The Effect of Temperature on the Emission of Electron Field Currents from Tungsten and Molybdenum

Abstract

Electron field currents from the central portion of long molybdenum and tungsten filaments about 2.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ cm in diameter have been studied. The field currents were first made stable to about 5 percent by long-continued conditioning treatments of temperature and high voltage under high vacuum conditions. Thermionic emission measurements gave the values 4.32 and 4.58 volts for the work function of the molybdenum and tungsten, respectively, in good agreement with the accepted values for the clean metals. Emission measurements were then made at fields varying from about 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts/cm to about 1\ifmmode\times\else\texttimes\fi{}${10}^{6}$ volts/cm and at temperatures varying from 300\ifmmode^\circ\else\textdegree\fi{}K to about 2000\ifmmode^\circ\else\textdegree\fi{}K. Down to about 1600\ifmmode^\circ\else\textdegree\fi{}K the thermionic currents completely masked the field currents. Thermionic emission values below 1600\ifmmode^\circ\else\textdegree\fi{}K were obtained by extrapolation. Thus the field currents at the lower temperatures were separated from the thermionic currents. Where necessary, corrections were made for the decrease in the voltage gradient accompanying the thermal expansion of the filament. The field currents were found to be independent of temperature to within 5 percent from 300\ifmmode^\circ\else\textdegree\fi{}K to 1400\ifmmode^\circ\else\textdegree\fi{}K. At temperatures higher than 1400\ifmmode^\circ\else\textdegree\fi{}K the data are consistent with the assumption that the current consists of a thermionic current plus a current which is independent of temperature. However, because of the exponential change of thermionic current with temperature a small effect of temperature on the field current could not be distinguished at temperatures higher than 1400\ifmmode^\circ\else\textdegree\fi{}K. From the theory of Fowler and Nordheim, $\ensuremath{\beta}$, a factor introduced by surface irregularities, is found to be 120 for the tungsten cathode and 47 for the molybdenum one. Thus for tungsten, Houston's theory of the temperature effect is in approximate agreement with the negative results of these experiments.