Mk mmmm . _ - f REESE LIBRARY cl 'in i; i f. UNIVERSITY OF CALIFQ%NIA\ Received . Accessions No. ^Q ' , Shelf No. db s?e W^Sf^&i^MM -' -"'MMfNW^ .;/^r= sS Hi FRICTIONAL ELECTRICITY BY THOMAS P. TREGLOHAN HEAD MASTER ST JAMES'S SCIENCE AND ART SCHOOLS, KEYHAM, DEVONPORT UNIVERSITY LONDON LONGMANS, GREEN, AND CO. 1886 7 PRINTED BY SFOTTISWOODE AND CO., NEW-STREET SQUARE LONDON PREFACE. THE present volume has been prepared with the same idea as the 'Text-Book of Magnetism/ to meet the syllabus of the Science and Art Department for the Elementary Stage, and to satisfy the requirements of the second stage of one of the Physics Courses of the 'Mundella Code,' as laid down by the Education Department. The same plan is followed in this work as in the 1 Magnetism/ viz., thorough explanation, clear diagrams, and numerous experiments to illustrate every phase of the subject. The experiments are adapted for the teacher's use in illustrating his lectures, or for the pupil's private practice for his own special benefit. The diagrams should be carefully drawn, and every division of the subject thoroughly mastered. All the questions which have been set in the Ele- mentary Stage since 1867 are collected at the end of the book, and numbers are attached referring to the page of the work where the explanations may be found. A supplementary chapter has been added on l Suggestions as to Apparatus/ and the author trusts this may be found useful to many students. The author is indebted to other writers on electricity in one or two instances for short extracts and diagrams. T. P. T. ST. JAMBS'S SCIENCE AND ART SCHOOLS, KEYHAM, DEVONPORT : June, 1885. SYLLABUS OF SCIENCE AND ART DEPARTMENT. SUBJECT IX. PART II. First Stage, or Elementary Course. FRICTIONAL ELECTRICITY. Electrification by friction. Two distinct electrical states and their mutual relations. Positive and negative electrification. Simultaneous and equal development of the two electrical states. Action of electrified bodies on each other. Conductors and non-conductors. Effect of connecting a conductor. Experiments illustrating the relation between a primary electrical charge and the correlative induced charge. The Leyden jar and similar arrangements. Distribution of electricity on conductors. Action of points. Properties of hollow conductors. Frictional electrical machines. Electric discharge. SYLLABUS FOR ELEMENTARY SCHOOLS. New Code, 1882. PHYSICS. STAGE II. ELECTRICITY (FRICTIONAL). Attraction of light bodies by rubbed sealing-wax and glass. Experimental proof that there are two forms of electricity. Attraction and repulsion. Gold leaf electroscope. Construction of electrophorus, electrical machine and Leyden jar. Explanation of atmospheric electricity. CONTENTS. INTRODUCTORY REMARKS. First reference to Electricity Dr. Gilbert's investigations Volta's discovery Further development Necessity for experiment Tyn- clall's opinion Guthrie's opinion .... pages 1-3 1. Electrification by Friction. Result of rubbing two bodies together Attraction of light bodies Experiments Influence of moisture on the charging of bodies Glass and sealing-wax representative substances Supports for glass and sealing-wax rods The Electric Pendulum Experi- < ments pp. 3-7 2. Two Distinct Electrical States and their Mutual Relation. Vitreous Electricity Resinous Electricity Positive and negative Experiments. Theories of Electricity One-fluid theory Two- fluid theory Numerical representation of two-fluid theory Numeri- cal representation of one-fluid theory .... pp. 7-12 3. Positive and Negative Electrification. Electricities of glass and sealing-wax rods examined Action on charged pith ball Experiments .... pp. 12-14 4. Simultaneous and Equal Development of the two Electrical States. One kind cannot be excited without the other Electroscopes The Electric Pendulum The Pith-ball Electroscope The Gold-leaf Electroscope The Proof Plane Carrier Ball Use of electroscope Method of charging : (a) positively, (/>) negatively Experiments Equal quantities in rod and rubber Faraday's experiment Experiments pp. I4~ 2 3 vi Contents. 5. Action of Electrified Todies on each other. Charged rods on neutral pith ball Charged pith balls on each other Strips of silk ribbon, and ribbons of gutta-percha electrified Elec- trical laws Experiments ...... pp. 23-25 6. Conductors and Non-conductors. Conductors Non-conductors Semi-conductors Lists of each Non- electrics and electrics Brass rod held in hand or by insulating handle Insulating Stool Human body earth-connected or insu- lated Method of testing conductors and non-conductors Con- ductors and insulators Conductivity of wire and silk string contrasted Wet string Insulation Conduction Experi- ments . . . . . . . . pp. 25-32 7. Effect of connecting a Conductor with the Earth. Conductors in earth-connection and insulated Result of touching con- ductor under inductive action of rod Method of charging conduc- torInduction Free Electricity Bound Electricity Specific inductive capacity Experiments . .'.'-. pp. 32-36 8. Experiments illustrating the relation between the Primary Electrical Charge and the Correlatively Induced Charge. Successive steps of inductive action On neutral pith ball Gold-leaf electroscope Faraday's Ice-pail experiments Action on conductors Two insulated balls Charged conductors on each other The Electrophorus Description Method of charging Continuous charge Experiments ...... pp. 36-46 9. Frictional Electrical Machines. Kinds of frictional machines Cylinder machine Description Charg- ing machine Positively Limit of charge Negatively Methods of showing tension Henley's Quadrant Electrometer Use Plate machine Ebonite or metal plate Method of charging plate ma- chineWinter's machine Theories of charging prime conductor Experiments pp. 46-58 Contents. vii 10. The Leyden Jar and similar arrangements. Condensation of electricity Condenser explained Three forms of condensers Epinus's Method of charging Disruptive and spon- taneous discharges Discharge Slow Instantaneous Discharg- ing rod Explanation of discharge Discharge through human body Strength of charge Highest specific inductive capacity Franklin's Pane Charging and discharging Experiments Leyden Jar History Description How charged Positively Negatively Effect of insulating jar Connecting inner and outer coating with earth Discharge Slowly Instantaneously Residual charge Where electricity is stored Jar with movable coatings Charging and discharging Leyden Batteries Ordinary form Charging and discharging Universal Discharger Effects of discharge Cascade arrangement Charging and discharging Experiments Unit Jar Use Harris's Lane's Electrometer Condensing Electroscope Experiments. ....... pp. 59-84 11. Distribution of Electricity. Charge on outside of conductor Methods of testing Distribution on sphere Cylinder Ellipsoid Disc Cube Density where greatest Quantity, area, density, tension Subdivision of charge Equal spheres Unequal spheres Relation between area and density Electrical tension pp. 84-89 12. Distribution on Hollow Conductors. Same on solid or hollow balls Biot's experiment Hollow conductor tested with proof plane Faraday's Butterfly Net Cubical room Electroscopes protected Illustration of distribution Exceptions to general law Experiments ..... pp. 89-95 13. Action of Points. Points attached to and presented towards charged bodies Tension of prime conductors reduced Electrical aura Use when attached to prime conductor and rubber Action of needle on charged rod Action of point in conductor either turned towards or away from a charged body Point projecting from pith ball Electrical whirl Electrical inclined plane Experiments . . , pp. 95-99 viii Contents. 14. Electrical Discharges. Three forms of spark Luminous effects Spark and glow discharges St. Elmo's Fire Illuminated designs Leyden Jar Luminous Leyden Jar Electric Egg Geissler's Tubes Chemical effects Volta's Pistol Ozone Heating effects Liquids ignited Gas lit Platinum wire heated Gunpowder and gun-cotton exploded Mechanical effects Card punctured Glass and wood Leyden Jar pierced Magnetic effects Needles or strips Action on Gal- vanometer Physiological effects Effect from prime conductor Leyden Jar Leyden Battery TyndalPs remarks Lichtenberg's figures Effect of heat White-hot iron ball Bright-red Dull-red Action of flames Evaporation Induced currents Use of flat spirals Secondary and tertiary currents Duration of spark Velocity of Electricity Illustrations of velocity Experi- ments ... . . . . pp. 99-H2 15. Electrometers. Use Simplest forms Chief electrometers Coulomb's Torsion Balance and experiment Peltier Electrometer Sir Wm. Thomp- son's Quadrant Electrometer Experiments . . pp. 112-115 16. Atmospheric Electricity. Electricity of air Lightning and thunder Thunderstorm and ful- minating pane Kinds of Lightning Thunder Crash and peal Distance of discharge Return shock Lightning-conductors-^-Use Area of security Experiments . . . . pp. 115-121 17. Suggestions as to Apparatus for Frictional Electricity, pp. 121-125 18. Brief History of the development of Frictional Electricity. pp. 125-127 Questions in Frictional Electricity which have been set by the Science and Art Department from 1867 to 1885, and references to the pages of the work where the Answers may be found . pp. 128-135 UNIVRSITY! TEXT-BOOK OF FRICTIONAL ELECTRICITY. THE development of this branch of physics has been the result of the careful observation of simple experiments by scientific minds during the last three centuries. Until the reign of Queen Elizabeth, A.D. 1600, little was known of Frictional Electricity beyond the fact that, when amber was rubbed, it attracted light bodies, so as to cause them to stick to it. This was first mentioned about 600 B.C. by the Greek philosopher Thales, and is considered to be that from which electricity derived its name ; for the Greek word for amber is elektron, in which we can see the resemblance to the word now used. After an interval of more than 2,000 years, Dr. Gilbert, an eminent scientist and physician to Queen Elizabeth, gave the science a fresh im- pulse by his experiments with glass, resin, spars, gems, &c. He proved that these substances, when rubbed, exhibited similar effects to those which amber did. From these in- vestigations the science has been gradually and successfully unfolded by a long list of thoughtful experimenters, each adding some new principle which he had successfully investigated, or laying down some new law which he had conclusively established. From an accidental occurrence in 1790 A.D., and the reflections of Volta connected therewith, Electricity was ^ B 2 Text-Book of Fractional Electricity. studied as the result of chemical action on dissimilar metals, which were placed in liquids, and connected in circuit. This was the dawn of current, or as it is known from its first investigator, Voltaic Electricity, which for some time made steady progress ; but within the last few years has taken marvellous strides, giving us the electric telegraph, electric engines, electric light, electric railways, &c. What it is destined to accomplish for mankind in the subsequent history of the world is a problem, which the most penetrating intellect cannot presume to predict. It is with- out doubt one of the most mighty forms of energy, and, as time rolls on, will no doubt be still more wonderfully de- veloped. ' It should be the object of everyone studying this subject to test every phase of it by as many experiments as possible, that the mind may be familiarised with the simple pheno- mena, which have led to marvellous discoveries. Professor Tyndall, in his work on Electricity, writes concerning the art of experimenting : ' 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. 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.' Professor Guthrie, the examiner in Sound, Light, and Heat, and Magnetism and Electricity, says in his report on the Examination of Training Colleges in the annual report of 1882 : * In many cases the teaching appears to have been mainly theoretical. Many of the answers show that the examiner, while understanding the conventional description of a piece of apparatus, has probably never seen it, and cer- tainly never used it. This won't do ; and for a very small sum many bits of apparatus may be purchased or made, and where this subject is taught the teacher should allow the Attraction of Light Bodies. 3 pupils to try the experiments for themselves, that they may gain confidence, and he might inform them of the best means of obtaining or making little things for their own use. I therefore venture to point out that for training colleges sufficient laboratories are necessary. No description, how- ever conscientious, can take the place of seeing, handling, and using physical apparatus.' 