like a white cloud, or the milky-way on a clear star- 
 light night. It seldom reaches the whole length of 
 the vessel, but is generally only like a lucid ball at the 
 end of the wire.' 
 
 Of the two appearances here described the former is 
 now known as the electric brush, and the latter as the 
 electric gloiv. Both can be produced in unconfined air. 
 The glow is sometimes seen on the masts of ships, and it 
 is mentioned by the ancients as appearing on the points 
 of lances. It is called St. Ermo's or St. Elmo's fire, after 
 the sailor's saint, Erasmus, who suffered martyrdom at 
 Gaeta at the beginning of the fourth century. 
 
 The purple colour of the diffused light in attenuated 
 air was noticed by Hauksbee. The colour depends 
 upon the residue of attenuated gas, or vapour, through 
 which the discharge passes. If it be an oxygen-residue
 
 Electric Light in Vacua. 
 
 93 
 
 the light is whitish, if it be a hydrogen-residue the light 
 is red, if a nit'.ogen-residue the light is purple, exactly 
 resembling that displayed at times by the aurora bore- 
 alis a colour doubtless due to the discharge of elec- 
 tricity through the attenuated nitrogen of the air. 
 
 Electric light in vacuo is readily produced by the 
 fritstion of an amalgamated rubber against the outside 
 
 FIG. 52. 
 
 Di an exhausted tube. The light also is produced by 
 the friction of mercury within a barometric vacuum. 
 The discharges through tubes many feet in length and
 
 94 Lessons in Electricity. 
 
 exhausted by an air-pump are very fine. The double, 
 barometer tube of Cavendish also yields a truly splendid 
 bow of light, when a strong electric discharge is sent 
 through it. For this experiment fig. 52 shows the best 
 arrangement p is the prime conductor of an electrical 
 machine, I an insulated metal ball, connected by a wire 
 with the mercury trough A. The trough B is connected 
 by a wire with the earth, c and (/ mark the height of 
 the mercurial columns. When the machine is worked 
 sparks pass from p to I, a vivid bow of light at each 
 passage stretching from c to c'. By causing I to ap- 
 proach p, the discharges become more frequent, but 
 more feeble ; by augmenting the distance p I, the 
 sparks become rarer, but more strong. When very 
 strong, a bow of dazzling brilliancy accompanies every 
 spark. 1 
 
 Small tubes for these experiments are best obtained 
 from philosophical instrument makers. 
 
 27. Lichtenberg's Figures. 
 
 Lichtenberg devised a means of revealing the con- 
 dition of an electrified surface by dusting it with powder. 
 Ked lead, in passing through muslin, is positively elec- 
 trified ; flower of sulphur is negatively electrified. 
 Whisking a fox's brush over a cake of resin, and draw- 
 ing over the surface the knob of a Leyden jar, positively 
 charged, the resin is rendered in part negative and in 
 part positive. Dusting the mixed powder over the sur- 
 face, the sulphur arranges itself over the positive places, 
 and the red lead over the negative places, a very beauti- 
 ful pattern being the result. 
 
 ' It is well to hare the interval PI at some distance from the bow, 
 so that the light of the spark shall not impair the effect of the disehargs 
 ipon the eye.
 
 Surface Compared with Mass. 95 
 
 This experiment of Lichtenberg's constituted the 
 germ of Chladni's important acoustical researches. 
 ' Chladni's figures' were the direct offspring of ' Lich- 
 tenberg's figures.' 
 
 28. Surface Compared with Mass. Distribution 
 of Electricity in Hollow Conductors. 
 
 Monnier proved that the charge of a conductor de- 
 pended upon its surface, and not upon its solid contents. 
 An anvil weighing 200 Ibs. gave a smaller spark than 
 a speaking trumpet weighing 10 Ibs. A solid ball of 
 lead gave a spark only of the same force as that obtained 
 from a piece of thin lead of the same superficies, bent 
 into the form of a hoop. Finally Monnier obtained a 
 strong spark from a long strip of sheet lead, but a very 
 umall one when it was rolled into a lump. 
 
 Le Eoi and D'Arcy showed that a hollow sphere 
 accepted the same charge when empty as when filled 
 with mercury, which augmented its weight 60-fold. 
 All this proves the influence of surface as distinguished 
 from mass. 
 
 The distribution of electric! tyMs well illustrated Ly 
 the deportment of hollow bodies. Impart by your 
 carrier (fig. 15) successive measures of electricity to the 
 interior of an insulated ice-pail, or a pewter pot. On 
 testing the interior of the vessel with the carrier and 
 an electroscope no electricity is found there ; but it is 
 found on the external surface. A hat suspended by 
 Bilk strings answers as well as the ice-pail. 
 
 This experiment with the hat is a very instructive 
 one. The hat may be charged either with Cottrell's 
 rubber or with your rubbed glass tube.
 
 96 
 
 Lessons in Electricitn. 
 
 Notice, when testing, that you take your strongest 
 charges from tbe edges and not from the round or H;it 
 surface of the hat. The strongest charge of all is com- 
 municated to the carrier by the leaf of the hat. 
 
 The successive charges may be communicated to 
 the hat by a metal ball suspended by silk. The charged 
 ball, on touching the interior surface, becomes com- 
 pletely unelectric. 
 
 Franklin placed a long chain in a silver teapot 
 which he electrified. Connecting his teapot with a 
 pith-ball electroscope he produced a divergence. Then 
 lifting the chain by a silk string he found that over 
 
 '
 
 Physiological Effects of the Electrical Discharge. 97 
 
 the portion outside the teapot the electricity diffused 
 itself, this withdrawal of the electricity from the 
 electroscope being announced by the partial collapse 
 of the divergent pith-balls. 
 
 The mode of repeating this experiment is shown in 
 Sg. 53, where T is the teapot, supported on a good 
 glass tumbler G, and connected by the wire w with the 
 electroscope E. The effect is small, but distinct. 
 
 The greatest experiment with hollow conductors was 
 made by Faraday, who placed himself in a cubical 
 chamber built of laths and covered with paper and wire 
 gauze. It was suspended by silk ropes. Within this 
 chamber he could not detect the slightest sign of elec- 
 tricity, however delicate his electroscope, and however 
 strongly the sides of the chamber might be electrified. 
 
 29. Physiological Effects of the Electric Discharge. 
 
 The physiological effect of the electric shock has 
 been studied in various ways. Graham caused a number 
 of persons to lay hold of the same metal plate, which was 
 connected with the outer coating of a charged Leyden 
 jar, and also to lay hold of a rod by which the jar was dis- 
 charged. The shock divided itself equally among them. 
 
 The Abbe Nollet formed a line of one hundred and 
 eighty guardsmen, and sent the discharge through them 
 all. He also killed sparrows and fishes by the shock. 
 The analogy of these effects with those produced by 
 thunder and lightning could not escape attention, nor 
 fail to stimulate enquiry. 
 
 Indeed, as experimental knowledge increased, men's 
 thoughts became more definite and exact as regards the 
 relation of electrical effects to thunder and lightning.
 
 98 Lessons in Electricity. 
 
 The Abbe Nollet thus quaintly expresses himself: * II 
 any one should take upon him to prove, from a well- 
 connected comparison of phenomena, that thunder is, 
 in the hands of Nature, what electricity is in ours, and 
 that the wonders which we now exhibit at our pleasure 
 are little imitations of those great effects which frighten 
 us ; I avow that this idea, if it was well supported, 
 would give me a great deal of pleasure.' He then 
 points out the analogies between both, and continues 
 thus : ' All those points of analogy, which I have been 
 some time meditating, begin to make me believe that 
 one might, by taking electricity as the model, form to 
 one's self, in relation to thunder and lightning, more 
 perfect and more probable ideas than what have been 
 offered hitherto.' l 
 
 These views were prevalent at the time now referred 
 to, and out of them grew the experimental proof by the 
 great physical philosopher, Franklin, of the substantial 
 identity of the lightning flash and the electric spark. 
 
 Franklin was twice struck senseless by the electric 
 shock. He afterwards sent the discharge of two large 
 jars through six robust men ; they fell to the ground 
 and got up again without knowing what had happened ; 
 they neither heard nor felt the discharge. Priestley, 
 who made many valuable contributions to electricity, 
 received the charge of two jars, but did not find it 
 painful. 
 
