We saw earlier that a charged rod brought near to the cap of an electroscope causes the leaf to diverge from the plate, showing that a charge has been induced on both of them. The following experiment provides more information about the charges which are induced on an insulated conductor when a charged rod is brought near it (Fig.32.2).
(a) Two insulated brass spheres A and B are placed together so that they touch one another and thus form, in effect, a single conductor.
(b) A negatively charged rod is now brought near to A. As a result, a positive charge is induced on A and a negative charge on B.
(c) Still keeping the charged rod in position, sphere B is moved a short distance from A.
(d) The charged rod is now removed and A and B are tested for charge. The test is carried out as follows. Sphere A is brought near to the cap of a positively charged electroscope. An increase in divergence shows that it is positively
charged. Similarly, sphere B produces an increase in divergence when it is brought near to the cap of a negatively charged electroscope, thus showing it to be negatively charged. If the whole experiment is carried out again using a positively charged rod as the inducing charge, the induced charges on A and B are reversed .
. The process by which charges are induced on A and B will be explained after we have dealt with the electron theory of electricity. In the meantime, it is interesting to compare electrostatic induction with magnetic induction described on page 345. There it was seen that the induced pole on the end of the steel nearest to the inducing pole was of opposite polarity. In the case of electrostatic induction the induced charge on the end of the conductor nearest to the inducing charge is of opposite sign. There is, however, an important difference. While we are able to separate positive charges from negative charges, it is impossible to isolate magnetic poles.