379. =Induction.=--A remarkable influence is exerted by an electrified body upon another body in its usual state when brought near it, and this influence is called induction. I will illustrate this by Fig. 259. Let A be a metallic ball standing on a glass pillar, and charged with positive electricity. Let B be a metallic cylinder supported upon two glass pillars. Now if A be placed near B, but not near enough for the electric spark to pass from it to B, it will destroy the equilibrium of the two electricties in B, the negative electricity being accumulated at the end near A, and the positive at the remote end. This is because the positive electricity in A repels its like in B and attracts the unlike fluid. You observe that there is a pair of pith balls suspended at each end of B, and also at the middle. The two balls at the positive end repel each other because they are charged with the same electricity, and so with the balls at the negative end. But the balls hanging from the middle are not affected, because they are on middle ground between the two electricities. Here is no communication of electricity from A to B, but only an influence upon the quiescent balanced electricities of B. Accordingly, if the surplus electricity of A be discharged by putting the hand or any good conductor upon it the influence will cease, the equilibrium in B will be restored, and the pith balls will all hang straight down. The same effect will be produced if A be withdrawn to a distance from B, and the influence will be renewed if A be brought near again. [Illustration: Fig. 260.] If instead of one conductor we use two, B and C, Fig. 260, and have them in contact, we shall have the negative electricity on B and the positive on C. Now if we withdraw C from B we may have the two electricities separate, B being charged with the negative and C with the positive. [Illustration: Fig. 261.]