 This experience agrees with mine. Some time ago 
 I stood in this room with a charged battery, of fifteen 
 large Ley den jars beide me. Through some awkward- 
 ness on my part I touched the wire leading from the 
 battery, and the discharge went through me. For a 
 1 Priestley's History of Electricity,' pp. 51-52.
 
 Atmospheric Electricity. 99 
 
 lensible interval life was absolutely blotted out, but 
 there was no trace of pain. After a little time con- 
 sciousness returned ; I saw confusedly both the audience 
 and the apparatus, and concluded from this, and from 
 my own condition, that I had received the discharge. 
 To prevent the audience from being alarmed, I made 
 the remark that it had often been my desire to receive 
 such a shock accidentally, and that my wish had at 
 length been fulfilled. But though the intellectual 
 consciousness of my position returned with exceeding 
 rapidity, it was not so with the optical consciousness. 
 For, while making the foregoing remark, my body pre- 
 sented to my eyes the appearance of a number of 
 separate pieces. The arms, for example, were detached 
 from the trunk and suspended in the air. In fact, 
 memory, and the power of reasoning, appeared to be 
 complete, long before the restoration of the optic nerve 
 to healthy action. 
 
 This may be regarded as an experimental proof that 
 people killed by lightning suffer no pain. 
 
 30. Atmospheric Electricity. 
 
 The air at all times can be proved to be a reservoir 
 of electricity, which undergoes periodic variation. We 
 have seen that ingenious men began soon to suspect 
 a common origin for the crackling and light of the 
 electric spark, and thunder and lightning. The greatest 
 investigator in this field is the celebrated Dr. Franklin. 
 He made an exhaustive comparison of the effects of 
 electricity and those of lightning. The lightning flash 
 he saw was of the same shape as the elongated electric 
 spark ; like electricity, lightning strikes pointed objects
 
 KIIJ Lessons in tiiectncity. 
 
 in preference to others : lightning pursues the path of 
 least resistance : it burns, dissolves metals, rends bodies 
 asunder, and strikes men blind. Franklin imitated all 
 these effects, striking a pigeon blind, and killing a hen 
 
 FIQ. 54. 
 
 and turkey by the electrical discharge. I place before 
 y*m in fig. 54, with a view to its comparison with a dis- 
 charge of forked lightning, the long spark obtained 
 from an effective ebonite machine, furnished with a 
 conductor of a special construction, which favours 
 length of spark. 
 
 Having completely satisfied his mind by this com- 
 parison of the identity of both agents, Franklin pro- 
 posed to draw electricity from the clouds by a pointed 
 rod erected on a high tower. But before the tower 
 could be built he succeeded in his object by means of 
 a kite with a pointed wire attached to it. The elec- 
 tricity descended by the hempen string which held 
 the kite, to a key at the end of it, the key being sepa- 
 rated from the observer by a silken string held in 
 the hand. Franklin thus obtained sparks, and charged 
 a Leyden phial with atmospheric electricity. 
 
 But, spurred by Franklin's researches, an observer 
 in France had previously proved the electrical character 
 of lightning. A translation of Franklin's writings on
 
 Atmospheric Electricity. 101 
 
 t.'ie subject fell into the hands of the naturalist Buffon, 
 who requested his friend D'Alibard to revise the trans- 
 lation. D'Alibard was thus induced to erect an iron 
 rod 40 feet long, supported by silk strings, and ending 
 in a sentry-box. It was watched by an old dragoon 
 nanitd Coiffier, who on the 10th of May, 1752, heard a 
 clap of thunder, and immediately afterwards drew sparks 
 from the end of the iron rod. 
 
 The danger of experiments with metal rods was 
 soon illustrated. Professor Eichmann of St. Peters- 
 burg had a rod raised three or four feet above the 
 tiles of his house. It was connected by a chain with 
 another rod in his room ; the latter rod resting in a glass 
 vessel, and being therefore insulated from the earth. 
 On the 6th of August, 1753, a thunder cloud discharged 
 itself agaiuut the external rod ; the electricity passed 
 downwards along the chain ; on reaching the rod below, 
 it was stopped by the glass vessel, darted to Richmann's 
 head, which vas about a foot distant, and killed him 
 on the spot. Had a perfect communication existed 
 between the lower rod and the earth, the lightning in 
 this case would have expended itself harmlessly. 
 
