A New Method of Investigating Thermionic Cathodes

Under certain conditions, the electron current from a thermionic cathode is determined by the positive ion current reaching it. Two methods of using this relation to measure the thermionic emission of a cathode in a gaseous discharge are described. The constants, $b_0=30,000\mathrm{}$, $A=10\mathrm{}$ for a thoriated tungsten filament, and $b_0=22,700\mathrm{}$, $A=88\mathrm{}$ for an oxide-coated cathode, were obtained by use of these methods. The zero-field emission of an oxide-coated cathode was found to be only about 10 percent of the current at which the cathode was normally operated in practical discharge tubes. The increased current is due to the influence at the cathode of an external field which is established by an increase in the rate of generation of positive ions. The emission increases linearly with field up to fields of 150 volts/cm. Use is made of an auxiliary discharge to determine the relative ionizing power of electrons of different velocities. With cumulative ionization predominating, the total ionizing power increases linearly with the accelerating potential of the electron for voltages above the resonance potential and up to about two or three times this value.