1, Electrification by Friction. By Friction is meant the rubbing of one body against another body. When this takes place electricity is produced on the bodies rubbed together, and under certain conditions it may be accumulated, so that its presence may be perceived by its influence on F IG> x< other neutral or elec- trified bodies. To exhibit this we must resort to experiment. A body in which electricity is apparent is said to be excited, charged, or electrified, and it will then attract light bodies, such as bran, small bits of paper, Small bitS Of Excited Rod attracting light bodies. pith, bits of gold leaf, and other very light substances (fig. i). As previously mentioned, amber was the first substance tested, but afterwards glass, sealing-wax, resin, gutta-percha, ebonite, vulcanite, gems, and spars were similarly examined. Experiments. Take piece of amber, glass rod, ebonite rod, stick of sealing-wax, stick of sulphur, and sheet of coarse brown paper, and rubbers of silk, fur, and flannel. Excite amber with flannel, glass with silk, ebonite with fur, sealing-wax and sulphur with flannel, brown paper with fur or coat-sleeve. Test each excited body by holding them in succession just above such light bodies as those mentioned, and 4 Text-Book of Frictional Electricity. certain effects will result. In some cases the light bodies, particularly if they have points about them, will be attracted to the electrified body, and will stick to it. In other cases, as in the tiny pith-balls, there will be attraction and repulsion, frequently repeated, causing them to jump to and be driven from the excited body. Note. It should be stated at the outset that the moisture in the atmosphere has a very bad effect upon experiments in Frictional Electricity ; and, even with the greatest care, it sometimes prevents an experiment being successfully per- formed. To counteract this deposit of moisture on the bodies to be excited they must be slightly warmed, and the rubbers well dried. Great care is required with glass rods, as they are more liable to this dewy deposit than the other substances mentioned. Brown paper is best excited, when it is heated until it just begins to smoke, and then smartly rubbed. Two of these substances, glass and seating-wax, soon became selected as representative substances, the former when it was rubbed with silk, and the latter when rubbed with flannel ; for it was found that the electricity developed on glass and sealing-wax under these conditions exhibited opposite effects on similarly excited substances when they were brought within the influence of one another. For ex- ample, glass rubbed with silk repelled glass rubbed with silk, but attracted sealing-wax rubbed with flannel ; while sealing-wax rubbed with flannel repelled sealing-wax rubbed with flannel, but attracted glass rubbed with silk. In order that such experiments may be performed, the glass, or sealing-wax, must be supported so as to be exactly balanced, that it may be able easily to move horizontally. This may be done by closing the ends of the tube in a blow- pipe flame, finding the middle of the tube by balancing it on a knife-blade or other equally sharp edge, heating this spot in a blow-pipe flame, and making a slight depression while the glass is soft, with a compass leg or other sharp- pointed instrument. Electric Pendulum. 5 To support it a stand may be made, consisting of a block of wood 3 in. square, having a hole in the centre into which a piece of glass tubing 6 in. long and \ in. in diameter is placed. The upper end of the tubing should be filled with shell-lac for about \ in., and then the hot eye of a needle FIG. 2. FIG. 3. Supports for Excited Rods. pushed into the centre of the tube. This needle-point forms the pivot on which the supported body turns (fig. 2). There is another method of suspending rods by means of wire rests, made as represented in the diagram (fig. 3), and fastened to a ribbon of silk for support. The different actions of glass and sealing-wax, when excited by silk and flannel, on neutral bodies may be plainly seen by their separate influences on a pith-ball suspended by a thread of raw silk. When a glass rod rubbed with silk is brought near the pith-ball, the little ball is attracted to the glass, touches it, and is immediately repelled, and will not for a time again approach the excited glass. If sealing-wax rubbed with flannel be now brought near the repelled pith- ball, the ball will be attracted strongly towards the wax, but in a very short time will be again repelled. An excited glass rod will now attract it. By placing excited rods of glass and wax on either side of the pith-ball it will swing backwards and forwards to wax and glass alternately, as it is repelled from one and attracted towards the other. This forms the Electric Pendulum. 6 Text-Book of Frictional Electricity. Experiment. Exciteglass rod with silk, bring it near suspended pith- ball ; notice attraction, contact, then repulsion. Excite sealing-wax with flannel, bring it near the charged ball ; the pith-ball is first attracted into contact, and then repelled ; the excited glass rod now attracts it. Excite both glass and wax, and place them on each side of the pith- ball, sufficiently near to affect it. Let it first be attracted by the glass ; Attraction and Repulsion of Pith-Ball. it is then repelled to the wax, and again repelled to the glass, and so forth several times. Had it been attracted to the wax first, it would have been repelled to the glass, and from thence again to the wax, and so on, as in the former case. Show this. Heavier bodies are also attracted by highly excited rods, as when glass is rubbed with amalgamed silk, or an ebonite rod is excited with fur. Experiments. Take empty egg-shell, which has had its contents blown through holes in the end, let it lie on table, bring excited rod near it ; as it moves towards rod, gradually withdraw the rod, the egg- shell will follow it. Make a cylinder of cartridge-paper 6 in. long, 3 in. diameter, close ends, this may be drawn after excited rod similar to egg-shell. Balance lath on rounded end of test tube, approach rod to end, notice attraction ; draw it round by the attraction of the rod. Two Distinct Electrical States. 7 Attach feather to a fibre of raw silk ; let it be attracted to glass rod ; it will soon be repelled ; pursue it with the rod ; it will be driven from it, and seem to float in the air. Rub sticks of ebonite, sulphur, gutta-percha, amber, &c., with fur ; and show that they act similarly on the suspended pith-ball, and balanced lath. Heat coarse brown paper, and rub it briskly either with the coat- sleeve or with a clothes-brush, it becomes strongly excited, and will attract readily the bodies previously mentioned. Excite brown paper highly ; lift it by one corner, and place it against the wall ; it will cling to it, and remain there, when the hand is taken away. Hold it over head ; notice attraction of hair. Warm a drawing-board ; lay a sheet of foreign paper on it ; rub briskly with bottle indiarubber ; if the sheet be then lifted by a corner, it will cling to the -wall as the brown paper did. A ribbon of gutta-percha, drawn rapidly between the fingers, or a film of collodion rubbed with the hand, will show an attractive influ- ence on light bodies. So will a strip of narrow white ribbon drawn between vulcanised indiarubber tubes placed over the fingers, and it will also stick to the wall if placed against it when strongly excited. Test these. 2, Two Distinct Electrical States and their Mutual Relation. From these experiments it will be perceived that the electricity derived from glass rubbed with silk differs in its action on the suspended pith-ball from that developed on sealing-wax rubbed with flannel, one attracting what the other repels, and repelling what the other attracts. By other experiments it may be seen that all electrified bodies exhibit influences similar to one or other of these representative bodies. For a long time it was considered that only one kind of electricity could be produced on glass, when it was rubbed with any rubber, and this was named vitreous electricity from vitrum, the Latin word for glass. It was also thought that sealing-wax only gave one kind when similarly rubbed, and this was termed resinous elec- tricity, because resin formed a large portion of the substance of the sealing-wax. Recently, however, it has been shown 8 Text-Book of Frictional Electricity. that, when glass was rubbed with catskin, the same kind ot electricity was excited on glass, which was thought to be peculiar to sealing-wax, and other resinous substances ; also that when sealing-wax was rubbed with gun-cotton, the electricity considered peculiar to glass or vitreous electricity was excited on the wax. The names vitreous and resinous thus lost their significant meaning, and new names, positive and negative, were substituted by Franklin. Vitreous elec- tricity received the name positive, or + (plus) electricity, and resinous electricity, negative or (minus) electricity. The excited bodies belonging to each class may be easily determined by testing them with the electric pendulum. To do this the pith-ball must be charged with glass rubbed with silk. It is strongly attracted by sealing-wax rubbed with flannel. These are termed unlike electricities. All resinous substances, or other substances excited negatively, or charged with negative electricity, will also strongly attract the pith-ball charged by glass rubbed with silk, which is vitreous or positive electricity. On the other hand, all bodies positively excited will act similarly to the glass rod rubbed with silk, they will repel it. Again, if the pith-ball be charged negatively by sealing- wax rubbed with flannel, the glass rod rubbed with silk, which is positively charged will strongly attract it ; but any substance negatively excited will repel it. Experiments. Charge pith-ball with sealing-wax rubbed with flannel. Excite glass rod with catskin ; strong repulsion. This proves the electricities to be alike. Next charge pith-ball with glass rubbed with silk, then excite sealing-wax with gun-cotton ; notice strong repulsion ; similar electricity. Charge pith- ball with glass rubbed with silk ; excite sealing-wax rubbed with flannel ; bring it near charged ball ; strong attraction. Test ebonite rubbed with fur in the same way ; strong attraction ; negatively charged. Rub foreign paper with indiarubber on a warm drawing-board, it repels the pith- ball charged with glass rubbed with silk ; similarly charged ; positive electricity. Charge ball with sealing-wax rubbed with flannel ; bring near it glass rod rubbed with silk ; strong attraction, positive. Test strip of white ribbon drawn between vulcanised indiarubber tubes on Theories of Electricity. g the fingers in the same way ; strong attraction ; positively charged. Rub glass rod with catskin ; test with sealing-wax charged pith-ball ; repulsion ; negatively charged. Several other experiments of a similar character will be found on p. 14. It will be necessary here to test various substances with different rubbers, so that each statement may be confirmed by observation, and not taken for granted because stated to be true. To account for these differences of electrical condition, theories or ideas were put forward by scientific men. The chief of these were, the ' One Fluid Theory of Franklin/ and the ' Two Fluid Theory of Symmer.' Theories of Electricity, (a) One Fluid Theory. (b) Two Fluid Theory. (a) One Fluid Theory. The ' One Fluid Theory ' was suggested by Franklin ; and he endeavoured to explain all electrical phenomena under that idea. According to this theory all bodies contain a certain quantity of a peculiar fluid, which he termed electricity. As long as the body had only its definite quantity, it was in what he considered its neutral condition, and would exhibit no electrical influence on other neutral bodies. When by any means, such as friction, chemical action, or otherwise, more electricity was produced on the body than it would have in its neutral condition, it became positively charged, or excited ; and the higher as more electricity was added. It then had its own quantity + electricity received from some other body acting upon it, and was thus said to be charged with + or positive electricity. When electricity was taken from the neutral body, it became charged negatively, or had elec- tricity, or less electricity remaining on it than its own neutral quantity. (b) The Two Fluid Theory. This is due to Symmer. He considered that the neutral condition of all bodies was 10 Text-Book of Frictional Electricity, owing to two fluids existing in them in equal quantity and of enormous amount. As soon as this equilibrium was dis- turbed to any extent the body became charged positively or negatively, according to the nature of the rubber and rubbed surfaces. The process of charging he thought to be due to the decomposition of the two fluids in rod and rubber, one of which accumulated on the surface of the rod and the other on the surface of the rubber. If the rod accumulated posi- tive from the rubber, it gave back an equal quantity of negative to it, so that the amount of electricity remained constant in both, but was re-arranged, one body becoming positively charged and the other negatively, or vice versa. Both these theories may be represented numerically, and a clearer idea of the charging of bodies conveyed to the pupil. We will deal with the Two Fluid Theory first. Suppose rod and rubber each to contain 1,000 units of electricity when in the neutral condition, and that this results FIG. 5. FIG. ( Neutral. Rod and Rubber. Excited. from both having 500 units of positive and 500 units of negative (fig. 5). Let the rod be glass and the rubber silk. After a little friction the neutral electricity is decomposed in each, and the units are re-arranged. It is found by experi- ment that in this case positive accumulates on the rod and negative on the rubber. This may be explained in the following way. Suppose the friction liberates one unit of Numerical Representations. 