 In 1749 Franklin proposed lightning conductors. 
 He repeated his recommendation in 1753. He was op- 
 posed on two grounds. The Abbe Nollet, and those who 
 thought with him, considered it as impious to ward off 
 heaven's lightnings, as for a child to ward off the 
 chastening rod of its father. Others thought that 
 the conductors would 'invite 'the lightning to break 
 upon them. A long discussion was also carried on as to 
 whether the conductors should be blunt or pointed. 
 Wilson advocated blunt conductors against Franklin, 
 Cavendish, and Watson. He so influenced George III.,
 
 102 Lessons in Electricity. 
 
 hinting that the points were a republican device to 
 injui'e his Majesty, that the pointed conductors on 
 Buckingham House were changed for others ending 
 in balls. Experience of the most varied kind has 
 justified the employment of pointed conductors. In 
 1769 St. Paul's Cathedral was first protected. 
 
 The most decisive evidence in favour of conductors 
 was obtained from ships ; and such evidence was 
 needed, to overcome the obstinate prejudice of seamen. 
 Case after case occurred in which ships unprotected by 
 conductors were singled out from protected ships, and 
 shattered or destroyed by lightning. The conductors 
 were at first made movable, being hoisted on the ap- 
 proach of a thunderstorm ; but these were finally aban- 
 doned for the fixed lightning conductors devised by the 
 late Sir Snow Harris. The saving of property and 
 life by this obvious outgrowth of electrical research is 
 incalculable. 
 
 31. The Returning Stroke. 
 
 In the year 1779 Charles, Viscount Mahon, after- 
 wards Earl Stanhope, published his * Principles of 
 Electricity.' On the title-page of the book stands the 
 following remark : * This treatise comprehends an 
 explanation of an electrical returning stroke, by which 
 fatal effects may be produced even at a vast distance 
 from the place where the lightning falls.' 
 
 Lord Mahon's experiments, which are models of 
 scientific clearness and precision, will be readily under- 
 stood by reference to the principles of electric induction, 
 with which you are now so familiar. It need only be 
 noted here that whenever he speaks of a body being
 
 The Returning Stroke. 
 
 103 
 
 plunged in an ' electrical atmosphere,' he means that 
 the body is exposed to the inductive action of a second 
 electrified body, which latter he supposed to be sur- 
 rounded by such an atmosphere. 
 
 A few extracts from his work will give a clear notion 
 of the nature of his discovery : 
 
 'I placed an insulated metallic cylinder, AB, fig. 55, 
 
 Fio. 55. 
 
 within the electrical atmosphere of the prime con- 
 ductor [P c] when charged, but beyond the striking dis- 
 tance. The distance between the near end A of the 
 insulated metallic body and the side of the prime con- 
 ductor was 20 inches. The body A B was of brass, of a 
 cylindrical form, 1 8 inches long by 2 inches in diameter. 
 I then placed another insulated brass body EF, 40 inches
 
 104 Lessons in Electricity. 
 
 long by about 3| inches in diameter, with its end B at 
 the distance of about one-tenth of an inch from the end B 
 of the other metallic body AB. I electrified the prime 
 conductor. All the time that it was receiving its plus 
 charge of electricity there passed a great number of 
 weak (red or purple) sparks from the end B of the near 
 body A B into the end E of the remote body E r.' 
 
 Make clear to your mind the origin of this stream 
 of weak red or purple sparks. It is obviously due to 
 the inductive action of the prime conductor p c upon 
 the body AB. The positive electricity of AB being 
 repelled by the prime conductor, passed as a stream of 
 sparks to E F. 
 
 ' When the prime conductor, having received its 
 full charge, came suddenly to discharge, with an ex- 
 plosion, its superabundant electricity on a large brass 
 ball L, which was made to communicate with the earth, 
 it always happened that the electrical fluid, which had 
 been gradually expelled from the body A B and driven 
 into the body E F, did suddenly return from the body E F 
 into the body A B, in a strong and bright spark, at the very 
 instant that the explosion took place upon the ball L. 
 ' This I call the electrical returning stroke? 
 For the two conductors Lord Mahon then sub- 
 stituted his own body and that of another person, both 
 of them standing upon insulating stools. He continues 
 thus : 
 
 ' I placed myself upon an insulating stool E (fig. 56), 
 so as to have my right arm A at the distance of about 20 
 inches from a large prime conductor ; another person, 
 standing upon another insulating stool K, brought his 
 right hand F within one-quarter of an inch of my left 
 hand B.
 
