Abstract
Although extensive theoretical and experimental work of many laboratories has been done on the so-called cathode, it does not yet appear to have produced a useful device. If, however, the thickness of the insulating layer in a sandwich type cathode is increased to several times the mean free path of the hot carriers, appreciable current can be drawn for extended periods. The most highly developed cathode of this type is found in the MgO cold cathode tube. In this tube the sandwich is composed of the nickel cathode sleeve, a layer of NiO, and a layer of porous MgO. The slow starting and extreme spread in velocity of emitted electrons of the MgO tube has initiated development of an analog which has the metal-oxide-metal structure of the proposed tunnel cathodes, but has proven capable of D.C. emission of microamperes for periods of several hours.
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