1 1 positive in the rubber which passes to the rod, it at the same time liberates one unit of negative in the rod which passes to the rubber. Each still retains the same total units of electricity (1,000), but in an altered condition, the rod having positive in excess and the rubber negative (fig. 6). The two units of positive in the rod and negative in the rubber are now free to affect neutral or excited bodies. If a larger number of units be liberated by the friction, it may be easily perceived and numerically represented, that the rod and rubber will be in a higher state ot electrical excitation by having a larger number of positive and negative units free, that is remaining after the number of the units of the oppo- site kind is subtracted from the higher number which is produced. In every case we must understand that the body is not charged entirely with positive or negative, but that there is always a certain amount of neutral electricity in all bodies, and that the strength of the positive or negative charge depends upon the number of positive or negative units in excess of the equal quantities of opposite electricities, which keep a portion of the electricity of a body in the neutral condition. Note. Any number of units may be assumed to repre- sent the neutral condition of a body. High numbers have been used that it may be easily seen how a large quantity of neutral electricity may exist in a body, and yet it may be highly charged with free positive or free negative electricity. It must be remembered that whatever electrical changes take place in bodies, the total number of units of electricity will remain the same, only differently arranged. This statement will enable the student to understand many difficulties which could not be well understood without it. The" ' One Fluid Theory ' may be numerically represented in the following way : To account for the charging of the rod and rubber by the ' One Fluid Theory,' let us assume that each body con- tains 500 units of a peculiar fluid named electricity, and that 1 1 Text-Book of Frictional Electricity. this is the necessary quantity to keep each in its neutral condition (fig. 7). If either of these receive more than this amount, it will be 'charged with the excess positively, and if it lose any, with the loss negatively. On rubbing the rod we will assume that it receives two units of electricity from the rubber. The condition of each will then be represented by fig. 8, and it FIG. 7. FIG. 8. Neutral. Rod and Rubber. Excited. will be seen that the one has more than its usual quantity, and the other less ; or, in other words, one is positively and the other negatively charged. Any other units may be as- sumed to represent the neutral condition of the bodies, and higher units to express the positive and negative electrifica- tion besides those used here for the purpose of illustration. Other Ideas. Some of our leading scientists consider the electrical excitation to be due to a peculiar condition of the particles of the excited body, or the ether which sur- rounds the particles and fills the interstices between them. All, however, are merely suppositions, but afford means for the mind to apprehend in some degree this extraordinary form of energy. 3. Positive and Negative Electrification. It was at first considered that the electricity developed on glass was positive, and on sealing-wax and similar sub- stances negative ; experiment, however, proved that the Positive and Negative Electrification. 13 character of the rubber in some cases changed the electrical state of the body rubbed, for negative was found to be pro- duced on vitreous bodies, and positive on resinous. The kind of electricity with which a body is charged may be easily determined by testing it with the electric pendulum. For this purpose the pith-ball should be charged with a known kind of electricity say, glass rubbed with silk, which we know is positive. In this condition it is strongly attracted by sealing-wax rubbed with flannel, which is also known to be negative. All negatively-charged bodies will act similarly on the pith-ball, charged positively, as the sealing-wax does ; they will attract it. On the other hand, if the pith-ball be charged with sealing-wax rubbed with flannel, it will be nega- tive, and will be strongly attracted by the positive electricity which is produced on a glass rod rubbed with silk. Bodies charged with positive electricity also attract the negatively- charged ball. This attraction, however, must not be relied on as indicating definitely a different kind of electricity, for a neutral body would also attract the pith-ball charged with either positive or negative ; but the attraction in the case of the neutral body would not be so strong as in the other cases. It is better, therefore, to depend upon repulsion than attraction for determining the kind of electricity with which a body is charged. We have previously seen that when the pith-ball was brought into contact with an excited rod, it was immediately repelled, and would continue to be driven from the rod, while both were electrified similarly. Now, any body charged with the same kind of electricity as the excited rod would act similarly on the charged pith-ball ; it would repel the ball. We thus conclude that when the pith- ball has been charged by a glass rod rubbed with silk, if re- pulsion takes place, when another charged body is brought near it, that that body is charged with positive electricity ; or again, when the pith-ball is charged with sealing-wax rubbed with flannel, which is negative, all electrified bodies which repel the ball in this condition are negatively charged. 14 Text-Book of Frictional Electricity. Negative electricity is best excited on resinous substances, and positive on vitreous. The best rubber for resinous bodies is fur, and for vitreous, silk (amalgamed). Experiments. Prove all these statements : Charge pith-ball with glass rod rubbed with silk for each substance tested, and notice strong attraction of pith-ball, when the following excited substances are brought near it : Sealing-wax rubbed with flannel. Ebonite rubbed with fur. Sulphur rubbed with flannel. Brown paper rubbed with clothes-brush. Gutta-percha rubbed with flannel. Ribbon of gutta-percha drawn between dry fingers. Vulcanite, or vulcanised indiarubber, rubbed with fur. Glass rod rubbed with catskin. Charge for each substance the pith -ball with sealing-wax rubbed with flannel. Notice strong attraction from the following excited sub- stances : Glass rod rubbed with silk. Sealing-wax rubbed with gun-cotton. Vulcanite rubbed with gun-cotton. Ebonite rubbed with gun-cotton. Ribbon of white silk drawn between fingers, having two pieces of vulcanite placed over them. Foreign paper rubbed with caoutchouc. Collodion rubbed with the hand. Next, test each substance mentioned in its neutral condition with a positively and negatively-charged ball ; and notice that attraction also takes place, but not so strongly as when oppositely charged bodies were near. Test, again, the character of the electricity on each body in the order already given, and satisfy yourself that your results are accurate by the repulsion of the similarly-charged bodies. If in the first case the pith-ball be charged by sealing-wax rubbed with flannel, repulsion will be apparent as each excited body is presented to it, while the second list will repel the ball if it be charged by the glass rod rubbed with silk. 4, Simultaneous and Equal Development of the two Electrical States. One kind of electricity cannot be excited in a body without the interchange of an equal quantity of electricity Electroscopes. 1 5 of the opposite sign from the one body to the other. This may be easily understood by reference to the numerical representation of the electrical condition of a charged body, p. 10. But before we examine this portion of our subject fully, it will be necessary to understand the construction and action of Electroscopes. Electroscopes. These are apparatus which enable us to see by certain definite effects, whether a body is excited or not ; and if so, to determine with what kind of electricity it is charged. Electroscopes are usually one of three kinds : a. The Electric Pendulum. b. The Pith-Ball Electroscope. c. The Gold Leaf Electroscope. a. The Electric Pendulum. This has been fully de- scribed, p. 5. The insulated pith-ball enables us to see that a body is charged by the attractive influence which FIG. 9 . it exerts on the ball. Again, if the ball be charged (~\ with a known kind of electricity, all bodies charged similarly will repel it, while all charged with the opposite kind will strongly attract it. The charge of the pith-ball does not last long, and so it cannot be relied on as an invariable test for any length of time. The electricity is conducted away from the ball by the surrounding air, and, if this be ^ damp or dusty, the discharge is very rapid. b. The Pith-Ball Electroscope. This has two Pendulum, pith-balls instead of one, and they are securely inclosed in a well-dried glass jar or bottle, to prevent the discharge of the electricity from the balls into the surrounding air. Through the cork, but carefully insulated from it by gutta-percha, shellac, sealing wax, &c., passes a brass rod, the end of which extends into the middle of the jar. To this end of the rod two pith-balls are attached by cotton, and hang at equal distances from it about two inches. To the other 1 6 Text-Book of Frictional Electricity. end of the rod is fastened a brass ball, which receives the electrical charge ; and thus communicates it to the balls, by means of the brass rod and cotton. These immediately show that they are both charged with the same kind of FIG. 10. electricity by the repulsion which takes place. Had silk been used instead of cotton to connect the balls they would not have received the necessary charge, neither would they have been affected by the passage of electricity through the rod. Also, if the brass rod be not well insulated from the cork and bottle, the charge will not pass into the balls. For this purpose, and as a secure means of insulation, the Pith-Bail Eiectroscope.stopper should be made of gutta-percha or vulcanite, and generally the cork and neck of the vessel coated with shellac varnish. When the balls are charged with a certain kind of electricity the condition of another charged body may be easily detected by the movement of the balls, as the body approaches the knob at the end of the rod outside the vessel. Should the electricity of the body be similar to the charge of the balls, they gradually diverge further, as the rod approaches ; but, if of opposite character, they gradually collapse, returning to their previous position on the withdrawal of the excited rod or body. c. The Gold Leaf Electroscope. This is similar in its construction to the Pith- Ball Electroscope, as regards the jar, stopper of some insulating substance, or cork with a hole to insulate the brass rod, passing through it into the vessel. To the end of the rod within the jar is fastened a flat cross piece of brass about f in. long, and on each side of this is stuck a strip of gold leaf or Dutch metal about 2 in. long and f in. wide. These should be exactly parallel and equal, and just long enough so that they may not touch the sides of the vessel, when they are extended to the furthest extent. The stopper of the glass jar or bottle should fit Gold Leaf Electroscope and Proof Plane. 17 FIG. securely, so that moisture may be prevented from getting into the jar, as this interferes with the perfect action of the Electroscope by causing the leaves to collapse quickly after a charge of elec- tricity of either kind has been communicated to them. The Gold Leaf Electroscope (fig. n) is the most delicate of either of those described, and is liable to damage from the sudden divergence of the leaves, if a very highly charged body be brought near it. To pre- vent this a simple apparatus has been made which has received the name of the Proof Plane. The Proof Plane consists generally of a small metallic disc, fastened to an insulating rod of some kind. A piece of tin i in. in diameter stuck into a thin stick of sealing- wax will do well ; or two pieces of silvered paper cut out having the same diameter and stuck together will form a very good collector. This should be stuck with gum to a strip of glass (^ in. wide) coated with shellac varnish. It is well for experimenting to have two Proof Planes of the shape given in the diagrams (figs. 12, 13). The circular one should have a diameter of an inch, and the other should be first made an oblong of 3 in. by 2 in., and have the angles rounded off. The former is used to obtain a small quantity from a highly charged body, and the latter to collect a larger quantity from a body less highly charged. When the Proof Plane is in contact with the excited body, it forms for the time a portion of it, and is conse- quently charged with the same electricity. On its removal a portion of the body is, as it were, torn off to be tested. This Gold Leaf Electroscope. FIG. 12. FIG. Proof Planes. i8 Text-Book of Frictional Electricity* exhibits by its action oh the Electroscope the kind of electricity with which the body is charged. Sometimes instead of the Proof Plane a pith ball coated with tin foil is used. This is hung by a fibre of silk, and is named the ' carrier ball? Use of the Electroscope. The Electroscope generally used is the Gold Leaf Electroscope, as it exhibits most readily the presence of an excited body ; for when a highly charged body is some distance away from it, the leaves show signs of divergence, FIG. 14. FIG. 15. FIG. 16. Electroscope. Divergence. Collapse. which increases as the body approaches (fig. 15), but de- creases again as it is withdrawn (fig. 16). This is in conse- quence of the charged body acting upon the neutral electroscope. As soon as it is sufficiently near to influence the electroscope the two electricities are decomposed, and an amount of electricity of the opposite kind is drawn towards the disc, and held there by the presence of the excited body; while a certain portion of the same kind of electricity is repelled into the leaves ; and as there are no means for interchange with the earth, the two leaves become charged with similar electricity, and, like the similarly-charged bodies previously described, they repel one another. To determine the kind of electricity with which a body is charged, the electroscope must first be charged with either positive or Charging tlie Electroscope Positively and Negatively. 