Page:Electrical Engineering Volume 1.djvu/20

1452 When two dissimilar substances are placed in contact, one of them always assumes the positive and the other the negative condition, although the amount may sometimes be so small as to render its detection very difficult.

Electrified bodies with similar charges are mutually repellent, while electrified bodies with dissimilar charges are mutually attractive.

2209. Table 71 gives a list called the electric series, where the substances are arranged in such order that each receives a positive charge when rubbed with any of the bodies following, and a negative charge when rubbed with any of those which precede it:

1. Fur. 2. Flannel. 3. Ivory. 4. Crystals 5. Glass.

6. Cotton. 7. Silk. 8. The body. 9. Wood. 10. Metals.

11. Sealing-wax. 12. Resin. 13. Sulphur. 14. Gutta-percha. 15. Gun-cotton.

For example, glass when rubbed with fur receives a negative charge; but when rubbed with silk, it receives a positive charge.

2210. The electroscope is an instrument for detecting static charges of electricity and for determining their condition, whether positive or negative; but not for measuring the intensity of the charges.

The pith-ball suspended by a silk thread acts as a simple electroscope. A more sensitive electroscope is shown in Fig. 902, and consists of two gold leaves suspended within a glass jar J, which serves to protect them from drafts of air and to support them from contact with the earth. The gold leaves a are supported side by side in the jar by a brass rod or wire b which passes through a cork in the mouth of the jar. The upper end of the brass rod is furnished with a fiat metallic