Electrification by Impact; Measurement of the Charge Produced by Collision between a Metal and a Dielectric.—After briefly discussing the unsuccessful attempts which have been made to formulate a satisfactory theory to explain electrification by friction, the author suggests that impact of dielectric upon metal, without sliding friction, may cause an electrical effect whose laws will shed light upon the frictional phenomenon. An apparatus is described for measuring the electric charge produced when a disc or sphere of dielectric material collides with a metal disc. The charges obtained in this manner ranged from 0.16 to 9.83 e.s.u., and produced potentials of 2.41 to 183.8 volts upon the metallic systems employed. These charges are of the same order of magnitude as those obtained by friction. The experiment was performed with various metals and dielectrics, and in every instance the metal received a positive charge. In no case was there any evidence of the erratic variation which others have found to be characteristic of electrification by friction. Curves are given which show the variation of charge with velocity of impact and with the mass of the impinging system. The charge produced by a single collision increases with each of these factors, but the velocity of impact was found to exert a greater influence than the mass of the moving body, in determining the amount of the charge. In certain cases velocities were attained at which the electrification due to a single impact reached a maximum value.