1 9 negative electricity, and the effect noticed on bringing a positively or negatively excited body near it. Method of charging the Electroscope : (a) Positively. Bring a negatively excited body near the electroscope, such as sealing-wax rubbed with flannel. This decomposes the neutral electricity of the electroscope, attracting the positive to the disc, but repelling the negative into the leaves, which diverge with free negative electricity. This free negative will pass to the earth, if a way be provided, and an equal quantity of positive will enter the electroscope from the earth at the same time. That this may take place, the electroscope must be put into conducting connection with the earth, either by touching the metallic portion with the hand, or with some suitable substance held in the hand. When this is satisfactorily done, which is seen by the collapse of the leaves, the hand or earth connection is removed ; and then the excited rod is withdrawn. Care must be taken that the rod is not withdrawn before the hand is removed, otherwise the repelled negative will again enter the elec- troscope from the earth, and an equal quantity of positive leave it for the earth, thereby again producing the equal quantities of positive and negative, which restore the previous neutral condition of the electroscope. In this case the leaves remain collapsed when the hand is removed, and show no signs of electrical charge. But if the hand be withdrawn, and then the rod removed, the positive electricity, which was held by the presence of the rod, will spread itself over the electroscope, and that amount of free positive will charge it more or less, which will be shown by the diver- gence of the gold leaves. If the charge be little the diver- gence will be slight ; but, if high, the leaves will spread further apart. (fr) Negatively. Bring a positively excited body near the disc of the electroscope. Touch it while the leaves are repelled with positive electricity, and the excited body is near. As in t'he previous case, positive leaves and negative 20 Text-Book of Frictional Electricity. enters the electroscope. On removing the hand, and then the excited body, the electricity, which was attracted to the upper portion of the electroscope, is now free to spread FIG. 18. FIG. 17. Neutral. Positive Divergence. Earth Contact. Negative Charge. over disc, rod, and leaves, and its free negative charge is again exhibited by the divergence of the leaves. Experiments. Exhibit electric pendulum, bring neutral bodies near, no effect on pith -ball. Excite rods of various kinds, bring them in succession near the neutral pith-ball ; three effects always take place : attraction, charge, repulsion. Charge pith-ball positively with glass rod rubbed with silk ; the kind of electricity of the body to be tested can be easily perceived as it is brought near the pith-ball ; if the body be positively charged the pith-ball will be repelled ; but if negatively charged, the ball will be strongly attracted. Test this. Charge pith-ball negatively with sealing-wax rubbed with flannel ; negatively charged bodies will repel the ball, but positively charged will strongly attract it. Prove that this is so. Exhibit pith-ball electroscope, bring neutral bodies near the knob, and into contact with it : no movements of the pith balls noticeable. Excite rods positively or negatively ; bring them in succession near the knob : notice repulsion of balls from the free electricity of the same kind driven into them by the presence of the excited body. Withdraw the rod ; the balls again assume the neutral condition and come together. Bring negatively charged body into contact with the knob ; the Experiments with Gold Leaf Electroscope. 2 1 electroscope will then be charged with negative electricity, and the balls will be immediately repelled, having similar negative charges. If the body be positive, the electroscope will be charged positively, and the balls will repel one another in this case with positive electricity. Note. By contact the balls will have the same kind of electricity as the electrified body has. Experiments. Charge the electroscope without contact. Bring excited body near the knob notice repulsion of balls touch knob or metallic portion of electroscope with hand, rod being still near, balls again come together ; remove hand, then remove rod ; the balls now diverge with the opposite electricity to that of the exciting body : if that be positive, the balls will be charged negatively ; but if negative, the balls will be charged positively. Exhibit gold leaf electroscope. Bring neutral bodies of different kinds near and into contact with the disc : no movement apparent in the leaves. Excite bodies slightly : notice gradual but feeble divergence as excited body approaches the disc, and collapse as gradually as the excited body is withdrawn. Excite bodies more highly : greater divergence is now apparent, violent repulsion as the body is near. Sometimes in this way the leaves are broken, and the electroscope spoiled. Show use of proof plane here. Take charge with small one from strongly charged rod ; rub both sides of it over rod, bring it into contact with the disc of the electroscope : the electroscope is thus charged with the same kind of electricity as the excited body. A gold leaf electroscope is seldom charged by contact, unless the body be very feebly electrified ; if the body be highly charged, the proof plane is always used. The action of an excited body on the gold leaf and pith- ball electroscopes should here be exhibited and contrasted, so that the delicacy and sensitiveness of the former may be fully perceived. Experiments. Charge gold leaf electroscope without contact. First, positively. Test it with positively charged bodies, greater repulsion of leaves. With negatively charged bodies collapse, partially or wholly. Second, negatively. Greater repulsion as negatively charged bodies are brought near, but gradual collapse when the body is positively excited, 22 Text- Book of Frictional Electricity. In these cases, also, it is generally advisable to use the proof plane, for by means of it a more delicate test may be made. That electricity is produced at the same time in equal quantities in the rubber and rubbed body may be plainly shown by allowing the rubber to remain wrapped around the rod, and placing both together on the disc of the elec- troscope. While they continue together the leaves of the electroscope are not affected ; but, on withdrawing the rod, the leaves diverge, owing to the influence of the electricity on the rubber charging the electroscope. If the rod be placed again within the rubber, the leaves collapse. This may be 'easily shown by rubbing sealing-wax with flannel. On removing the rod without allowing the electricity to escape, the leaves will remain charged with the same elec- tricity as the flannel has, and on the approach of the excited wax the gold leaves will fall together, indicating the presence of electricity of the opposite kind, and proving the existence of opposite electricities in rod and rubber. Faraday arranged a suitable means of illustrating this simultaneous and equal development of the two electricities by using a rod of shellac, and placing over a portion of it a flannel cap, which had a silk string attached to it (fig. 21). On rubbing the flannel round the shellac a few times, electricity of opposite cha- racter was excited in both. When the flannel cap was removed by the silk string, both flannel and shellac retained their respective charges : and when Shellac rod and Flannel cap. . & . , . presented to the electric pendulum, charged positively, the rod attracted the pith-ball and the flannel repelled it. Again, when both were brought at the same time on the disc of a neutral gold leaf electroscope, no divergence took place, proving thereby that the two elec- tricities excited in each are equal in quantity. Action of Electrified Bodies on each other. 23 Experiments. Rub sealing-wax with flannel; let wax remain in flannel, and place both on disc of electroscope, no movement in leaves ; draw out wax, leaves diverge with positive electricity of flannel ; place wax again in former position, leaves are neutralised and collapse. Rub rod again, and place both on disc as before ; draw out wax, leaves are again charged positively ; next remove flannel from disc with vulcanised tubes over fingers, or with two pieces of sealing-wax, or gutta-percha, or similar substances, then bring the excited wax near the electroscope : notice the leaves collapse. Get thick round piece of shellac, gutta- percha, sealing-wax, or vulcanite ; rub portion of end, about two inches, with a flannel cap, having a silk string attached to it ; remove cap by silk string ; test kind of electricity in rod and rubber by means of the positively charged pith-ball of the electric pendulum, and equal quantity in each by placing both together on disc of uncharged gold leaf electroscope : no divergence. 5. Action of Electrified Bodies on each other. It has been previously shown that when a positively or negatively excited or charged body is brought sufficiently near a neutral pith-ball suspended by silk the ball is first attracted, next charged with the same kind of electricity by contact with the body, and lastly repelled. If the ball be charged, and a heavier neutral body be brought near it, the ball will be drawn to the neutral body, and be partially or wholly neutralised by the contact. When two suspended pith -balls are charged from the same excited body, as, for example, glass rubbed with silk, or sealing-wax rubbed with flannel, and are brought near one another, they repel one another ; but, if one be charged from a glass rod rubbed with silk, and the other from sealing- wax rubbed with flannel, they will strongly attract each other. A striking means of illustrating this is to draw two pieces of narrow white silk ribbon between vulcanite tubes placed over the fingers. They will thus be charged positively, and will, when liberated from the tubes, stand widely apart, Strips of thin gutta-percha, drawn between the fingers, are excited negatively, and will repel one another from being similarly charged. If, however, the two. positively charged 24 Text-Book of Frictional Electricity. ribbons be brought near the two negatively excited strips of gutta-percha, strong attraction will take place, and the ribbon and gutta-percha will in each case stick together. These results are always apparent under similar circum- stances an excited body always attracts, or is attracted by a neutral body ; a positively charged body repels a posi- tively charged body ; a negatively charged body repels a body similarly excited ; while bodies positively and nega- tively charged attract one another. The regularity of these actions has led to the establish- ment of the laws, which apply to electrified bodies, and are termed the electrical laws. These laws are the following : 1. Neutral bodies are attracted by, or attract electrified bodies. 2. Like electricities repel. 3. Unlike electricities attract. In these we can trace a similarity to the magnetic laws. 1. Magnetic substances are attracted by, or attract, both poles of a magnet. 2. Like poles repel. 3. Unlike poles attract. Experiments. Excite glass rod with silk ; show influence on neutral pith-ball. Excite sealing-wax with flannel ; test similarly. Charge one pith-ball positively and another negatively ; bring finger near them ; notice the ball is attracted by the hand in both cases ; they will also be attracted by any other neutral body. Charge both balls positively ; show that they repel. Charge both negatively ; exhibit repulsion in this case. Charge one positively and another negatively ; bring them near : the balls attract one another. Place two vulcanised indiarubber tubes over tops of two forefingers ; draw two strips of narrow white silk ribbon between the tubes ; the strips will stand widely apart in consequence of being charged with the same kind of electricity. Each strip is strongly attracted to a neutral body near it, e.g. the wall, human body, desk, &c. Draw two ends of a gutta-percha ribbon through dry fingers ; notice strong repulsion. Conductors. Non-Conductors. Semi-Conductors. 25 Bring excited strips of gutta-percha to excited ribbon : notice effect strong attraction. Show also here the action of a magnet on a compass needle, so as to impress the similarity of action between electricity and magnetism. 6. Conductors and Non-Conductors. All bodies fall under one or other of these divisions. They are either conductors or non-conductors. Conductors are those substances which allow electricity to pass freely and rapidly through or over them. The more readily this takes place, the better conductor a body is con- sidered to be. As the metals convey electricity most freely, they are the best conductors. Conductors enable the elec- tricity which is generated on any portion of them to spread itself over the whole surface of the body, or to be conveyed to an object at a distance from the excited body. Non-conductors check or prevent the passage of elec- tricity through or over them, and cause it to accumulate or remain on the body, or parts of the body, on which it is excited. They insulate it, and in consequence are also termed insulators. Some substances, however, do not conduct electricity very freely, nor insulate it very perfectly ; they thus seem to occupy an intermediate stage between the two, and are named semi-conductors. Semi-conductors are a class of substances which do not allow electricity the free passage of conductors, nor offer the resistance of non-conductors. They afford means for a gradual passage along them, and are in reality imperfect conductors and imperfect insulators. The following sub- stances are familiar examples of each class : CONDUCTORS NON-CONDUCTORS SEMI-CONDUCTORS. (Non-electrics). (Electrics or Insulators). The metals. Fatty oils. Alcohol. Well-burnt charcoal. Chalk. Ether. Graphite. Indiarubber. Glass powder. Acids. Gutta-percha. Dry wood. 26 Text -Book of Frictional Electricity. CONDUCTORS NON-CONDUCTORS SEMI-CONDUCTORS. (Non-electrics). (Electrics or insulators). Saline solutions. Dry paper. Marble. Water. Feathers. Paper. Snow. Hair. Straw. Living vegetables. Silk. Living animals. Glass. Flame. Wax. Smoke. Sulphur. Steam. Resin. Amber. Shellac. Parafine. The two main subdivisions may be illustrated in the following manner. If any conductor, held in the naked hand, be rubbed, electricity is generated, but conveyed along its substance through the hand, arm, and body into the earth, as fast as it is produced ; and no trace of elec- tricity is apparent in the conductor, even when it is tested by the most delicate electroscopes. On the other hand, if a non-conductor be similarly treated, the portion rubbed soon becomes highly charged ; whereas the other end of the body shows no sign of excitation, for the electricity is pre- vented by the nature of the substance from passing along it. From these two opposite effects being apparent in all bodies, they were at first divided into non-electrics and electrics^ the former corresponding to the conductors, and the latter to the non-conductors. Non-electrics were so named because it was thought, at that time, that they could not be electrified. Subsequent investigation, however, has shown that, if such bodies be separated from the earth by means of a non-conductor or insulator, or if they be, as is usually termed, insulated, what were known as non-electrics may be excited. A brass or other metallic rod, held in the bare hand, cannot be electrified, if violently rubbed for a long time ; but if it be fastened to an ebonite, gutta-percha, glass, or other non-conducting handle (fig. 23), or if one end be wrapped Insulation of Conductors. 