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LXVI. The Variation of Thermionic Currents with Potentials. 

 By Horace Lester, Ph.D., Instructor in Physics, Princeton 

 University *. 



INITIAL thermionic currents from hot surfaces differ 

 from the currents found after steady conditions have 

 been obtained. The peculiarities of these currents have been 

 studied by several investigators. Richardson f found that 

 the positive emission was not permanent, that it did not 

 " saturate " easily, and that the currents increased with the 

 temperature according to an exponential law, at least for a 

 limited range of temperature. He found the carriers to be 

 of atomic magnitude, and considered them to be produced 

 by thermal dissociation of impurities in the hot anode. A 

 further investigation of the initial positive emission has been 

 undertaken more recently by Richardson and Sheard. They 

 were prevented from completing their experiments, but a 

 preliminary account J of the results obtained has been pub- 

 lished in a note in the ' Physical Review.' They found that 

 the positive currents decayed with time, the rate of decay 

 depending on the temperature, and that the current-potential 

 relation at constant temperature followed a steep curve up to 

 about 40 volts, at which point a bend occurred towards the 

 potential axis, and beyond which there was a nearly linear 

 increase up to 400 volts. According to Richardson's theory 

 referred to above one would expect saturation to occur 

 .at much lower potentials than those mentioned, since the 

 number of negative ions or electrons present is negligible. 

 Saturation for positive currents does not ordinarily occur, 

 although Richardson and Sheard found that with time the 

 slope of the curve above 40 volts decreased ; so that approxi- 

 mate saturation was obtained eventually. 



At the suggestion of Professor Richardson, the writer 

 undertook the continuation of this investigation. As a result, 

 the above-mentioned peculiarities of the current-voltage 

 curves for the initial positive emissions have been confirmed, 

 and similar phenomena have been found to occur with the 

 initial negative currents. The further conclusion of Richardson 

 and Sheard that the increase in the current beyond 40 volts 

 is due to secondary ionization caused by the bombardment 

 of a film at the opposite electrode has not been verified. It 

 is possible that such an effect may occur, but it seems clear 



* Communicated by Prof. O. W. Richardson, F.R.S. 

 t Phil. Mag. July 1903, Sept. 1904. 

 % Phys. Rev. vol. xxxiv. p. 392 (1912). 



