Distribution of charge over the surface of a conductor
A proof plane and gold-leaf electroscope may be used to investigate the distribution of charge over the surface of a conductor, by pressing the proof plane into contact with the surface at various places in turn and then transferring the charge to the electroscope (Fig. 32.12). The divergence of the leaf will give a rough measure of the amount of charge transferred, and hence some idea of the surface density of the charge.
Surface density is defined as the quantity of charge per unit area of surface of a conductor. The experiment is usually carried out with a flat circular proof plane of the type shown on the left in Fig. 32.3. Strictly speaking, a set of proof planes should be used, each having the same area but shaped so as to fit the curvature of the surface at the various places chosen for test. Fig. 32.12 also shows how charge is distributed over the surface of conductors of