428 MEASUREMENT OF ELECTROMOTIVE FORCES. 



proportional to e' and to e-e'\ but in this case the resistance 

 and the electromotive force correspond to the closed battery.* 



1023. SEAT OF THE ELECTROMOTIVE FORCE. In any battery, 

 open or closed, the electromotive force should be regarded as the 

 algebraical sum of the differences of potential existing in the circuit. 

 With the exception of those which are due to the Thomson effects 

 (276), these differences of potential should be sought for at the 

 various bounding surfaces of solids and liquids. 



Volta ascribed the seat of the electromotive force solely to the 

 contact of the two metals, the function of the liquid being merely 

 to reduce the metals of two successive pairs to the same tension, 

 or, in other words, to the same potential. An ingenious experiment 

 of Sir W. Thomson seems to confirm Volta's theory, f 



Two insulated half-circles, or two half-rings, the one of zinc and 

 the other of copper, are placed below a light needle suspended 

 horizontally. They are brought almost in contact, and so that 

 the surface of separation is exactly in the plane of equilibrium 

 of the needle. When two points of the ring are joined by any 

 conductor, and the needle is positively electrified, it turns towards 

 the copper, and if it is negative towards the zinc. When the 

 apparatus is well adjusted, these deflections on either side are 

 equal for equal charges and of opposite sign to the needle. Ex- 

 periment demonstrates then the general fact of the difference of 

 potential of two metals in contact (186). But if, instead of con- 

 necting the two rings by means of a solid conductor, they are 

 connected by a drop of water or of alcohol, the needle remains 

 in the plane of equilibrium whatever be its charge, proving that 

 the two metals are at the same potential. 



In order to determine the difference of potential of the two 

 metals, Sir W. Thomson connects them respectively with the two 

 points of a conductor traversed by a current (1007), and seeks that 

 position of the movable contact which reduces the needle to zero. 

 He thus found that the difference of potential changes with the 

 state of the metals. For instance, it increases considerably when 

 the copper is oxidised by heating it and when the zinc plate is 

 carefully scraped. 



1024. MEASUREMENT OF THE ELECTROMOTIVE FORCE OF 

 CONTACT. CASE OF Two METALS. A general method of deter- 



* KEMPE. Handbook of Electrical Testing, p. 195. 



t Sir W. THOMSON. Reprint of Papers on Electricity and Magnetism, 

 p. 317. JEN KIN. Electricity and Magnetism, p. 48. 