 The Returning Stroke. 
 
 105 
 
 4 When the prime conductor began to receive ita 
 plus charge of electricity, we felt the electrical fluid 
 running out of my hand B into his hand F. 
 
 FIG. 56. 
 
 ' When we separated our hands B and F a little, the 
 electricity passed between us in small sparks, which 
 sparks increased in sharpness the farther we removed 
 our hands B and F asunder, until we had brought them 
 quite out of a striking distance. The intervals of time 
 between these departing sparks increased also the 
 more the distance between our hands B and F was in- 
 creased, as must necessarily be the case. 
 
 ' As soon as the prime conductor came suddenly to 
 discharge its electricity upon the ball L, the super- 
 abundant electricity which the other person had re- 
 ceived from my body did then return from him to me 
 in a sharp spark, which issued from his hand F at the 
 very instant that the explosion of the piime conductoi 
 took place upon the ball L.
 
 106 Lessons in Electricity. 
 
 ' I still continued upon the insulating stool E, and 1 
 desired the other person to stand upon the floor. The 
 returning stroke between us was still stronger than it 
 had yet been. The reason of it was this : the other 
 person being no longer insulated, transmitted his super- 
 abundant electricity freely into the earth. I conse- 
 quently became still more negative than before. 
 
 ' Now, when the returning stroke came to take 
 place, not only the electricity which had passed from 
 my body into the body of the other person, but also the 
 electricity which had passed from my body into the 
 earth (through the other person), did suddenly return 
 upon me from his hand F to my hand B, at the same 
 instant that the discharge of the prime conductor took 
 place upon the ball L. This caused the returning stroke 
 to be stronger than before.' 
 
 Lord Mahon fused metals, and produced strong 
 physiological effects by the return stroke. 
 
 In nature disastrous effects may be produced by the 
 return stroke. The earth's surface, and animals or men 
 upon it, maybe powerfully influenced by one end of an 
 electrified cloud. Discharge may occur at the other 
 end, possibly miles away. The restoration of the elec- 
 tric equilibrium by the return shock may be so violent 
 as to cause death. 
 
 This was clearly seen and illustrated by Lord Mahon. 
 Fig. 57 is a reduced copy of his illustration. A B c is 
 the electrified cloud, the two ends of which, A and c, 
 come near the earth. The discharge occurs at c. A 
 man at F is killed by the returning stroke, while the 
 people at D, nearer to the place of discharge, but farther 
 from the cloud, are uninjured. 
 
 With the view of still further testing your know-
 
 The Returning Stroke. 107 
 
 ledge of induction, I have here copied a portion of this 
 admirable essay ; but the entire memoir of Lord Mahon 
 
 FIG. 57. 
 
 would constitute a most useful and interesting lesson 
 in electricity. 
 
 For our own instruction we can illustrate the return 
 shock thus : Connect one arm of your universal dis- 
 charger, fig. 49, with a conductor like c, fig. 20, and 
 the other arm with the earth. Bring c within a few 
 inches of your prime conductor, but not within striking 
 distance ; on working the machine a stream of feeble 
 sparks will pass from point to point of the discharger. 
 Let the prime conductor be discharged from time to 
 time by an assistant ; at every discharge the returning 
 stroke is announced by a flash between the points of 
 the discharger at 8, If gun-cotton with a little 
 fulminating powder scattered on it, or a fine silver wire, 
 be introduced between the points of the discharger, the 
 one is exploded and the other deflagrated.
 
 108 Lessons in Electricity. 
 
 The stream of repelled sparks first seen may be en- 
 tirely abolished by establishing an imperfect connexion 
 between the conductor c and the earth: a chain resting 
 upon the dry table on which the conductor stands 
 will do. The chain permits the feebler sparks to pass 
 through it in preference to crossing the space s ; but 
 the returning stroke is too strong and sudden to find 
 a sufficiently open channel through the table and chain, 
 and on the discharge of the prime conductor the spark 
 is seen. 
 
