The Element of Time in the Photoelectric Effect

Time of appearance and cessation of the photoelectric effect from a potassium hydride surface.—A method has been devised which has made possible the study of the time variation of the photoelectric emission from a metal surface illuminated by light flashes of ${10}^{-8\mathrm{}}$ sec. duration. The experimental arrangement has also yielded information on the speed of operation of the Kerr cell electro-optical shutter described in earlier work and has made possible for the first time the observation of the steepness of wave fronts traveling along wires resulting from spark discharges. Photoelectric emission begins in less than 3×${10}^{-9\mathrm{}}$ sec. after the beginning of the illumination of a potassium hydride surface. The light shutter closes less abruptly than it opens and the experimental observations indicate that the sum of the time required for the shutter to close plus the time during which the photoelectric emission persists after cessation of irradiation is less than ${10}^{-8\mathrm{}}$ sec. A wave traveling along a wire resulting from the sudden change of potential of one end by a spark discharge is so steep that the time necessary for about half the wave-front to pass a point 6 meters along the wire is 4.5×${10}^{-9\mathrm{}}$ sec. Theoretical considerations bearing on these results are discussed.