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. [Illustration: 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. 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. [Illustration: 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. 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.