The Time Factor

Author: J.B. Hoag

It has been shown by direct measurements that there is practically no delay between the time light falls on the cathode and the time of emission of electrons. Furthermore, when the light is turned off, the photoelectrons cease to come out of the cathode almost instantaneously. If there is any time delay for a phototube, it is less than one one-hundred-millionth of a second.

Suppose the light which falls on the cathode were turned on and off successively at an increasing rate; then, in the case of a gas-filled photoelectric tube, it would be found that the photoelectric current gradually decreased, by as much as 20 per cent when the frequency is raised to 10,000 cycles per second. This is due to the gas and not to any time delay of emission of the photoelectrons from the cathode.