27 around carefully with sheets of vulcanised caoutchouc, or layers of silk, the metallic substance may soon be excited by rubbing. A conductor, however, although it be insulated, will not become so highly charged as a non-conductor when similarly rubbed, for in the former case the electricity, as soon as it is generated, spreads over the whole surface of the conductor ; whereas in the latter case it is confined to the small portion of the non-conducting substance which has been excited. The charging of a non-electric or conductor may be clearly seen by striking the disc or ball of an electroscope with fur or flannel. Elec- tricity is thus generated and conveyed through the rod, which is insulated in passing through the cork to the leaves. These gradually diverge in consequence r0 Q^ s th of the charge communicated to them, and the kind handle. of electricity may be tested by methods described in p. 21. The human body is also a good conductor ; and, while in contact with the earth, allows the electricity a free pas- sage, and shows no trace of charge, however rapidly it may be generated ; but if the person FlG 23 be placed upon an insulating stool, which is a mahogany board on four glass supports (fig. 23), and then be struck with flannel, fur, or a fox's brush, he becomes so charged that on presenting the hand to the disc of the electroscope the leaves are . , ,-. 11 ^ i . Insulating Stool. violently repelled. On bringing the knuckle near the pith-ball of the electric pendulum the ball is first attracted, then charged, and afterwards repelled by the similar electricity of the insulated body. Or, again, if another person, standing in earth contact, bring his knuckle near any part of the charged insulated person's body, sparks may be drawn from him or her. A sheet of gutta-percha, caoutchouc, ebonite, or vulcanite, or 28 Text- Bo ok of Frictional Electricity. a disc of any non-conducting substance placed on the ground for a boy or girl to stand upon, will serve the same purpose as an insulating stool ; or a dry, warm drawing- board placed on four dry glass tumblers will also do. The best means of exhibiting the difference between the passage of electricity through a conductor and an insulator, is to charge an electroscope, and touch the disc with sub- stances selected from the list of conductors mentioned on pp. 25, 26. These cause the leaves to collapse instantly, showing that the electricity, which previously charged the electroscope, had been conducted through the conductor and the body into the earth. The electroscope must be charged, separately for each of these experiments. If the electroscope be again charged for the testing of the non-con- ductors, and substances be taken from the list and presented to the disc, as in the previous case, no movement whatever is noticeable in the leaves ; they still remain charged, prov- ing that no electricity passes away through the body which touches the electroscope. All substances may be easily classified by submitting them to this test. If it cause the leaves of the electroscope to collapse instantly, we may certainly consider it to be a good conductor ; but if the leaves remain divergent, and show no signs of collapse, the substance is as certainly proved to be an insulator. Should the charge be observed to pass away gradually, not rapidly, as through a conductor, or be apparently checked, as by a non-conductor, the substance should then be included amongst those bodies which are known as semi-conductors. The difference in the passage of electricity for a distance of several feet or yards through a conductor or non-conduc- tor may be strikingly illustrated by using a long thin copper wire, and a string of silk of the same length. Each should be attached in turn to an electroscope by one end, and wound around a glass or ebonite rod by the other. On exciting Insulation. the rod with the rubber, while rod and rubber are in con- tact no movement is apparent in the leaves of the electro- scope in either case ; but, as soon as they are separated the leaves diverge instantly, and widely, when the wire is used ; but they are unaffected when silk is the connecting medium ; thus proving that the wire conducts the electricity, but the silk checks or resists its passage. Care should be taken that pure silk is used, otherwise the leaves will show a gradual divergence, owing to its becoming a semi-conductor to a certain extent by its mixture with cotton. The conductivity of string, when it is wet or dry, may be similarly shown ; and from this the effect of dampness and moisture perceived. The influence of a conductor and a non-conductor may also be contrasted by using the electric pendulum to exhibit the latter, and a pith-ball sus- pended by cotton from a wire, bent similarly to the stand of the pendulum, but in earth contact for the former. The pith-ball of the electric pendulum is first attracted to an excited body, charged, and then repelled ; whereas the ball in earth contact is attracted to the excited body, discharges the whole of the electricity if the body be a conductor, or a portion of it if it be a non-conductor, after which the pith- ball is liberated in its neutral condition, as at first. Insulation. To enable a conductor to become charged with electricity, it must be insulated, that is, it must be separated from the earth by some non-conducting substance. This is generally done by supporting conductors on glass rods, and in order to render the insulation complete, the glass rods should be coated with shellac varnish to prevent moisture being deposited upon them from the atmosphere. On account of the hygroscopic nature of glass, under certain conditions moisture is deposited in a dewy film very rapidly upon it. This soon destroys the insulation, by changing the insulator into a conductor. Varnishing glass rods is a remedy for this. With every precaution, however, a con- ductor does not hold its charge for any considerable time, 3O Text- Bo ok of Frictional Electricity. for the moisture and the tiny dust particles in the atmo- sphere assist in gradually reducing the charge. Some substances, as ebonite, gutta-percha, sealing-wax, &c., insulate better than glass, as they are not so hygroscopic, and require no shellac varnish to be used on them. Conduction. The first experiments on conduction and insulation were performed by Stephen Gray. At first he noticed that a cork stuck in a glass rod attracted light bodies when the rod was excited. He then stuck rods of deal about six inches long into the cork, and these also became electrified on exciting the glass rod. From these he passed to packthread, and found that light bodies were attracted at the end of long lengths of thread. He increased the lengths of the thread, and still the attraction was noticeable. In one experiment he used upwards of 700 feet, and suspended it with thread, but no electricity passed through it. When silk loops were used to support it Brass ball attached . ir,i 1,1 i , , J to excited rod by instead of thread, the electricity passed readily to the end of it. On substituting a thinner wire loop for one of the silk the electricity failed to reach the end of the thread. He then came to the conclusion that the silk prevented the passage of the electricity to the earth, and so confined it to the thread ; but the wire loop, when it was in contact with the thread, permitted the elec- tricity to pass to the earth by that means. He thus discovered that the silk was an insulator, but the wire a conductor. Continuing his experiments, he also examined other substances, and arranged them under two divisions, insula- tors and conductors. Experiments. Rub brass rod held in the hand with fur ; touch disc of the electroscope ; no movement of leaves ; conductor^ non-electric. Rub ebonite rod with fur ; bring it near the electroscope ; leaves diverge ; non-conductor or electric. Experiments with Conductors and Non-Conductors. 31 Charge the electroscope for each experiment with either kind of electricity ; touch the disc with as many substances as possible from the list of conductors ; the leaves collapse immediately the disc is touched ; electricity conducted away to the earth ; bodies conductors. Charge electroscope again for experiments with non-conductors ; touch the disc with as many as possible of those which are given in the list ; no collapse of leaves ; electricity not conducted away from the electroscope ; substances non-conductors. Charge electroscope again, and show gradual discharge, when the disc is touched with the semi-conductors. Rub brass rod with fur ; no movement in leaves when it is brought to the electroscope. Fix insulating handle to brass rod, such as ebonite, gutta-percha, or glass, or wrap one end in sheet-caoutchouc, or in layers of silk ; brass rod may now be excited by rubbing, or striking rapidly with fur. Rub with fur ; touch electroscope ; leaves diverge. Hold electroscope in the hand ; strike the disc a few times with flannel, fur, silk, &c. , notice divergence of the leaves, as the electro- scope becomes charged. Let a lad stand on the ground, and hold his hand just above the disc of the electroscope, then strike him with a fox's brush or with fur, no divergence takes place. Place him on the insulating stool, and strike him similarly ; leaves now diverge freely. Try it also with lad standing on a sheet of gutta-percha or vulcanised caoutchouc. Bring knuckle near electric pendulum ; notice effect. Let another lad present his knuckle to him, and take a spark. Fasten long thin copper wire to electroscope ; wrap other end around ebonite rod ; rub rod, and notice condition of leaves, as rubber remains in contact with rod ; separate rubber from rod ; strong diver- gence. Attach silk string similarly, after excitation, and separation of rubber from rod ; no divergence of the leaves whatever. Test dry string and wet string similarly, and notice effects. Show the need of insulation for a conductor. Place a conductor on a brass support, no charge can be imparted to it. Test again, when supported on a clean glass rod ; the conductor is charged, except under very favourable conditions, with difficulty. Place conductor on glass rod coated with shellac varnish, charged easily. Notice this. Explain and understand the cause of these dif- ferent effects. Push cork into the end of a glass tube ; excite tube ; electricity also charges the cdrk ; show it by repelling similarly charged pith-ball, and by its action on the leaves of the electroscope. Text-Book of Frictional Electricity. Stick rod of deal about 6 in. long into the cork ; rub tube again ; show repulsion of pith-ball, and divergence of the leaves, as before, when wood is brought near them. Suspend small brass ball at the end of a wire, and again at the end of a silk string, and show their effect on an electroscope. Attach long length of thin copper wire to elec- troscope and rod. Excite rod, leaves diverge, touch wire in any place with conductor ; notice collapse of leaves ; touch with insulator, leaves unaffected. 7. Effect of connecting a Conductor with the Earth. In the last division of the subject it has been shown that, in order that a conducting substance may be electrified, it must not be in contact, or in conducting connection with the earth, but must be insulated. That a conductor also may retain the charge which it receives from the action of an electrified body, it must be kept from earth connection by insulation. The accompanying diagrams will make this clear. Let the positively excited rod, A, be brought in succession near the conductors B and c, one of which (fig. 25) is sup- FIG. 25. FIG. 26. Conductor on brass support. Conductor on glass connected with earth by wire. ported on a rod of brass, and the other (fig. 26) on a rod of varnished glass, but connected with the earth by means of a wire. Both of the conductors are thus in earth contact. Now, as soon as the electrified body is brought near enough to act upon the conductor, the first effect produced is to decompose a certain amount of neutral electricity in the conductor ; negative, the opposite kind of electricity, is- Action on Insulated Conductors. 