Fig. 208.—Induction.
Let us take another illustration. Here we have a horizontal metal rod, cc′, insulated on a glass stand. Two balls of cork are attached at both ends of the rod by metallic wires. Hold a rod of resin, r, which has been made negatively electrical, and apply it to one pair of the cork balls. The positive electricity will be attracted at c′ and the negative repulsed, and fly away at c. If we remove the resin the equilibrium will be again established, and the balls will fall to a vertical position.
Fig. 207.—Electrical induction.
We can also by drawing off the negative electricity by the finger at c, while the resin rod is still held to the other side, c′, fill the whole of the metal rod with the positive electricity when the finger and the resin have been removed respectively first and last. The balls will then fly in opposite directions again, in consequence of the repulsion exercised by the positive poles.
The “Electrophore,” or “Electrophorus,” we have already learned to make for ourselves, as also the Leyden Jar. But we give cuts of them. The former is very simple, and can be made by mixing two parts of shell-lac and one of turpentine, and pouring the mixture upon a metal plate. If this be rubbed with a cat’s skin when dry, and a metal cover with a glass handle be placed upon it, it will be found that the positive and negative electricity are collected on the lower and upper surfaces of the plate respectively, and can be drawn away with a spark as before, and made use of.
Fig. 209.—Electrophorus.
The Leyden Jar requires a little more detailed description, as it is to it we are indebted for our Battery. It is a common glass bottle or jar, coated both inside and out with tinfoil nearly as high as the shoulder, a a. The mouth should be firmly closed with a bung of wood, g g; a hole should be bored in the bung, through which a brass rod is tightly pushed. The rod, too, is topped by a brass knob, and a brass chain is attached to the other extremity. The interior of the tinfoil receives positive electricity, and the exterior negative when the jar is charged from the “Electrophorus.” To discharge the jar and create a shock it is necessary to put one hand on the outside, and the other on the knob of the jar. A brilliant spark and a severe shock will result if the jar has been fully charged. It is as well to be cautious when trying this experiment. The effect of the shock may be felt by any number of persons joining hands, if one at one end of the row, and one at the other end, touch the knob and the outside of the jar simultaneously.