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.
The “cover” may be replaced, touched, and once more removed, and will thus yield any number of sparks, the original 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.
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.
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. 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 no 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.
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.