To show that equal and opposite charges are produced by friction
Whenever an ebonite rod is rubbed with fur the fur becomes charged as well as the ebonite, but with electricity of opposite sign. The same remarks apply to the case of glass rubbed with silk, or indeed to any pair of substances. Moreover, the two
opposite charges produced by friction are equal in magnitude. Normally it is not easy to show that fur becomes charged, since it is a partial conductor owing to the slight amount of moisture in it and so the charge leaks away through the hand. However, it is possible to show that the charge on the fur is equal and opposite to that on the rod if special precautions are taken, as in the following experiment (Fig. 32.24).
An ebonite rod is discharged completely by drawing it quickly through air above a gas flame and afterwards it is fitted with a small fur cap to which a thread of silk or other insulating material is attached (Fig. 32.24 (a)). The rod is then held in one hand while the cap is rotated several times round the rod by means of the silk thread. This causes both rod and cap to become charged by friction. The capped end of the rod is now placed inside a metal can standing on the cap of a gold-leaf electroscope, and it is observed that no divergence of the leaf occurs (Fig. 32.24 (b)). This implies that, either:
(i) there is no charge on either the rod or the fur cap; or
(ii) the rod and fur cap possess equal and opposite charges.
Condition (ii) is shown to hold in this case by removing the rod and leaving the fur cap inside the can (Fig. 32.24 (cj). Immediately, the leaf diverges, indicating that, when both rod and cap were together inside the can, they possessed equal and opposite charges which exactly neutralized one another.