Contact Difference of Potential. 303 



numbers give the contact difference in volts relative to a plate 

 of uncleaned copper. Judged by their appearance, the un- 

 cleaned copper and zinc plates must have lain in the laboratory 

 for manv years. 



" * Volts. 



Freshly cleaned zinc 0*826 



Uncleaned zinc 0'466 



Freshly cleaned copper 0*060 



Uncleaned copper 0*000 



The effect of cleaning was the same in the case of both 

 copper and zinc, but much more marked for the latter. With 

 lapse of time the surface of a metal, whether from oxidation 

 or from some other cause, changes so as to make the metal 

 more electronegative. To get the maximum contact difference 

 between two metals, the electropositive one must be quite 

 clean, and the electronegative one must have as much of the 

 opposite quality as possible. Thus, according to the above 

 results, while the contact difference between freshly cleaned 

 zinc and freshly cleaned copper was 0*766 volt, that between 

 freshly cleaned zinc and uncleaned copper was 0*826 volt. 

 It is probable that the surface of a metal exposed to the air 

 approximates to a state in which the potential of the metal is 

 equal to that of the air in contact with it. 



It is perhaps not too much to assume that, whatever be the 

 cause of contact difference of potential, it has its seat in 

 the surface and not in the interior of the metal. Possibly, 

 therefore, some light may be thrown on it by other electrical 

 effects at the surface of metals. One such effect is that of 

 ultra-violet light, and the experiment herein described was 

 devised with the object of obtaining measurements of the 

 contact difference of potential between zinc and copper while 

 and after the former was exposed to rays of ultra-violet 

 light. The result of the experiment was to show that, with 

 a powerful source of ultra-violet light, there is a decided 

 electronegative increase, which disappears slowly when the 

 source is removed. 



To understand what, on the electron theory, takes place 

 when a metal becomes more or less electropositive, let us 

 suppose we have two metals, A and B, in contact at the 

 same temperature and connected to earth, that is kept at 

 the same constant potential. From considerations bearing 

 on the Peltier effect at the junction of two metals, the 

 number of free electrons per unit volume in A will differ 

 very slightly from the number per unit volume in B. We 

 will suppose these numbers equal. If, now, by any means A 

 be made more electropositive^ there will be a flow of negative 



