In this case, there will be a tendency to equilibrium, caused by attraction. The earth is supposed to be a vast reservoir of electricity, from which a quantity can be drawn to fill up a deficiency, and which is always ready to receive an excess from other bodies. Every body in nature has its own natural quantity of electricity, and when an object is negatively electrified, or has a deficiency in its normal quantity, there is a tendency to receive a supply from any convenient source. Such an object would receive electricity from the earth if means were afforded; and a body positively electrified, would tend to part with its excess in the same manner. Where such facilities for establishing electrical equilibrium are afforded, the result is the passage of a current of electricity.
Conductors.—Sensible effects can be produced by electricity at great distances from the source, provided there be a medium of communication, that is, good conductors to transfer it. When a glass rod is rubbed with a piece of silk, it becomes charged with an excess of, or positive, electricity, and at the same time the silk becomes charged with negative electricity.
The glass rod will retain the positive electricity upon it for some time, unless touched with the wet hand, a wet cloth, a metal, &c., when it will instantly cease to be electrified. The electricity is then said to have been conducted away, and the bodies which allow it to run off the glass are called conductors of electricity. Metals, water, the human body, charcoal, damp wood, and many other bodies are conductors.
Those bodies which conduct electricity hardly at all, such as the air, silk, glass, sealing wax, gutta percha, india rubber, &c., are termed nonconductors or insulators.
Strictly speaking, all substances conduct electricity in some degree, and a nonconductor is merely a bad conductor.
In the following table the bodies are arranged in their order of conductivity, i.e. each substance conducts better than that which precedes it; the first-named body is the best insulator, and the last-named one is the best conductor.
- Dry air.
- Ebonite.
- Paraffin.
- Shellac.
- India rubber.
- Gutta percha.
- Resin.
- Sulphur.
- Sealing wax.
- Glass.
- Silk.
- Wool.
- Dry paper.
- Porcelain.
- Dry wood.
- Stone.
- Pure water.
- Rarefied air.
- Sea water.
- Saline solutions.
- Acids.
- Charcoal, or Coke.
- Mercury.
- Lead.
- Tin.
- Iron.
- Platinum.
- Zinc.
- Gold.
- Copper.
- Silver.
Though two substances are near one another in the above list, they do not necessarily approach one another in their power of conducting. For instance, taking the conducting power of pure silver as represented by the number 100, then
| Pure | Copper will be equal to 99·9, | |
| Gold will be equal to 78·0, | ||
| while | Zinc | will be only equal to 29·0, |
and pure water, which is half-way down the list, will offer 6,754 millions more resistance than silver to the passage of the electric current.