If we take away the rod again the ball will return to its neutral or non-electric state, showing that the charge was temporarily induced by the presence of the electrified rod.  Again, if, as in figure 7, we have two insulated balls touching each other, and bring the rod up, that nearest the rod will become negative and that farthest from it positive.  It appears from these facts that electricity has the power of disturbing or decomposing the neutral state of a neighbouring conductor, and attracting the unlike while it repels the like induced charge.  Hence, too, it is that the electrified amber or sealing-wax is able to attract a light straw or pithball.  The effect supplies a simple way of developing a large amount of electricity from a small initial charge.  For if in figure 6 the positive side of the ball be connected for a moment to earth by a conductor, its positive charge will escape, leaving the negative on the ball, and as there is no longer an equal positive charge to recombine with it when the exciting rod is withdrawn, it remains as a negative charge on the ball.  Similarly, if we separate the two balls in figure 7, we gain two equal charges—­one positive, the other negative.  These processes have only to be repeated by a machine in order to develop very strong charges from a feeble source.

Faraday saw that the intervening air played a part in this action at a distance, and proved conclusively that the value of the induction depended on the nature of the medium between the induced and the inducing charge.  He showed, for example, that the induction through an intervening cake of sulphur is greater than through an equal thickness of air.  This property of the medium is termed its inductive capacity.

The Electrophorus, or carrier of electricity, is a simple device for developing and conveying a charge on the principle of induction.  It consists, as shown in figure 8, of a metal plate B having an insulating handle of glass H, and a flat cake of resin or ebonite R. If the resin is laid on a table and briskly rubbed with cat’s fur it becomes negatively electrified.  The brass plate is then lifted by the handle and laid upon the cake.  It touches the electrified surface at a few points, takes a minute charge from these by contact.  The rest of it, however, is insulated from the resin by the air.  In the main, therefore, the negative charge of the resin is free to induce an opposite or positive charge on the lower surface and a negative charge on the upper surface of the plate.  By touching this upper surface with the finger, as shown in figure 8, the negative charge will escape through the body to the ground or “earth,” as it is technically called, and the positive charge will remain on the plate.  We can withdraw it by lifting the plate, and prove its existence by drawing a spark from it with the knuckle.  The process can be repeated as long as the negative charge continues on the resin.