I [ 279 ] 



XXXIV. Ihe Effect of an Electric Current on the 

 Photoelectric Effect. 



To the Editoi's of the Philosophical Magazine. 

 Gentlemen, 



IX the June number of the Philosophical Magazine, 

 Mr. Shenstone has described experiments in which a 

 current through a plate or film o£ bismuth appeared to pro- 

 duce a considerable increase in the photoelectric emission 

 from that metal. He concludes that his results are best 

 explained by assuming that the electric field applied to the 

 bismuth causes an orientation of the elementary crystals of 

 the metal, and that this orientation is responsible for an 

 increased photoelectric actiyity. It seems to the writer that 

 the possibility of the effect being due to the evolution of 

 gases from parts of the apparatus, which become warmed by 

 the current through the bismuth, needs to be taken into 

 consideration. 



No mention is made in the paper of precautions being 

 taken to remove occluded gases from the glass and metal 

 parts of the apparatus before making the tests, and if this 

 were not done a gas-pressure sufficient to cause a consider- 

 able increase in the measured current might exist so long as 

 parts of the apparatus were warm, even with the Langmuir 

 diffusion-pump in action. On stopping the heating, the 

 supply of gas would practically cease and the action of the 

 pump would quickly improve the vacuum and thus reduce 

 the measured current in a manner similar to that recorded by 

 Mr. Shenstone. In the case of the experiments with the bis- 

 muth plate, this is stated not to have been appreciably warmed 

 by the current through it, but it is possible that the leads, 

 and the glass or cement near them, became warm and 

 emitted gas when the current was passed. In the case of 

 the films through which much smaller currents were passed, 

 but which were more strongly heated, since they ultimately 

 burned out, gas would probabty arise mainly from the 

 film itself and from the substance on which it was deposited. 



The current measured as the " photoelectric current " in 

 these experiments is that carried by the electrons liberated 

 from the bismuth under the influence of the mercury arc 

 used as a source of ultra-violet light, together with that due 

 to any other source of ionization in the apparatus. The 

 most important of such sources is no doubt the bombard- 

 ment of the residual gas in the apparatus by the photo- 

 electrons from the bismuth. The difference of potential 



