Page:Electrical Engineering Volume 1.djvu/19

Rh charges mutual repulsion will take place whenever they are brought near each other. If a stick of sealing-wax, electrified by being rubbed with fur, is approached to another pendulum, the same results will be produced—the ball will fly towards the wax, and after contact will again be repelled. But the charges respectively developed in these two cases are not in the same condition. For if after the pith-ball has been touched with the glass rod and repelled, the electrified sealing-wax be brought in the vicinity, attraction takes place between the ball and sealing-wax. Similarly, if the pendulum be charged with the electrified sealing-wax, the ball will be repelled by the wax and attracted by the glass rod.

We have, therefore, to distinguish between two kinds of electrification—that produced by rubbing glass with silk and that produced by rubbing sealing-wax with fur. To make this distinction clear, the following designations have been adopted:

An electric charge excited upon glass by rubbing it with silk has been termed a positive charge (+), and that developed on resinous bodies by friction with flannel or fur a negative charge (–).

2206. Neither charge is produced alone, for there is always an equal quantity of both charges produced, one charge appearing on the body rubbed, and an equal amount of the opposite charge upon the rubber.

2207. The intensity of the charge developed by rubbing the two substances together is evidently independent of the actual amount of friction which takes place between the bodies. For, in order to obtain the highest possible degree of electrification from two dissimilar substances, it is only necessary to bring every portion of one surface into intimate contact with every particle, or every portion of the other surface; when this is done, no extra amount of rubbing can develop any greater charge upon either substance.

2208. From these experiments are derived the following laws: