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No. 2. DETERMINATION OF elm FROM MEASUREMENTS OF THERMIONIC CURRENTS.' By SAUL DUSHMAN. IN a recent paper on "The Effect of Space Charge and Residual Gases on Thermionic Currents in High Vacuum,"2 Dr. I. Langmuir has shown that the thermionic current from a heated metallic surface in high vacuum is limited by (a) temperature, (b) space charge. With a given potential difference between the electrodes, the current from the heated cathode increases with the temperature in accordance with the equation of Richardson, _b i = aV/eT. (.I ) However, above a certain temperature, this current becomes constant; further increase in temperature does not cause any corresponding increase in thermionic current. The temperature at which this limitation occurs increases with increase in anode potential. It was shown in the above paper that this effect is due to the existence of a space charge produced by the emitted electrons, and it was furthermore deduced that at a fixed cathode temperature, the electron current ought to increase with the three-halves power of the voltage (until the saturation current as defined by the Richardson equation is attained), that is, for electrodes of any shape, the space charge current i = k V3/2,
Physical Review – American Physical Society (APS)
Published: Aug 1, 1914
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