33 attracted towards the rod, and positive, the similar kind, is repelled to the earth. That the number of electrical units may remain constant in the conductor, the same quantity of negative is considered to pass from the earth to the con- ductor, as positive to leave the conductor for the earth. On removing the rod, the repelled positive returns from the earth, and the equal amount of negative passes at the same instant to the earth, thus again producing the neutral condition of the conductor as it was before its disturbance by the presence of the electrified body. Should the conductor be perfectly insulated by being fixed on some non-conducting support, such as ebonite, gutta-percha, sealing-wax, or glass FIG. 27 . coated with shellac varnish, the effect of the approach of an elec- trified body towards it is simply to decompose and rearrange the two electricities, as shown in the diagram (fig. 27). On the ex- cited rod A approaching B, its first effect, as before explained, is to decompose the two electricities, and, in accordance with the electrical laws the negative is drawn towards A, while the positive is repelled from it. As there is now . ^w FIG. 28. earth Insulated Conducto. no con- nection, the inter- change between the conductor and the earth cannot take place as be- fore. The repel- led positive, how- ever, is in SUCh a Decomposed Conductor touched with the hand. condition that it will exchange for an equal quantity of negative (fig. 28) if any means be provided for conducting 34 Text- Book of Frictional Electricity. connection with the earth or any conductor larger than itself. The positive is liberated by the presence 01 the positively excited rod, and is thus rendered free to pass to the earth or other large conductor ; while the negative is held captive by the attractive influence of the rod, and is unable to move from the part of the conductor, where it is produced, until the rod be withdrawn or discharged. When in this condition it is termed bound. It may here be stated that the electrical action which takes place between an excited body and a conductor, when they are not in contact with each other, is named Induction, and that the decomposed electricity of a conductor, when under the induc- tive influence of an electrified body, is either free or bound. Free Electricity. Electricity is termed free, when it is so far liberated from the neutral condition in which it pre- viously existed, that it is enabled to exhibit its attractive and repelling influences on outside bodies, or to pass to the earth or other conductor, if a suitable conducting means be provided for its passage. The charge excited on a rod by friction is free electricity ; so is that which causes the leaves of the electroscope to diverge, and the electricity which passes to the earth from a conductor through the inductive action of an electrified body, so also is that which is apparent at the further extremity of an insulated conductor when under induction, and the charge of the prime conductor of the electrical machine. Bound Electricity. Electricity is considered bound when, after it is separated from its neutral condition, it is held cap- tive or paralysed by the action of some electrified body near it, and is thereby rendered powerless to exert its influence on outside bodies. Under certain conditions, on the with- drawal of the electrified body, the liberated electricity charges the conductor, and is immediately transformed from bound to free ; for then it is capable of affecting other charged or neutral bodies. Specific Inductive Capacity. 35 The electricities of both rod and rubber are bound, after they are rubbed together, until separation takes place. The electricity on the disc of the electroscope is bound while it is under induction, and will not pass away although the disc be brought into earth contact. That also which is produced on the end of the conductor near the inducing body is also bound ; and so are the electricities on each side of a con- denser or Leyden jar. Bound electricity is always of the opposite character to that which retains it. Specific Inductive Capacity. It was established by Pro- fessor Faraday, that for an excited body to act upon a con- ductor by induction, some substance must exist between the two through which the electricity may be imparted. In our examples on induction, air is understood to be present. Some substances, as, for example, varnished glass, sealing- wax, ebonite, &c., allow this action to take place at the same distances much more freely than air. Those sub- stances which enable electricity to act most strongly through them are said to have the highest specific inductive capacity. Dry air is superior to moist air in this respect. A conductor may be charged either positively or negatively similar to the method of charging the electroscope, pp. 19, 20. Experiments. Place conductor on brass rod ; test it with electro- scope ; no action on leaves. Bring positively excited rod near con- ductor ; withdraw it ; test conductor again with electroscope : no charge. Support conductor on insulating rod of varnished glass, ebonite, or other non-conducting substance ; attach thin copper wire to the end of the conductor and the disc of the electroscope. Bring excited rod near the conductor ; notice the divergence of the leaves of the electroscope as the rod approaches. Withdraw rod ; notice gradual collapse of the leaves as the distance increases. Refer to similar action taking place in all bodies under induction, although in most cases invisible. Repeat the last experiment, and while the leaves are divergent remove the wire with some insulating substance, such as vulcanite tubes over fingers ; test the character of the charge ; show it to be positive or negative according as the leaves become more divergent by a glass rod rubbed with silk, or stick of sealing-wax excited with flannel. Let pith-ball rest against the end of the conductor away from the D 2 36 Text -Book of Frict zonal Electricity. portion next the rod ; bring excited rod near conductor ; notice the repulsion of the pith-ball, being charged with the free electricity of the conductor. Withdraw rod, electricities recombine ; conductor neu- tralised, ball also neutralised, and is attracted towards conductor, after which it hangs vertically as at first. Repeat a portion of this experi- ment, and while the pith-ball is repelled by the free electricity, remove it from the influence of the conductor, and test its character, as in the previous example. Bring glass rod rubbed with silk to insulated con- ductor ; touch the conductor in any part with the finger while it is under induction, remove the hand, and then withdraw the exciting rod ; the conductor will remain charged with the opposite electricity from that of the exciting body. Test this by showing the repulsion of the pith-ball of the electric pendulum, charged negatively ; take a portion of it with a proof plane, and test with an electroscope charged nega- tively ; leaves diverge further same electricity. 8, Experiments illustrating the relation between the Primary Electrical Charge and the Correlatively Induced Charge. As explained in the last division of the subject, an ex- cited or charged body acts inductively on a neutral body, and the successive steps of that action are : 1. The decomposition of the neutral fluids. 2. The rearrangement of the two electricities ; the opposite kind being attracted towards, and the same kind repelled from the electrified body. Should the charged body be withdrawn, the two elec- tricities recombine, and the conductor returns again to its neutral condition. This may be plainly shown by the action of a charged body on the gold-leaf electroscope. As the body approaches the electroscope the leaves gradually diverge, but collapse again as it is withdrawn. A similar change in the condition of the electricity of the body must be imagined under all cases of induction. We will examine first the action of a positively excited rod on the pith-ball of the electric pendulum (fig. 29). As the rod approaches the pith-ball, near enough to affect it, the first influence is to decompose the neutral electricities of the Illustrations of Inductors. 37 ball, and the next is to rearrange them ; negative electricity is attracted to the part of the ball nearest the rod, and positive is repelled to the part away from it. Now, from the action of the law of inverse squares, the attraction is greater between the positive rod and FlG 29 the negative part of the ball, than the repulsion between the rod and the positive part, and therefore the ball is attracted towards and touches the rod. It then gives a portion of its negative to, and receives an equal amount of positive from the rod, and thus becomes charged with a certain quantity of free positive. The ball is n j r . i_ j j ji Action on Neutral Pith-Ball. now repelled from the rod, and this repulsion continues until the free positive is discharged, either by the moisture of the atmosphere, the dust particles in the air, or by touching some body which permits its discharge. This is what is known as charging by contact. An electroscope is similarly acted upon by an excited body, as may be understood from the accompanying dia- gram (fig. 30). If the rod be brought into contact with the disc or ball, equal quantities of negative pass from the electroscope to FIG. 30. FIG. 31. Action on Gold Leaf Electroscope. Inductive action from Proof Plane. the rod and positive from the rod to the electroscope. The electroscope thus becomes charged with a greater quantity 38 Text-Book of Frictional Electricity. of free positive, and the leaves diverge further in conse- quence. With a highly charged rod this is a dangerous ex- periment to attempt, as it is likely to break the leaves, particularly if the electroscope be a sensitive one. To avoid this, the proof plane is used, and by means of it small charges may be removed from the surface of the charged body to be tested by the electroscope (fig. 31). When this is done, the influence on the electroscope is similar to that exerted by the rod, but the leaves are much less violently disturbed. Faraday's Ice-pail Experiments. The character and quantity of the induced electricity may be clearly seen and easily detected by what is known as Faraday's Ice-pail Experiment. The ordinary ice-pail is FIG. 32. made of pewter, and is about seven inches in diameter and ten inches deep (fig. 32). This must be insulated, and connected on the outside by means of a copper wire with a gold-leaf electroscope. On bringing a posi- tively charged brass ball, held by a long silk string, into the interior of the ice-pail, negative electricity is induced on the inner One insulated ice-pail, and positive on the outer surface of the vessel. The free positive of the exterior of the pail causes the leaves of the electroscope to diverge gradually as the ball is lowered into it, until it has sunk about three inches into the vessel, after which no further divergence takes place. If now the ball be lowered, so as to touch the bottom of the pail, and then be withdrawn, the ball will have become neutralised by interchanging with the negative of the pail, but the leaves of the electroscope will not diverge further ; thus proving that the charge, produced on the outside of the pail by induction from the charged ball, was equal to that which it received by contact with it. Faraday varied the experiment by using four ice-pails, Faraday's Ice-pail Experiments. 39 placed in one another (fig. 33), but separated from each other by layers of shellac, thus securing the necessary insu- lation. The outside of the outer pail was again insulated, and connected by a thin copper wire with a gold-leaf electro- scope. When the charged ball was brought into the inner pail of the series, the leaves of the electroscope diverged in the same manner as they would have done if only one pail were used. This arose from the inductive action of the inner pail on the one adjacent to it, that one on the next, &c., until the free positive of the outer vessel through the wire connection charged the electroscope, and caused the leaves to diverge. If now the ball be withdrawn, the leaves collapse, showing that the decomposed electricities in the pails had again recombined, and the series returned once more to the neutral condition. If, however, before withdrawal, the ball be allowed to touch the inner pail, it is charged with the same kind of electricity as the charge of the ball, and, as in the previous case, the ball is neutralised, but the leaves show no further divergence. If now the inner positively charged pail be removed by some insulating substance, the remaining pails are neutralised, and the leaves immediately fall to- FIG. 33. gether, but diverge again on its being replaced. If the series be connected in order from the outer pail, as i and 2, 2 and 3, 3 and 4, or i, 2, 3, 4, no alteration in the leaves of the electro- scope will be apparent. Again if, while the pails are under the inductive action of the charged Four insulated ice-pails, ball, the inner pail, 4, be connected with the earth, the free positive escapes, and the remaining pails at once return to their neutral condition, which is seen by the collapse of the leaves. The negative of the inner pail is unable to disturb the neutral condition of the other pails, because it is held bound by the presence of the positively charged ball. Text-Book of Frictional Electricity. Should 3 be connected with the earth, 2 and i immediately become neutralised; and, lastly, if 2 be placed in earth connection, i is restored to the neutral condition. Action on Conductors. A cylindrical conductor may be used to exhibit the in- fluence of an excited body upon it by attaching a small elec- FIG trie pendulum to each ^ end of it. Suppose a ""*% positively charged rod be brought near the con- ductor, the pith-balls on the ends immediately rise the one nearest the rod charged with nega- tive electricity, and the one away from it charged with positive (fig. 34). If the rod be withdrawn, the pith balls instantly fall, and the conductor is again neutralised. Should a negatively charged rod be used, the pith balls also rise, but in this case charged with opposite electricities to the former example (fig. 35). On the withdrawal of the rod the two elec- tricities again recombine. The kind of electricity charging the pith-balls in each case is shown by the repulsion, which is notice- Effect of the presence of a Negatively able when the Y are tested Excited Rod. by a glass rod rubbed with silk and a sealing-wax rod rubbed with flannel. When two equal brass or tin-foil-covered wooden spheres are in contact and acted upon inductively by an excited body, one ball becomes charged positively and the other negatively Effect of the presence of a Positively Excited Rod. FIG. 35. Inductive Action on Conductors. the nearer one with the opposite kind and the further with the same kind of electricity as that of the inducing body. These two charges are nearly equal, but not exactly so, as the intensity is modified by the 'law of inverse squares,' which is applicable to electricity as to magnetism (see ' Mag- netism/ p. 22). The ball next the inducing body would have a slightly higher charge of the opposite kind of elec- tricity to the other. If they be separated while they are under the inductive influence of the charged body each will FIG. 36. FIG. 37. Inductive Action on two Balls in contact. Separation of Balls while under Induction. retain for a time its own particular charge, which may be tested by means of the electroscope, and the character and quantity of each easily exhibited. If, however, when in this condition, they be brought very FIG. 38. FIG. 39. Action of Conductors on each other when under the influence of a Charged Body. near or into contact, they immediately neutralise one ano ther by the interchange of electricity from ball to ball. When two or more insulated conductors are placed near Text-Book of Frictional Electricity. one another (figs. 38, 39), the decomposed electricity of the first influences the next, and this the next, &c., similarly to the action of a magnet on neighbouring pieces of soft iron. (See Magnetism, p. 17). The Electrophoms. A most important piece of apparatus for exhibiting the induced charge obtained from the action of the primary charge is the electrophorus. This consists of two circular plates, one of which is about 2 inches less in diameter than the other. The larger forms the base, or portion of the FIG. 40. FIG. 41. COLLECTING EoccitCi Cahe GENERATING CAKEl") apparatus to be excited, and must be of some insulating substance generally a cake of resin or ebonite contained in a tin dish which is named the ' sole.' This forms the FIG. 42. FIG. 43. generating plate. The smaller is the conductor, and must be of metal. It is generally made of brass or tin. This is known as the collecting plate. To this plate an insulating handle is attached, and the apparatus is complete. Charging Electrophorus. 