Difference of Potential of Distilled Metals. 341 



of beeswax and resin which had been subjected to prolonged 

 heating to drive off vapours. The bulb B, provided with 

 electrodes, served to test the vacuum during the course of 

 the experiments. Its diameter was about 15 cm. 



The method of experiment was as follows : — The electrode 

 E connected to the electrometer, previously earthed, was 

 insulated. Then the disk, connected to earth through the 

 •spindle, was rotated. Since the disk is always equidistant 

 from E, no change will take place in the deflexion of the 

 electrometer as the disk is rotated, provided that the portion 

 of the disk opposite E is always of the same metal. But if a 

 part of the disk covered by another metal comes opposite to 

 E, the deflexion of the electrometer will alter. By 

 applying a suitable potential to the disk (through the 

 spindle), the deflexion can be brought back to its original 

 value. The potential necessary to do this is the contact 

 difference of potential between the two metals on the disk. 

 The method was tested by electroplating a brass disk of the 

 same size as the glass disk with copper on one part and zinc 

 on the other, and measuring the contact difference of 

 potential between them. It was found to be *75 volt, which 

 agrees well with the values given by other methods. 



Two typical sets of results will be given. The apparatus 

 was exhausted by means of the molecular pump. After 

 about 15 minutes, a patch of zinc was distilled on to a portion 

 of the disk. The contact potential between this patch and 

 the rest of the platinum surface was measured as described 

 above. 



f 2 uihis. after Zn electropositive I #f>1 lf 



(a)\ distillation. to Pt by j -i voir. 



[ 10 mins. later. „ '23 „ 



Air was then admitted to the " rough vacuum J ' of the 

 molecular pump to a pressure of 1 cm. 



( Zn electropositive | . 601 



to Pt by J 



I f) mins. later. ,, '79 ,, 



(b)i& „ .. „ "87 ,. 



j 5 ,. „ „ '92 ., 



! 5 .. „ „ -86 „ 



15 „ „ H '80 „ 



Rough vacuum pumped out. 



5 mins later. 



Zn electropositive"! 01 ... 

 toPtby j-Slrolt. 



•83 



•87 



