43 2 MEASUREMENT OF ELECTROMOTIVE FORCES. 



places the liquid in a siphon (Fig. 211) which on one side is widened 

 out as a funnel, the liquid being level with the edge of the funnel. 

 The surface of the liquid forms one plate of the condenser, the 

 other being the copper plate B, which, by means of the platinum 

 wire /, is in connection with the metal M to be investigated. This 

 dips in the second limb of the siphon, and is also put to earth by a 

 platinum wire/". 



If the connections / and p" are broken, and, raising the plate B, 

 it is connected with an electrometer by the wire /', the charge is 

 proportional to the difference of potential V l of the copper and 

 of the liquid that is to say, to the sum of the electromotive 



Fig. 211. 



forces of contact of copper with the metal M, and of this again 

 with the liquid L, 



The funnel is emptied, and instead of the surface of the liquid, a 

 plate of the metal M is used, which is connected by a platinum 

 wire with B and with the earth ; for the same distance the charge 

 observed corresponds to the difference of potential 



whence 



If as lower plate A (Fig. 210) we take the surface of the liquid in 

 question, connected by a siphon with the metal of the plate A', 

 Ayrton and Perry obtain the electromotive force of contact of the 

 metal or of the liquid. 



