=Electrification by Induction.=--An insulated conductor, charged with either kind of electricity, acts on bodies in a neutral state placed near it in a manner analogous to that of the action of a magnet on soft iron; that is, it decomposes the neutral electricity, attracting the opposite and repelling the like kind of electricity. The action thus exerted is said to take place by _influence_ or _induction_. The phenomenon of electrification by induction may be demonstrated by the following experiment: In fig. 16, let the ebonite rod be electrified by friction and slowly brought toward the knob of the gold leaf electroscope. The leaves will be seen to diverge, even though the rod does not approach to within a foot of the electroscope. [Illustration: FIG. 16.--Experiment to illustrate electrostatic induction. The leaves will diverge, even though the charged ebonite rod does not approach to within a foot of the electroscope.] This experiment shows that the mere _influence_ which an electric charge exerts upon a conductor placed in its vicinity is able to produce electrification in that conductor. This method of producing electrification is called _electrostatic induction_. As soon as the charged rod is removed the leaves will collapse, indicating that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood. =Nature of the Induced Charge.=--This is shown by the experiment illustrated in fig. 17. Let a metal ball A be charged by rubbing it with a charged rod, and let it then be brought near an insulated metal cylinder B which is provided with pith balls on strips of paper C, D, E, as shown. The divergence of C and E will show that the ends of B have received electrical charges because of the presence of A, while the failure of D to diverge will show that the middle of B is uncharged. Further, the rod which charged A will be found to repel C but to attract E. [Illustration: FIG. 17.--Experiment illustrating the nature of an induced charge. The apparatus consists of a metal ball and cylinder, both mounted on insulated stands, pith balls being placed on the cylinder at points C, D, and E.] From these experiments, the conclusion is that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of opposite sign. =The Electrophorus.=--This is a simple and ingenious instrument, invented by Volta in 1775 for the purpose of procuring, by the principle of induction, _an unlimited number of charges of electricity from one single charge_.