The thermionic current produced by the emission of free negative electrons from the surface of heated metals is described in detail. Its theoretical magnitude is calculated for certain definite cases. The limitation of thermionic currents by space charges around the cathode (in high vacua) is explained. In reviewing the bibliography of the subject, it is shown that the preponderance of opinion before the most recent and careful experiments was to the effect the thermionic currents could not be

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... btained in a pure vacuum. The incorrectness of this conclusion is experimentally proven. The degree of cleanliness of the electrodes and the completeness of exhaustion required to produce these thermionic currents in regular fashion is unusual. With true thermionic currents, the cathode does not disintegrate, and there is no blue glow in the path of the cathode rays even at the highest voltages. An Xray tube using thermionic currents is described. A rectifier for high voltage alternating current ("kenotron") is considered, its operating characteristics being given. By inserting a third fine wire grid electrode in a kenotron, an amplifying device ("pliotron") is obtained. Its theory, construction, and characteristics are given. Its use in radio receiving stations as a detector or amplifier is described. The "exponential" method of tuning, involving the use of radio frequency pliotron amplifiers in cascade, is shown to have given remarkable selectivity. The pliotron may also be used as a powerful generator of radio frequency energy; or for the modulation or control of such energy. A 20-W radio telephone transmitter, and a 500-W radio telephone outfit are each described in detail.