The manner in which a body is electrified depends upon its nature and condition; but we may accept as a general axiom,—but by no means as a law,—that when two bodies are rubbed together, that which gets the hotter in the process takes the negative kind of electricity. In the following list the substances have been so arranged that each is negatively electrified by those preceding, and positively by those succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers; 5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a list of conducting and non-conducting bodies in their order:—
| Conductors. | ||
|---|---|---|
| Metals. | Pure water. | Hot air. |
| Lime, coal, or coke. | Vegetable tissues. | Steam. |
| Saline mixtures. | Animal tissues. | Rarefied air. |
| Non-Conductors. | ||
| Ice. | Dry gases. | Diamond. |
| India-rubber. | Paper. | Glass. |
| Marble. | Wool. | Wax. |
| Porcelain. | Silk. | Sulphur. |
| Resin. | Shell-lac. | |
Fig. 205.—Positive and negative.
Fig. 206.—The Battery.
It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies which are good and bad conductors are good and bad conductors of electricity. Water is a good conductor, air is a bad one; were it otherwise, electricity would escape from the ground into the air; as it is, the air manages in some degree to retain the electricity at the surface of bodies, for it is on the surface that we find the electric “fluid.”
We have mentioned electrical induction in a former experiment with the tea-tray. We will now explain it more fully, as a consideration of it will bring us to the electric spark, or lightning, with the account of the discovery of the Conductor and the Electrical Machine.
Let us look at the illustration next below. A B is a cylinder supported on a glass rod, and at each extremity is a small pith ball, a and b. The cylinder is in a neutral condition, as is evidenced at first by the pellets being in a vertical position. But suppose we bring a ball, C, towards the cylinder. C is charged with positive electricity, which attracts the negative to itself, and so repels the positive away at the opposite side. So the pellet at one side will be attracted to C, and the other will fly in an opposite direction.
