280 Effect of Electric Current on Photoelectric Effect. 



between the plate under test and the collecting electrode is 

 given in the paper as 14 volts, and this would enable the 

 photoelectrons to obtain sufficient energy to produce radia- 

 tion and possibly ionization on collision with gas-molecules. 

 Radiation produced in this way would act photoelectrically 

 on the cathode and give an increased emission of electrons 

 which, similarly, would give rise to more photoelectrons. 

 Thus the current measured by the electrometer might be 

 considerably increased by a slight rise of gas-pressure. 



If the apparatus were cold and had been pumped for a 

 long time, the equilibrium pressure of the gas would be low 

 and the electric current arising from its presence would be 

 negligibly small, but this current would become measurable 

 as the equilibrium-pressure of the gas increased when parts 

 of the apparatus warmed up during the passage of the 

 current through the bismuth. Moreover, after each change 

 in the current through the bismuth it would take some time 

 for an equilibrium-pressure of gas to be attained, so that we 

 should expect that the current measured by the electrometer 

 would only slowly reach a constant value. It would also be 

 expected that this constant value would at first increase with 

 each increase of the current through the bismuth. Thus the 

 changes in the electrometer current observed by Mr. Shen- 

 stone are such as might result from the presence in the 

 apparatus of a small quantity of gas arising from the heating- 

 effect of the electric current passed through the bismuth. 



An effect resembling that described by Mr. Shenstone 

 was obtained by the late Dr. E, R. Ladenburg in his experi- 

 ments on the effect of high temperature on the photoelectric 

 emission from platinum and other metals, which were begun 

 in the Gptvendish Laboratoiw in 1905. Ladenburg found * 

 that an increased photoelectric current was obtained as the 

 temperature or the metal under test was raised by an electric 

 current passed through it, and that this photoelectric current 

 reached a maximum value when the temperature of the 

 metal was about 100° C, after which it began to diminish. 

 This apparent change in the photoelectric activity, however, 

 gradually disappeared when the cathode was heated and 

 cooled several times, and Ladenburg concluded that the 

 increase obtained in the earlier series of observations was 

 due to water vapour. 



I am, Gentlemen, 



Yours faithfully, 



Frank Horton. 



* < Science Abstracts/ p. 339 (1907). 