 It was the action of the return shock upon a dead 
 frog's limbs, observed in the laboratory of Professor 
 Galvani, that led to Galvani's experiments on animal 
 electricity ; and led further to the discovery, by Volta, 
 of the electricity which bears his name. 
 
 32. The Leyden Battery, its Currents, and some 
 of their Effects. 
 
 In the ordinary Leyden battery described in 19 all 
 the inner coatings are connected together, and all the 
 outer coatings are also connected together. Such a 
 battery acts as a single large jar of extraordinary di- 
 mensions. 
 
 Wires are warmed by a moderate electric discharge ; 
 by augmenting the charge they are caused to glow ; 
 with a strengthened charge the metal is torn to pieces ; 
 fusion follows ; and by still stronger charges the wires 
 are reduced to metallic dust and vapour. 
 
 For such experiments the wire must be thin. 
 Without resistance we can have no heat, and when the 
 wire is thick we have little resistance. The mechanism 
 of the discharge, as shown by the figures produced, is
 
 The Ley den Battery. 109 
 
 different in different wires. The figure produced by 
 the dust of a deflagrated silver wire on white paper is 
 shown in fig. 58. 
 
 FJO. 58. 
 
 When the discharge of a powerful battery is sent 
 through a long steel chain with the ends of its links 
 unsoldered, the sparks between the unsoldered links 
 carry the incandescent particles of the steel along with 
 them. These are consumed in the air, a momentary 
 blaze occurring along the entire chain. Chain cables 
 have been fused by being made the channels of a flash 
 of lightning. 
 
 Eetaining our conception of an electric fluid, at 
 this point we naturally add to it the conception of a 
 current. It is the electric current which produces the 
 effects just described. In many of our former experi- 
 ments we had electricity at rest (static electricity), 
 here we have electricity in motion (dynamic elec- 
 tricity). 
 
 Sending the current from a battery through a flat 
 spiral (the primary) formed of fifty or sixty feet of 
 copper wire, and placing within a little distance of 
 it a second similar spiral (the secondary) with its 
 ends connected ; the passage of the current in the first 
 spiral excites in the second a current, which is competent 
 to deflagrate wires, and to produce all the other effects 
 of the electrical discharge. Even when the spirals are
 
 110 Lessons in Electricity. 
 
 some feet asunder, the shock produced by the secondary 
 current is still manifest. 
 
 The current from the secondary spiral may be 
 carried round a third ; and this third spiral may be 
 allowed to act upon a fourth, exactly as the primary did 
 upon the secondary. A tertiary current is thus evoked 
 by the secondary in the fourth spiral. 
 
 Carrying this tertiary current round a fifth spiral, 
 and causing it to act inductively upon a sixth, we obtain 
 in the latter a current of the fourth order. In this way 
 we generate a long progeny of currents, all of them 
 having the current sent from the battery through the 
 first spiral, for a common progenitor. To Prof. Henry 
 of the United States, and to Prof. Eiess of Berlin, we 
 are indebted for the investigation of the laws of these 
 currents. These researches, however, were subsequent 
 to, and were indeed suggested by, experiments of a 
 similar character previously made by Faraday with 
 Voltaic electricity. 
 
 Besides the electricity of friction and induction we 
 have the following sources and forms of this power. 
 
 The contact of dissimilar metals produces electricity. 
 
 The contact of metals with liquids produces elec- 
 tricity. 
 
 A mere variation of the character of the contact of 
 two bodies produces electricity. 
 
 Chemical action produces a continuous flow of elec- 
 tricity (Voltaic electricity). 
 
 Heat, suitably applied to dissimilar metals, produces 
 a continuous flow of electricity (thermo-electricity). 
 
 The heating and cooling of certain crystals produce 
 electricity (pyro-electricity).
 
 Conclusion. 1 1 1 
 
 The motion of magnets, and of bodies carrying elec- 
 tric currents, produces electricity (magneto-electricity). 
 
 The friction of sand against a metal plate produces 
 electricity. 
 
 The friction of condensed water-particles against a 
 safety valve, or better still against a box-wood nozzle 
 through which steam is driven, produces electricity 
 (Armstrong's hydro-electric machine). 
 
 These are different manifestations of one and the 
 same power ; and they are all evoked by an equivalent 
 expenditure of some other power. 
 