43 Method of Charging. To charge the electrophorus the cake must first be excited. This is usually done by warm- ing its surface and then striking it smartly two or three times, or rubbing it FlG 44 briskly with fur or flannel, which causes it to be electrified negatively. The con- ductor, held by the insulating handle, is now lowered gradually upon it. As this ap- proaches, the electri- city of the base acts inductively on the conductor, decomposes the neutral electricity of the metal plate, and attracts positive electricity to the lower face, but repels negative to the upper. Should the conductor be withdrawn again without being touched with the hand, or otherwise brought into earth contact, the two electricities will recombine, and the neutral condition be again produced. But if, while the conductor rests upon the excited cake and is under the inductive action of it, the upper surface of the conductor be touched by the finger, the free negative passes to the earth, and an equal quantity of positive enters the disc from the earth. The hand should then be removed, so as to break the earth connection, and the conductor be lifted by the insulating handle. It will then be found to be highly charged with free positive electricity, and if the knuckle be brought to the edge of the disc a spark will be seen, and a feeble crack heard, which is the result of positive passing from the disc to the knuckle, and negative at the same time passing from the knuckle to the disc. If a pith- ball, or a number of pith-balls, be laid on the conductor before it is charged, on charging it as described and lifting it from the cake, the pith-balls are immediately and violently 44 Text-Book of Frictional Electricity. repelled, being charged with the same kind of electricity as the conductor. The kind of electricity charging the disc may be tested by bringing it near an electric pendulum or gold- leaf electroscope charged positively. The free positive of the conductor may also by means of several charges be employed to charge a Leyden jar. These successive charges may be obtained from the same excited cake, for as the charging is by induction and not by contact, little electricity is lost by the cake when the disc is charged. On this point, Professor Guthrie writes : ' Scarcely any actual interchange between the + of the brass and the of the cake takes place, be- cause the two surfaces only touch one another in a few points, and the resinous substance, being an exceedingly bad conductor, does not permit the recharging of the discharged points.' To avoid the necessity of touching the collecting plate of the electrophorus every time it is required to charge it, a brass pin, or a series of pins, may be brought from the sole through the cake to its surface, so as to be in contact with the upper disc when it is placed upon the excited cake. In this way the free negative escapes from the collecting plate to the earth through the pin or pins, and an equal quantity of free positive passes by the same means from the earth to it. Sometimes a strip of tin-foil is brought from the sole over the cake to meet the edge of the upper disc, and this offers a similar passage for interchange with the earth as the pin arrangement does. In this way a rapid succession of charges may be communicated to, and sparks obtained from the collecting plate of an electrophorus, and a Leyden jar be charged most quickly from it. Experiments. Excite glass rod with silk ; bring it to the disc or knob of the electroscope ; notice divergence of the leaves. Withdraw it ; the leaves collapse. Excite sealing-wax with flannel ; test similarly : similar results. Test electric pendulum with excited glass and excited wax ; notice movements of pith-ball in each case. Experiments on Induction. 45 Test electroscope with highly-excited rod : the leaves diverge when the rod is some distance away. Draw a proof-plane over excited rod ; bring it near to, or on disc or ball of electroscope : similar action apparent. Obtain metallic vessel 7 inches in diameter and 10 inches deep. Insulate it. Attach wire to outside of the vessel and to the disc of electroscope. Notice divergence as positively charged ball is lowered ; withdraw ball : leaves collapse. Lower ball again, and let it touch the bottom of the vessel : no increase in divergence apparent after the ball has sunk below 3 inches. If possible, test four similar vessels, but of slightly different sizes, and insulated from one another and the earth. Charge the inner one posi- tively : the leaves of the electroscope diverge with positive electricity. Remove this charged inner vessel by means of silk handles, which should be fastened to it with sealing-wax : leaves collapse ; other vessels neutral- ised. Connect in succession, while under inductive influence of charged body, 4, 3, 2, i with the earth : the leaves collapse due to free electricity from each interchanging with earth and neutralising outer vessels. Fix a small electric pendulum at each end of a conductor ; bring excited rod near : the pith balls are repelled. Test kind of electricity of each ball by using positively and negatively charged rods ; notice re- pulsion : similar electricity. Bring two equal spherical balls into contact ; let an excited rod be brought near them, so as to decompose the electricity. Remove the further ball, then withdraw the rod : the two balls will be charged with opposite electricities ; that further away has similar electricity to the excited rod, and that near the opposite kind. Test this by means of proof-plane and electroscope. Charge balls again in this way, then bring them very near to one another : a feeble spark will be visible, and the balls become neutralised. Excite cake of electrophorus by rubbing with fur or flannel, or striking with a fox's brush ; bring conductor on excited cake, and with- draw it again without having brought it into earth contact ; test with electroscope : no trace of electricity. Bring conductor again on excited cake ; now touch it with finger, remove finger, and lift conductor by insulating handle ; present knuckle to conductor ; spark passes. Charge conductor again similarly, bring disc towards the electro- scope ; notice divergence of the leaves when it is some distance away. Charge electroscope with positive electricity ; the leaves diverge further as the disc is brought near it. Charge another electroscope 46 Text-Book of Frictional Electricity. negatively : the leaves collapse gradually as the disc approaches, The proof-plane should be used if the charge is very high. Fasten thin copper wire to the disc of the electrophorus, and attach the other end to the electroscope ; notice movement of leaves as the disc is brought on. the excited cake ; collapse as disc is touched, and strong divergence as disc is lifted from cake. Bend tin-foil from sole over edge of cake to meet disc ; excite cake, and obtain a succession of charges from excited cake ; charge Leyden jar by means of a series of sparks from charged disc. 9, Frictional Electrical Machines. There are two kinds of frictional electrical machines the cylinder machine and the plate machine. The essential parts of each are the rubber, the cylinder or plate on which the electricity is excited, and the prime conductor on which it is accumulated. In these respects they resemble the electrophorus, which is the first form of electrical machine. The Cylinder Machine was first developed by, and its present form is due to, Gordon ; while the ordinary plate ma- chine owes its origin some say to Planta, others to Van Marum. A later variety of plate machine was developed by Winter. The Cylinder Machine. The rubber of the machine consists of a long pad of horsehair covered with soft leather about three-quarters of the length of the cylinder. Before use it is smeared with bisulphide of tin, or an amalgam con- sisting of one part of zinc, one part of tin, and two parts ol mercury, laid on with lard, to develop the electricity more freely on the surface of the cylinder or plate. The rubber may be insulated by being fixed on a glass support coated with shellac varnish, or it may be fastened to a wooden stand, and thus be in earth connection. If the rubber be insulated, in order that the prime conductor may be charged, a wire or chain must pass from it to some conducting connection with the earth, as e.g. the ground, gas-fittings, &c., or the chain may be held in the hand of a person standing on the ground. A flap of silk extends from the rubber almost to the prime conductor to prevent the electricity escaping into the The Cylinder Machine. 47 atmosphere in its passage from one to the other. Sometimes a brass conductor is fixed at the back of the rubber for the accumulation of negative electricity. The Cylinder is of different sizes, but in all cases it is similarly blown, so that there may be projections at each end. Securely fastened to these by means of cement are wooden caps which carry the axles, and rest in two wooden supports which are firmly fixed into the wooden base. One axle is longer than the other, and to this longer one the handle for turning the cylinder is fixed. The Prime Conductor consists of a hollow cylinder of brass or tin, or it may be made of wood coated smoothly with tin-foil. It is placed about the same height as the FIG. 46. Comb or Rake. Cylinder Machine. rubber, but on the opposite side of the cylinder, and is sup- ported on a glass rod coated with shellac varnish. A row of points projects from the portion of the prime conductor facing the cylinder. These are sometimes termed the comb or rake, and are about inch long, and brought within about Jr inch from the surface of the cylinder. Both rubber and prime conductor are regulated by a foot, which moves in a groove and is secured by a screw. A knob is generally fixed to the prime conductor, when it is long in proportion to the width. Great care should be taken that no points or sharp edges form a portion of the prime 48 Text-Book of Frictional Electricity. conductor, except those turned towards the cylinder, other- wise the electricity will disappear as rapidly as it is produced. Charging the Cylinder Machine. This machine may be charged either positively or negatively. Its usual charge is positive, which is accumulated on the prime conductor ; but when it is charged negatively the negative is accumulated on a conductor, which is insulated and attached to the rubber. Positive Charge. To secure this the rubber must first be put into earth contact, and the prime conductor be insu- lated. On turning the cylinder the glass becomes charged positively by the friction with the rubber, and the latter negatively, in accordance with the theory adopted in explain- ing the charging of the glass rod with the silk rubber (p. 10). The rubber, however, being in earth contact, is enabled to interchange a certain amount of free negative electricity with the neutral of the earth, and thus to keep its units of elec- tricity constant, and to decompose a larger quantity of the electricity of the cylinder. The excited cylinder passes from the rubber, charged with positive electricity, towards the prime conductor, the flap of oiled silk preventing its being acted upon by the atmosphere. As it approaches the prime FIG. 47 . conductor in a high state of positive charge, it acts by in- duction on the prime conductor, attracting the negative but re- pelling the positive. The row of points of the prime conduc- tor, facing the cylinder, permits negative to rush out against the cylinder and positive to pass in Effect of turning Cylinder. f rOm it, thus SCCUHng equal Units of electricity in conductor and cylinder, but in an altered condition. The prime conductor, which was previously neutral, becomes rapidly and highly charged with positive, while the cylinder is as rapidly neutralised. It continues in this neutral state as it passes from the prime conductor to Charging Prime Conductor. 