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The MgO cold cathode is a new source of electrons with possible applications in various types of electron tubes. It consists of a thin layer of porous magnesium oxide on a nickel base. A strong electric field that exists across the layer while in operation is believed to produce the electron emission from the surface. Evidence supports the theory that avalanche multiplication occurs in the layer. This cathode glows with a pale blue luminescence during operation. The velocity distribution of the emitted electrons shows a peak at 13 electron volts. The outer surface potential has been measured and found to be of the order of 150 volts with respect to the nickel base. The emission is not self-starting, and starting means are discussed. Noise, life, emission density, and temperature range of operation are discussed in so far as present knowledge permits. An experimental design of an amplifier tube employing this cathode is described and the characteristics of the tube are given.