Bodies charged with the same electricity repel; if charged with different kinds they attract each other. The more readily displaced particles when bodies are rubbed become negatively electrified as a rule.
Similar electricities repel each other with a force inversely proportional to the squares of the distance between their centres, as established by Coulomb. So if the space between any two similarly electrified bodies be reduced by say one-half, the force of the repulsion will be increased four times. The rule for attraction is similar; so when two bodies are charged with opposite electricities, and the distance between them is increased, the attractive force is diminished in proportion as the square of the distance between them. Many confirmations of this theory were made by the late friend of our boyhood, Sir W. Snow Harris, and published in the Philosophical Transactions.
The following full list of conductors and non-conductors (copied from Professor Noad’s Text-book of Electricity, and compared with De La Rive’s Treatise) may be useful:—
| Conducting Bodies in Order of Conducting Power | Insulators in the Inverse Order of Insulating Power. | ||
|---|---|---|---|
| All the metals. | Dry metallic oxides. | ||
| Well-burnt charcoal. | Oils (heavier the better). | ||
| Plumbago. | Vegetable ashes. | ||
| Concentrated acids. | Transparent dry crystals. | ||
| Powdered charcoal. | Ice below 13° Fahr. | ||
| Dilute acids. | Phosphorus. | ||
| Saline solutions. | Lime. | ||
| Metallic ores. | Dry chalk. | ||
| Animal fluids. | Native carbonate of baryta. | ||
| Sea-water. | Lycopodium. | ||
| Spring-water. | Caoutchouc. | ||
| Rain-water. | Camphor. | ||
| Ice above 13° Fahr. | Silicious and argillaceous stones. | ||
| Snow. | Dry marble. | ||
| Living vegetables. | Porcelain. | ||
| Living animals. | Dry vegetables. | ||
| Flame smoke. | Baked wood. | ||
| Steam. | Leather. | ||
| Salts soluble in water. | Parchment. | ||
| Rarefied air. | Dry paper. | ||
| Vapour of alcohol. | Hair. | ||
| Vapour of ether. | Wool. | ||
| Moist earth and stones. | Dyed silk. | ||
| Powdered glass. | Bleached silk. | ||
| Flowers of sulphur. | Raw silk. | ||
The following may be added to the Insulators, viz.:—
| Transparent precious stones. | Sulphur. |
| Diamond. | Resins. |
| Mica. | Amber. |
| All vitrefactions. | Gutta-percha. |
| Glass. | Shell-lac (or gum-lac). |
| Jet. | Ebonite. |
| Wax. |
There are, as we know, two kinds of electricity, the static and dynamic; and when the latter state is instantaneous, it is referred to as the “electric discharge,” which occurs when opposite electricities seek each other, and the bodies return to a state of equilibrium or neutralization. “These bodies, if insulated, obtain no more electricity after the spark has passed; but if there be a constant source of negative electricity supplying one, and a constant source of negative electricity supplying the other, there will be a succession of sparks; and if they communicate by a conductor there will be, through this conductor, an uninterrupted neutralization of a continual reunion of the two electricities, and this is what is termed the continuous dynamic state or electric current” (De la Rive).
