On Contact Electricity of Metals. 125 



The plate (a) was left in the apparatus all night, that is to say, in a 

 position sheltered from dust, and in air partially dried by the 

 sulphuric acid in the lower part of the case; the potential next 

 morning, 20 hours 3 mins. after the measurements given above, was 



+ 0-045 volt 



with same standard plate. It was thus nearly constant. The plate 

 (&) was left in a rack in the laboratory without special protection, 

 with the result that next morning the potential, owing to exposure to 

 the air of the room for about 20 hours, had fallen to 



-0-056 volt, 



a change of O'OOl volt. The plate (5) was now placed in the 

 apparatus, and remained nearly constant for 2 hours at least. Plate 

 (a) was left in the rack, and its potential fell 0"065 volt in about 2 

 hours. It is thus evident that at ordinary temperatures in a rather 

 dry atmosphere, and in a place protected from dust and light, the rate 

 of variation of the contact-potential of copper is very small ; in an 

 exposed place, however, it is no means negligible. 



32. A plate of block tin was polished on glass-paper. Its 

 potential varied with time as follows, the time being counted from 

 the completion of the polishing : 



Hours. Mins. Yolts. 



22 +0-515 



27 +0-520 



46 +0-535 



1 4 +0-535 



1 23 +0-515 



3 18 +0-495 



4 6 +0-495 



4 42 +0-495 



71 12 +0-465 



The plate was kept in the apparatus all the time and its potential 

 taken with a standard gold plate. 



33. In another experiment a tin plate polished on the finest 

 glass-paper gave, after hours 15 mins., 



+ 0-525 volt. 

 After 23 hours 30 mins. it gave 



+ 0-535 volt. 



This experiment was made in June, while the previous onejwas 

 made in December. Hence probably the greater rate of variation in 

 the former was due to a moister and less pure atmosphere. 