49 the rubber. Here the electricities are again decomposed, and the cylinder receives another positive charge. This is again neutralised on reaching the prime conductor, and thus regularly and uniformly do the charging and discharging ot the cylinder proceed. From this explanation it will be understood that, when the cylinder machine is at work, the part between the rubber and the prime conductor is posi- tively charged, and the portion between the prime conductor and rubber is in the neutral condition (fig. 47). Limit of the charge of the Prime Conductor. The positive of the prime conductor will only acquire a certain tension, which depends on the condition of the atmosphere ; and, when that point is reached, it will inter- change with the adjacent air as rapidly as it receives an additional charge. The presence of dust particles in the air also tends to reduce the tension. It is also very much affected by the condition of the machine itself. If it be cold, little effect will be apparent from the action of the excited cylinder on the prime conductor ; but, if the machine be well warmed, on turning the cylinder, the electricity of the prime conductor rapidly acquires high tension. To retain a uniformly high charge the cylinder must continue to turn, otherwise a more or less rapid discharge will take place into the surrounding atmosphere. This may be plainly shown by attaching a Henley's Quadrant Electrometer to the prime conductor, and noticing the rise and fall of the pith-ball under different conditions. Negative Charge. To obtain a negative charge the rubber must be insulated^ and the prime conductor put into con- ducting earth connection. It is also necessary, if a large quantity of negative electricity is to be accumulated, that a cylindrical brass conductor should be attached to the rubber. When the cylinder is turned, as before described, posi- tive electricity is generated on the glass, and negative on E 50 Text- Book of Frictional Electricity. the rubber. The free negative is unable to interchange with positive from the earth, and so it accumulates with increasing tension about the rubber, or the conductor attached to it. The cylinder then passes to the prime con- ductor highly charged with positive. Here by induction through the points, and by means of the earth connection, there is an interchange of electricity, positive escapes from the cylinder to the earth, and negative passes from the earth to the cylinder. This neutralises the positive charge of the cylinder, which again passes to the rubber to yield up an equal quantity of negative for a similar amount of positive as at first. The amount of charge may be ascertained, as in the previous case, by attaching a Quadrant Electrometer to the conductor at the back of the rubber. The tension is depen- dent on the state of the atmosphere as regards moisture at the time of the experiment Methods of showing Tension. The tension of the electricity of the prime conductor, or the force with which a discharge takes place towards an object near it, is easily determined by bringing the knuckle of the finger to the prime conductor. If the tension be high, the spark will be long, loud, sharp, and stinging ; but, if it be low, the spark, if any, will be feeble in all respects, and scarcely perceptible. A rounded knob at the end of a short rod of brass is the best means for testing this ; for then the operator does not feel the pricking or stinging of the high tension sparks, as they strike the knob held in the hand. The tips of the fingers when presented to the conductor reduce the tension of the discharge ; and if a sharp point be used no spark can be drawn from the prime conductor, and the pith -ball of the Electrometer hangs vertically, as if no charge had been communicated to it. The sharper the point, the more successful is this experiment. Henley 's Quadrant Electrometer. 51 Henley's Quadrant Electrometer. This is an apparatus which is used for exhibiting the tension of the electricity of a charged body. It consists of a brass rod about 6 inches long, arranged at the bottom to fit into small holes, which are sometimes left in electrical apparatus for that purpose, while the top terminates in a small knob (fig. 48). To this rod is attached a semicircle of ivory, wood, or cardboard, graduated into degrees ; and from its centre is suspended a light wooden rod, having a small FlG 48 pith-ball attached to the end of it. In its ordinary condition the pith-ball hangs verti- cally downwards. When the brass rod of the Electrometer is attached to an electrified con- ductor, it immediately becomes charged with similar electricity. The pith-ball, resting against the rod, is similarly charged, and im- mediately repelled. The force of repulsion is measured by the number of degrees, shown on the semicircle by the rod to which the pith- ball is attached, and this angle is an indication Henle , S Q drant of the tension of the free electricity in the body. Electrometer. The gradual loss of electricity into the atmosphere, after the prime conductor has been charged, is clearly seen by placing an Electrometer on the conductor, charging it with electricity of high tension, and then watching the gradual fall of the pith-ball, as the charge is being reduced. The effect of presenting the knuckle, or a point held in the hand, to a highly charged conductor is also clearly seen by the sudden fall of the pith-ball to its vertical position. The Plate Machine. The Plate Machine has the same essential parts as the cylinder, the only difference being that the cylinder is re- placed by a circular glass plate having a thickness of about 52 Text-Book of Frictional Electricity. \ in. The rubbers are securely fixed on both sides of the plate over each other, one pair at the bottom, and the other at the top of it ; and the branches of the prime conductor carry - FIG ing the points or comb, sometimes termed the rake, are situated midway be- tween the rubbers. In some machines there is a row of points on each side of the plate ; but in others only one row against the side of the plate nearest the prime conductor. From the rubbers above and below on both sides of the plate, oiled-silk flaps extend almost to the points of the prime conductor, to retain the positive charge as in the cylinder machine. The prime conductor is insu- lated, and the rubbers are in suitable earth connection by means of the wooden supports. Positive electricity is thus easily and rapidly developed on the prime conductor. For experimental purposes the plate machine is better than the cylinder. An ebonite plate is sometimes used instead of a glass plate, and then the prime conductor is charged with negative electricity. A metal plate might be used instead of either, provided it be well insulated from the spindle, but as the generated electricity would spread over the whole surface of the plate, for the metal is a good conductor, instead of remaining on the portion of it where it was excited, the electricity from such an arrangement would have very low tension. Method of Charging. This is exactly similar to the charging of the cylinder, and need not be repeated here, if that be understood, see Winter's Machine. p. 48. The friction of the rubbers and plate decomposes the electricities of each, and the plate becomes charged positively. This positively excited plate causes positive to accumu- late on the prime conductor. The positive charge can only acquire a certain tension, for the atmosphere acts upon it in such a way that when that tension is reached, the air dis- charges as fast as the machine excites it. A machine thoroughly insulated, as, for example, placed on a table with a glass top, may have its prime conductor easily charged by allowing a sharp point, or a series of sharp points, to project from the back of the rubber. The free negative of the rubber is thus enabled to escape into the atmosphere, and an equal quantity of positive to enter through the points from the atmosphere, thus securing the neutral condition of the rubber which is necessary to continue the excitement of the plate. Winter's Machine. The Winter Machine is a modification of the Plate Machine, and was developed by Winter, of Vienna. It differs from it in having only one pair of rub- bers instead of two, and the prime conductor spherical in form instead of cylindrical. To this spherical conductor two rings are attached which are placed on each side of the plate. The portion of the rings towards the plate is grooved, and lined with metal, from which a series of points extend towards the plate, which assist in charging the prime conductor as in the other cases, or a ring of thin metal is fixed in the middle of the groove, and has its edge turned towards the plate. A ring, having a diameter of 3 or 4 feet, is sometimes fixed FIG. 50. Winter's Machine. 54 Text-Book of Frictional Electricity. into the top of the sphere to increase the surface of the prime conductor (fig. 50). This enables much longer sparks to be obtained from this machine than from the ordinary ones. The exterior of the ring is of polished mahogany, which covers a very stout brass wire. It is necessary that the ring and its connection should be perfectly smooth in all its parts, otherwise the electricity will escape, and the tension become rapidly reduced. This machine is charged similarly to the others, and like them may be charged either positively or negatively by connecting the rubber or the conductor with the earth, and insulating the part required to be charged. Theories of Charging the Prime Conductor. The theory which has been given to account for the charging of the prime conductor of the Electrical Machine is what is termed ( The Addition and Subtraction Theory j and is the most satisfactory explanation if the ' Two Fluid Theory ' of electricity be adopted. Two other theories have been advanced, both of which are worthy of attention, they are : a. The Addition Theory. b. The Subtraction Theory. a. The Addition Theory supposes that the excess 01 positive, which charges the prime conductor, is due to the passage of electricity from the cylinder or plate through the points to the conductor. It thus receives more electricity than it would have in its neutral condition ; and is said to be positively charged. This explanation is most consistent with the ' One Fluid Theory ' of electricity. b. The Subtraction Theory, The subtraction theory assumes, that, when the positively excited cylinder or plate approaches the prime conductor, it acts upon it inductively, attracting the negative, but repelling the positive. The position of the points, facing the cylinder or plate, permits the rapid and continuous* discharge of negative electricity Experiments with Electrical Machine. 55 from the prime conductor to the excited glass. This is sufficient in quantity to neutralise it. The positive electricity, remaining in the prime conductor, as the negative is being withdrawn, increases in tension, until it becomes so great as to interchange with the adjacent atmosphere, and so main- tain the conductor uniformly charged. The supporters of this theory contend that no positive enters the prime con- ductor, but that it is entirely charged by the withdrawal of the negative through the points. The conductor would thus be always positive, and the rake turned towards the glass negative. Experiments with the Electrical Machine. Well warm and wipe the machine with a clean warm duster before use. Connect the rubber with the earth, and, after having regulated the pressure of the rubber against the cylinder or plate, turn the handle. Bring the knuckle of the finger to the prime conductor of the machine notice spark. This is long, sharp, and stinging, when the weather is. dry and frosty ; but feeble when the air is damp. Hold a brass rod terminating in a knob to the charged prime con- ductor, better sparks are produced. Present a point towards it, and no spark will be noticeable. Fix Henley's Quadrant Electrometer to the prime conductor ; notice rise of pith-ball, as the machine is turned. Take spark from prime conductor with knuckle or knob at the end of brass rod, the pith-ball falls. Present a point to prime conductor, and turn machine, the pith- ball remains at rest ; remove the point away from the influence of the conductor, the pith -ball rises ; bring the point again towards it, the ball immediately falls. Charge the prime conductor, and notice the height to which the pith-ball rises. Cease turning the machine, and mark the gradual fall of the pith-ball, without any visible influence affecting it. The quick- ness of the fall indicates the invisible influences which are tending to reduce the tension of the prime conductor. Make a tassel of tissue paper, and fasten it to short rod of deal ; attach it to the prime conductor, the strips will repel one another, as the prime conductor becomes charged. Hold a pointed metallic rod in the hand, cover the point with the fore-finger, and present it to repelled strips notice effect ; remove the finger contrast result. A head of hair is sometimes used in a similar way. Text- Book of Frictional Electricity. Attach a point to prime conductor, and turn the machine : no spark can be obtained. Fix Electrometer on prime conductor, there will be no rise of the pith-ball, while the point is attached. Fasten tassel as before, there will be little or no repulsion of the strips. This arises from the reduction of the tension of the electricity of the prime con- ductor by the rapid interchange of electricity through the point. Various other experiments may be performed with the Electrical Machine, if the necessary apparatus be at hand such as : The Electrical Whirl. This consists of an arrangement similar to that represented in the diagram fig. 51. Four pieces of brass wire are FIG. 51. bent at right angles, and, terminating at #, b> c, d^ in sharp points, are fixed at right angles to one another in a circular piece of brass, e. This central portion has a hole in the middle of its lower face to rest on a pivot, that it may move with the slightest amount of friction. The support for this whirl is a short brass rod, ending in a sharp point. On turning the machine the whirl rotates in a direction opposite to that to which the points are turned. The speed of rotation is greatest when the tension is highest, and least when it is lowest. Electrical Inclined Plane. Another striking experiment to illustrate the effect of points is afforded by what is known as the Electrical Inclined Plane. A rectangular wooden base has four glass rods fixed into it, two at the front, and two at the back. The front rods are shorter than the back ones, as represented in the diagram fig. 52. To the top of each rod a brass ball is attached, and two parallel wires, fastened to the knobs, extend from each front to each back rod. Two thin wires, #, />, <:,