Within comparatively recent times a large amount of work has been done on the cathode fall of potential. As a result it is now regarded by many investigators that no sudden fall of potential occurs close to the cathode, at least in the case of low voltage discharges. A comprehensive treatment of recent work on this subject is to be found in a paper by Stücklen. Further, F. W. Aston, in his investigations on the cathode fall, came to the conclusion that practically the whole fall occurred in the dark space between the cathode and the negative glow, and that if there were any force in the negative glow it was negligible compared with that in the dark space. The potentials used by Aston across his discharge tubes were comparatively small, however, never exceeding 1000 volts. With voltages of this order he showed further that the electric force in the dark space was very nearly in linear relation with the distance from the negative glow.