 Conclusion. 
 
 Our experimental researches end here, I would 
 now bespeak your attention for five minutes longer. 
 The expensiveness of apparatus is sometimes urged as 
 an obstacle to the introduction of science into schools. 
 I hope it has been shown that the obstacle is not a real 
 one. Leaving out of account the few larger experi- 
 ments, which have contributed but little to our know- 
 ledge, it is manifest that the wise expenditure of a couple 
 of guineas would enable any competent teacher to place 
 the leading facts and principles of factional electricity 
 completely at the command of his pupils; giving them 
 thereby precious knowledge, and still more precious in- 
 tellectual discipline a discipline which invokes obser- 
 vation, reflection, prevision by the exercise of reason, and 
 experimental verification. 
 
 And here, if I might venture to do so, I would urge 
 upon the science teachers of our public and other schools 
 that the immediate future of science as a factor in English 
 education depends mainly upon them. I would respect-
 
 112 Lessons in Electricity. 
 
 fully submit to them whether it would not be a mistake 
 to direct their attention at present to the collection of 
 costly apparatus. Their principal function just now is 
 to arouse a love for scientific study. This is best done by 
 the exhibition of the needful facts and principles with 
 the simplest possible appliances, and by bringing their 
 pupils into contact with actual experimental work. 
 
 The very time and thought spent in devising such 
 simple instruments will give the teacher himself a 
 grasp and mastery of his subject which he could not 
 otherwise obtain; but it ought to be known by the head 
 masters of our schools that time is needed, not only for 
 devising such instruments, but also for preparing the 
 experiments to be made with them after they have been 
 devised. No science teacher is "fit to meet his class 
 without this distinct and special preparation before 
 every lesson. His experiments are part and parcel of 
 his language, and they ought to be as strict in logic, 
 and as free from stammering, as his spoken words. To 
 make them so may imply an expenditure of time which 
 few head masters now contemplate, but it is a necessary 
 expenditure, and they will act wisely in making pro- 
 vision for it. 
 
 To them, moreover, in words of friendly warning, I 
 would say, make room for science by your own healthy 
 and spontaneous action, and do not wait until it is 
 forced upon you by revolutionary pressure from without. 
 The condition of things now existing cannot continue. 
 Its simple statement suffices to call down upon it the 
 condemnation of every thoughtful mind. With refer- 
 ence to the report of a Commission appointed last year 
 to enquire into the scientific instruction of this country, 
 Sir John Lubbock writes as follows : ' The Commis-
 
 Conclusion. 113 
 
 sioners have published returns from more than a hundred 
 and twenty of the larger endowed schools. In more 
 than half of these no science whatever is taught ; only 
 thirteen have a laboratory, and only eighteen possess 
 any scientific apparatus. Out of the whole number, less 
 than twenty schools devote as much as four hours a 
 week to science, and only thirteen attach any weight at 
 all to scientific subjects in the examinations.' 
 
 Well may the Commissioners pronounce such a 
 state of things to be nothing less than a national 
 calamity ! If persisted in, it will assuredly be followed 
 by a reaction which the truest friends of classical 
 culture in England will have the greatest reason to 
 deplore.
 
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 One latest Improved Holtz Electrical Induc- 
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 plete, with catskin and piece of hard rubber, $25 00 
 
 One quart Leyden jar . . . . 1 50 
 
 One electrical flier . . . 1 25 
 
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 One insulated stool . V . . 3 00 
 
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 One electrical discharger . . 2 50 
 
 One Leyden jar with movable coatings . . 3 00
 
 ii Price-List of Apparatus. 
 
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 $68 25 
 
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 Manufacturers of Electrical and Philosophical Appara- 
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 Descriptive and Illustrated Circulars sent free. 
 
 N. B. This collection of apparatus is all-sufficient 
 to fully illustrate all of the experiments mentioned in 
 this book. 
 
 l^fT Every piece of apparatus is constructed in 
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 tested before sending from our shops. 
 
 J. & H. BERGE.
 
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 Personal Beauty. 
 
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 State-Tampering with Money and 
 
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 Parliamentary Reform : the Dangers 
 
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 " The Collective Wisdom." 
 Political ^Fetichism. 
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 From Freedom to Bondage. 
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 Index. 
 
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