Difference of Potential of Distilled Metals. 339 



surfaces, but also for high-speed electrons released by Rontgen 

 rays. Consequently, it may be inferred that the processes of 

 emission of electrons from metals by light and by Rontgen 

 rays have much in common. There is not the slightest 

 evidence for believing that the emission of high-speed 

 electrons from metals struck by Rontgen rays, depends in 

 any way upon the presence of occluded gases, and conse- 

 quently we should not expect the photo-electric effect to 

 depend, directly at least, upon the gases in the surface. The 

 value of Y may be a function of the amount of gas absorbed 

 in the surface, and on this view the complete removal of 

 occluded gases may cause V to increase to such a value that 

 the long wave-length limit of the photo-electric effect is 

 beyond the part of the spectrum used by the investigators. 



Ever since contact difference of potential between metals 

 was first observed, two essentially different theories as to its 

 nature have been advanced by physicists and, as yet, no 

 experiments have been made which can be said to support 

 one and disprove the other. According to one theory, two 

 metals in contact are at different potentials ; on this theory 

 the contact potential is a specific property of the metal itself. 

 The other theory maintains that the difference of potential 

 observed is the difference of potential between the gaseous 

 films on the surfaces of the metals and not between the metals 

 themselves. The difference of potential between the film and 

 the metal depends upon the nature of the metal and on the gas, 

 and hence this theory may be called the chemical theory 

 of contact potential. The great difficulty of removing or 

 altering the exceedingly tenacious films of gas which adhere 

 to metallic surfaces as soon as they are exposed to air, has 

 been an obstacle in the way of obtaining definite information 

 as to the nature of contact potential. 



The novel feature about the experiments described in this 

 paper is that the metals were prepared by distillation in vacuo, 

 and measurements of the contact potential were made before 

 the metals had been exposed to air at any appreciable 

 pressure. The apparatus used is shown in fig. 1. The 

 base is a brass plate about 35 cm. in diameter and 6 mm. 

 thick. Through the centre passes a vertical steel rod or 

 spindle, which is turned down to a point at the upper end. 

 A plate glass disk G with a circular hole in the middle is 

 supported by a brass cap C, which in turn rests on the steel 

 spindle in such a way that the cap and disk can rotate freely. 

 The position of the disk was controlled magnetically from 

 outside. An almost opaque film of platinum was deposited 

 on the under side of the glass disk from a platinum cathode. 



Z2 



