ELECTROMOTIVE FOKCES IN THE VOLTAIC CELL. 



47; 



any given locality has the value ordinarily assigned to it as the result of 

 experiment. 



The earliest attempt made to examine the question as to whether the 

 Volta effect depended on the atmosphere was made by Pfaff ' in 1829, who 



Fig. 9.— Pellat's Apparatus for experimenting in different Gases and at different 



Pressures. 



The movable plate is now the lower one, and it is pulled down bv an electromagnet e a little 

 way against the spnngs 1;, which tend to drive it up against 'the screw stops <■ Contact 

 is automatically broken at q the instant before separation. The bell jar has 35 litres 

 capacity, the diameter of each plate being 15 centimetres. It must be impossible to'emnW 

 anything like pure gases in a bell jar enclosing such a bulky mass of heterogeneous mate 

 rial ; and the pressure was found not to go below 2 or 3 centimetres of mercurv However 

 he has since made a smaller arrangement of 1 litre capacity, with plates 9 centimetre 

 diameter, and, what is more nnportant, with the electromagnet outside, arid nothing inside 

 but glass, mica, and metal. In this the pressure goes down to a millimetre But even 

 this is not all that could be wished. Moreover the experiments described had been made 

 with the larger apparatus. 



"^ dr y and dam P air > oxygen, nitrogen, hydrogen, carburetted hydrogen 

 and carbonic acid, and he found that there was no difference so Ion* as 

 no visible chemical action occurred; but it must be noted that the opposing 

 faces of his plates were varnished. De la Rive, on the other hand, asserted 



" Tfaff : Aim de Chlm., 2 ser. xli. 236. The metals he employed were copper, tin 



ana zinc. L r ' ' 



