Franklin called the electricity excited upon glass by rubbing it with silk positive electricity, and that produced on resinous bodies by friction with wool or fur, negative electricity.
The electricity developed on a body by friction depends on the rubber as well as the body rubbed. Thus glass becomes negatively electrified when rubbed with catskin, but positively electrified when rubbed with silk.
The nature of the electricity set free by friction depends on the degree of polish, the direction of the friction, and the temperature. If two glass discs of different degrees of polish be rubbed against each other, that which is most polished is positively, and that which is least polished is negatively electrified. If two silk ribbons of the same kind be rubbed across each other, that which is transversely rubbed is negatively and the other positively electrified. If two bodies of the same substance, of the same polish, but of different temperatures, be rubbed together, that which is most heated is negatively electrified. Generally speaking, the particles which are most readily displaced are negatively electrified.
In the following list, which is mainly due to Faraday, the substances are arranged in such order that each becomes positively electrified when rubbed with any of the bodies following, but negatively when rubbed with any of those which precede it:
1. Catskin. 2. Flannel. 3. Ivory. 4. Rock crystal. 5. Glass. 6. Cotton. 7. Silk. 8. The hand. 9. Wood. 10. Metals. 11. Caoutchouc. 12. Sealing wax. 13. Resin. 14. Sulphur. 15. Gutta-percha. 16. Gun cotton.
The Charge.—The quantity of electrification of either kind produced by friction or other means upon the surface of a body is spoken of as a charge, and a body when electrified is said to be charged. It is clear that there may be charges of different values as well as of either kind. When the charge of electricity is removed from a charged body it is said to be discharged. Good conductors of electricity are instantaneously discharged if touched by the hand or by any conductor in contact with the ground, the charge thus finding a means of escaping to earth. A body that is not a good conductor may be readily discharged by passing it rapidly through the flame of a lamp or candle; for the flame instantly carries off the electricity and dissipates it in the air.
Distribution of the Charge.—When an insulated sphere of conducting material is charged with electricity, the latter passes to the surface of the sphere, and forms there an extremely thin layer. The distribution of the charge then, depends on the extent of the surface and not on the mass.
Boit proved that the charge resides on the surface by the following experiment:
A copper ball was electrified and insulated. Two hollow hemispheres of copper of a larger size, provided with glass handles, were then placed near the sphere, as in fig. 4. So long as they did not touch the sphere, the charge remained on the latter, but if the hemispheres touched the inner sphere, the whole of the electricity passed to the exterior, and when the hemispheres were separated and removed the inner globe was found to be completely discharged.