DISCOVERY OF CONTACT ELECTRIFICATION 443
and tlic plate 7> so that Jl may be stopped in any position. When the crank is turned B passes alternately in front of and parallel to A and C. The contacts are so arranged that when Ji is parallel to A it is joined to earth and so charged oppositely to A. It is then revolved in front of C, at which instant C is joined to earth and is charged by induction oppo-
���sitely to B and like .4. As B again comes in front of A, A and C are joined and B is earthed. B now receives by induction a greater charge than before, since A has now received the greater part of C's charge. By successive revolutions of B the charge may thus be built up to any desired intensity upon A.
Bennett performed many experiments with this doubler, and after learning how to discharge it completely he tested the electrification induced upon metal plates by being placed in contact with various sub- stances, both solid and liquid. He concludes that different substances " have a greater or less affinity with the electrical fluid," and he then