duce electrification in that conductor. This method of producing electrification is called electrostatic induction.
As soon as the charged rod is removed the leaves will collapse, indicating that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.
Nature of the Induced Charge.--This is shown by the experiment illustrated in fig. 17.16
Let a metal ball A be charged by rubbing it with a charged rod, and let it then be brought near an insulated metal cylinder B which is provided with pith balls on strips of paper C, D, E, as shown.
The divergence of C and E will show that the ends of B have received electrical charges because of the presence of A, while the failure of D to diverge will show that the middle of B is uncharged. Further, the rod which charged A will be found to repel C but to attract E.
Fig. 17.--Experiment illustrating the nature of an induced charge. The apparatus consists of a metal ball and cylinder, both mounted on insulated stands, pith balls being placed on the cylinder at points C, D, and E.
From these experiments, the conclusion is that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of opposite sign.
The Electrophorus.--This is a simple and ingenious instrument, invented by Volta in 1775 for the purpose of procuring, by the principle of induction, an unlimited number of charges of electricity from one single charge.17
It consists of two parts, as shown in fig. 19, a round cake of resinous material B, cast in a metal dish or "sole" about one foot in diameter, and a round disc A, of slightly smaller diameter made of metal or of wood covered with tinfoil, and provided with a glass handle. Shellac, or sealing wax, or a mixture of resin shellac and Venice turpentine, may be used to make the cake.
Figs. 18 and 19.--The electrophorus and method of using. Charge B; place A in contact with B, and touch A (fig. 18). The disc is now charged by induction and will yield a spark when touched by the hand, as in fig. 19.
To use the electrophorus, the resinous cake B must be first beaten or rubbed with fur or a woolen cloth, the disc A is then placed on the cake, touched with the finger and then lifted by the handle. The disc will now be found to be charged and will yield a spark when touched with the hand, as in fig. 19.
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The "cover" may be replaced, touched, and once more removed, and will thus yield any number of sparks, the original18 charge on the resinous plate meanwhile remaining practically as strong as before.
The theory of the electrophorus is very simple, provided the student has clearly grasped the principle of induction.
Figs. 20 to 23.--Illustrating "how the electrophorus works."
When the resinous cake is first beaten with the cat's skin its surface is negatively electrified, as indicated in fig. 20. Again, when the metal disc is placed down upon it, it rests really only on three or four points of the surface, and may be regarded as an insulated conductor in the presence of an electrified body. The negative electrification of the cake therefore acts inductively on the metallic disc or "cover," attracting a positive charge to its under side, and repelling a negative charge to its upper surface, as shown in fig. 21.19
If, now, the cover be touched for an instant with the finger, the negative charge of the upper surface (which is upon the upper surface being repelled by the negative charge on the cake) will be neutralized by electricity flowing in from the earth through the hand and body of the experimenter. The attracted positive charge will, however remain being bound as it were by its attraction towards the negative charge on the cake.
Fig. 24.--Lines of force of a charged sphere and a conductor under induction. The negative electrification on the end a of the cylinder indicates that a certain number of lines end there, while the positive electrification on the end b similarly indicates that an equal number of lines set out from that end. It is one of the fundamental properties of a conductor that it yields instantly to the smallest electric force, and that no electric force can be permanently maintained within the substance of a conductor in which no current is passing. There can, therefore, be no electrostatic strain and no lines of force within the material of a conductor where the electric field has become steady. Hence the lines starting from b are entirely distinct from those ending at a. The two sets are equal in number because no charge has been given to the cylinder, either positive or negative, and therefore the sum of all the positive electrifications (or lines starting from b) must be equal to the sum of all the negative electrifications (or the lines ending at a). In all nine lines have been drawn at each end of the cylinder, leaving the thirteen lines emanating from the sphere which do not run on to the cylinder. If the cylinder be withdrawn to a distance from K, it (the cylinder) will be found to show no signs of electrification.
Fig. 22 shows the result after the cover has been touched. If, finally, the cover be lifted by its handle, the remaining positive charge
will no longer be "bound" on the lower surface by attraction, but will distribute itself on both sides of the cover, and may be used
to give a spark. It is clear that no20 part of the original charge has been consumed in the process, which may be repeated as often as desired. As a matter of fact, the charge on the cake slowly dissipates--especially if the air be damp. Hence it is needful sometimes to renew the original charge by again beating the cake with the cat's skin.
Fig. 25.--Faraday's ice-pail experiment. An ice-pail P connected with the gold leaves of an electroscope C, is placed on an insulating stand S. A charged conductor K, carried by a silk thread, is lowered into the pail, and finally touches it at the bottom. While it is being lowered the leaves of the electroscope diverge farther and farther, until K is well within the pail, after which they diverge no more, even when K touches the pail or is afterwards withdrawn by the insulating thread. After withdrawal, K is found to be completely discharged.
The labor of touching the cover with the finger at each operation may be saved by having a pin of brass or a strip of tinfoil projecting from the metallic "sole" on to the top of the cake, so that it touches the plate each time, and thus neutralizes the negative charge by allowing electricity to flow in from the earth.21
Figs. 26 to 29.--Explanation of Faraday's ice pail experiment. For simplicity the electroscope, insulating stand and silk thread have been omitted. Only the three principal conductors K, P, and the earth E are shown. In fig. 26 the ball K is sufficiently close to P to act inductively on it; six lines are shown as falling on P, and the other six as passing to E by different paths. Corresponding to the six lines falling on P from K, six others pass to E from the lower surfaces. In fig. 27 where K is just entering the pail, two lines only pass from K to E through the dielectric; the remaining ten fall on P, and ten others starting from the distant parts of P pass to E. In fig.
28, K is so far within P that none of its lines can reach E through the dielectric; they all fall on P and from the outside of P an equal
number start and pass through the dielectric to E. It is evident that in this position K can be moved about within P, without affecting the outside distribution in the slightest, and that even when K touches P as shown in fig. 29, and when, therefore, all lines between them disappear, the lines in the dielectric outside remain just as they are in fig. 28. K is now completely discharged, since lines no longer emanate from it, hence it can be removed by the silk cord without disturbing the electrification of P. If K be again charged and introduced into P it will be again discharged, for the fact that P is already charged will have no effect on the final result, provided when K touches P it is well under cover.
Since the electricity thus yielded by the electrophorus is not obtained at the expense of any part of the original charge, it is a matter
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of some interest to inquire whence is the source from which the energy of this apparently unlimited supply is drawn;22 for it cannot be called into existence without the expenditure of some other form of energy. The fact is, more work is done in lifting the cover