it, but that, on account of the tension acting along the tube, the tendency is for the tube to become as short as possible. When the two conductors,

ends of the line of force to tend to approach each other, and can only be kept apart by the interposition of a non-conductor, we shall be able to explain how it is that one of the kinds of electricity produced by induction remains on the body when the latter is put to earth, while the other kind of electrification escapes to earth.

In Fig. 434 let A represent the inducing body, which we may suppose charged with positive electricity, and B be an insulated conductor which is electrified by induction by A. Then some of the lines of force (shown by the full lines) which leave A will terminate on B, and B will therefore be negatively electrified at the part where these lines meet the surface. In addition a number of lines of force will leave B, and terminate on surrounding conductors, such as the walls of the room in which the two

bodies are placed. The part of B where these lines leave the surface will be positively electrified, the corresponding negative charge being on the walls. The lines of force which stretch from A to B, by their tension, cause the negative charge on B to accumulate on the side next A. The whole charge does not accumulate at the nearest point, however, because of the mutual repulsion which the lines of force exert on one another. It

is owing to this repulsion between the lines that the lines leaving the body B accumulate at the other end.

When the body B is put in conducting communication with the earth, i.e. with the bodies on which the lines of force which leave it terminate, owing to the action of the tension on the electrification itself, the latter will escape, but the negative electrification corresponding to the lines of force which leave A and terminate on B will not be able to reach A, since these two bodies are not in conducting communication. The distribution

of the charges will then be as shown by the lines of force in Fig. 435, where there are no lines of force leaving B, indicating that the charge on B is everywhere negative.

J

445. Faraday's Ice-Pail Experiment.-The production of electrification by induction can be very clearly investigated by means of the arrangement shown in Fig. 436. Faraday, to whom these experiments are due, used, in place of the hollow metal sphere A, a metal ice-pail, and owing to this circumstance it is generally known as Faraday's ice-pail experiment. The hollow metal sphere has an opening B, through which a small charged sphere D can be lowered into the interior. The hollow sphere is supported on an insulating stand, and is connected with an electroscope C. Let A be unelectrified, and suppose that we introduce the small sphere D, which is suspended by an insulating thread, and is

charged with Q units of positive electrification. Before D was introduced within A, the tubes of force which start from the charged sphere ter

B

A

FIG. 436.

The tubes of force which leave the outside of A correspond to a positive electrification, and the magnitude of this electrification is indicated by the magnitude of the divergence of the gold leaves of the electroscope. If now the vessel A is put in conducting communication with the earth, the tubes of force which stretch from its outer surface to the walls will be able to contract, the ends running along the conductor used to put A to earth. The tubes which terminate on the inside surface will, however, not be able to shrink, for the sphere D is not in conducting communication with the vessel A. Next let the vessel A be again insulated, and then lower the sphere D till it touches the inside of A. Now the tubes of force between D and the inside of A are able to shrink and vanish. When the vessel A was put to earth the leaves of the electroscope collapsed, showing that the vessel A had lost its free charge, and they remain collapsed even when the charged sphere is allowed to touch the inside of the vessel A. This therefore shows that the positive charge on D is exactly equal to the induced negative charge on A. Hence the charge induced on the inside of A is Q. Also, since there were tubes of force leaving D, and each of these tubes must have terminated on the inside of A, that is, on a surface on which the charge is -2, we see that this experiment proves that the charge on the portion of the surface on which each tube terminated was a unit of negative electricity.

Next remove D and again charge it with Qunits of positive electricity, that is, give it the same charge as before, and again introduce it within the vessel A. If now the sphere D be lowered till it touches the inside of A, it will be found that the separation of the leaves of the electroscope remains unaltered. We have now communicated a charge +Q to A, and since this produces the same deviation of the electroscope leaves as did the induced charge produced by the body charged with +Q units placed inside, we see that the positive charge produced by induction is equal to Qunits. But the previous experiment showed that the negative charge

produced by induction on the inside of A is also equal to Q units. Hence we see that the positive and negative charges produced by induction are equal. Thus our assumption that every tube of force starts from a portion of a conductor on which there is a unit positive charge, and terminates on a portion on which there is a unit negative charge, is justified.

Since every tube of force must have both a beginning and an end, it therefore follows that to every positive charge there must exist an equal negative charge, this charge being situated on the bodies on which the tubes of force which leave the positively charged body terminate. From this it follows that whenever we, by any means whatever, give a charge of one sign to a body an equal and opposite charge must at the same time be produced. This deduction may be proved experimentally by means of Faraday's ice-pail experiment, for if a small piece of sealing-wax is attached to an insulating handle and introduced within the vessel A, and is electrified by rubbing with a flannel pad attached to a second insulating handle, the electroscope will be unaffected. The reason is that the charges produced on the sealing-wax and the flannel are equal and opposite, and therefore they induce equal and opposite charges on the outside of the vessel A. On removing either the sealing-wax or the flannel the electroscope will be affected, for now the inducing charge inside A is all of one sign, and hence so also is the charge induced on the outside.

We can by this arrangement give the vessel A a charge, the magnitude of which is any given number of times the magnitude of some given charge. Thus suppose we have a negatively charged sphere, and that we bring the sphere D to within, say, six inches and then put it momentarily to earth. In this way D will obtain a charge of positive electricity of magnitude Q, say. Next introduce D inside A, and let it touch the bottom. In this way we shall communicate a charge of Q to A. If now D is again brought to within six inches of the negatively charged sphere earthed and then introduced within a as before, a further charge of 2 will be communicated to A, so that the total charge is 2 Q, and so on.

446. Difference of Potential.-If two conductors, one of which is charged positively and the other is charged negatively, are put in conducting communication, their state of electrification will become changed, so that if they originally possessed equal charges they will both, after being connected, exhibit no signs of electrification. If the charge on one was greater than that on the other, then, after being connected, the sign of the charge on the two will be the same as the sign of the charge which was originally the greater, while the sum of the charges now possessed will be equal to the difference of the two original charges. If, however, two bodies, each of which is charged with electricity of the same kind, are put in conducting communication, it does not follow that the charge on the body which was originally electrified with the larger charge will be decreased and that of the other