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have recently determined the thermionic constants for carbon, in the form of an extruded filament, as 4.35 zb 0.06 ev for the electronic work function and 48±25 amp/cm 2 deg 2 for the Richardson constant, A. They state that these values are in good agreement with "the most recent determination reported in the literature," i.e., that of Reimann 2 (0=4.34 ev, .4=30 amp/cm 2 deg 2 ). The purpose of this letter is to compare these results with other values published since Reimann's work and also with results obtained in this laboratory 3 on similar carbon filaments.Braun and Busch 4 investigated the thermionic emission from a graphite rod and found <£=4.39 ev and .4 = 15 amp/cm 2 deg 2 . Using a graphite tube, Bhatnagar 8 found 0=4.84 ev but did not determine the A value. Bhatnagar also makes the interesting observation that the initial measurement of work function resulted in 0=3.4 ev, but that with continued heat treatment (100 hours) the value increased and approached 4.84 ev assymptotically. It should be noted that this final value is considerably higher than the other figures quoted above.In the experiments here, a filament 0.0295-in. in diameter 6 was first outgassed in a separate tube by heating to a maximum temperature of 2525°K. At this temperature the vapor pressure 7 of carbon is about 2X10"" 4 mm Hg and in twenty minutes the walls of the tube were appreciably coated. This outgassing temperature is several hundred degrees hotter than that used by previous investigators. Following this treatment the filament was removed and mounted in a conventional tube with a cylindrical tantalum collector and guard rings for emission measurements. This tube was continually pumped and, after baking and outgassing the vacuum, was always at least a factor of ten better than the figure of 10~" 6 mm Hg reported by Glockler and Sausville for their tube which contained a ground-glass joint.Three Richardson plots were made after the filament had been heated to various temperatures. The results are shown in Table I. TABLE I. Thermionic constants as determined from three Richardson plots. No. I II III Maximum Temperature (°K) 1610 2125 2335 4> (ev) 4.44 4.61 4.59 A (amp/cm 2 deg 2 ) 22 45 48The constants were calculated by the method of least squares. The temperature was measured with an optical pyrometer and the spectral emissivity was assumed to be 0.85. This is a good approximation to the temperature scale of Prescott and Hincke 8 used by Reimann and by Glockler and Sausville. As a matter of fact, the choice of spectral emissivity has very little effect on the value of the work function, but does affect the A value. For example, if uncorrected brightness temperatures are used to calculate the constants for measurements III above, the results are 0=4.61 ev and ,4 = 81 amp/cm 2 deg 2 .The last two measurements above, i.e., those taken after heating at a high temperature, agree very closely and give average values of 0=4.60 ev and .4=46 amp/cm 2 deg 2 . These values are considered to be a reliable determination of the thermionic c...

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