176 SOURCES OF ELECTRICITY. 



A charge of electricity M may be imparted to the system of the 

 two plates, which raises the zinc to a potential V, for instance. In 

 this case the same difference of potential is still maintained at the 

 surface of contact ; when the plates are separated the charge on each 

 is made up of the charge m due to the difference of potential 

 of contact, together with part of the common charge M, which has 

 been divided between them according to the ordinary laws of dis- 

 tribution. 



The experiment presents great difficulties when it is attempted 

 to deduce the difference of potential from the magnitude of the 

 charges ; for the real distance of the plates depends on the degree 

 of polish of the surfaces ; the plates are often in contact in only a 

 small number of points, and the capacity of the system may vary 

 within considerable limits. 



More regular results are obtained by keeping the plates parallel 

 to each other at a certain distance e and connecting them by a 

 wire of copper or of zinc. The difference of potential SV pro- 

 duced at the point of contact, is maintained over the whole extent 

 of the two plates ; the distance of the corresponding layers is sensibly 

 equal to the thickness of the dielectric which separates them, and 

 the capacity of the system is inversely as this thickness. If, after 

 having suppressed the external contact, the plates are moved away 

 from each other, then, if S is the extent of surface, 



This experiment will enable us to determine the absolute value 

 of 8V. In all cases, if a constant distance e is maintained between 

 the plates, and if different metals are employed, the ratios of the 

 electromotive forces of contact may be determined. 



As a matter of fact, the experiments are very delicate, owing to 

 alterations in the results by very slight modifications in the state 

 of the surfaces. The very nature of the gas which constitutes the 

 dielectric seems to have a slight influence ; it may be that the layer 

 of gas, adhering to the metal, changes its physical properties, or that 

 some particular chemical compound is formed there. 



188. VOLTA'S LAWS. LAW OF CONTACT. However this may 

 be, Volta's ideas have been confirmed, by whatever progress has been 

 made in electricity, and the following law may be enunciated as the 

 law of contact: 



If two bodies are in contact at the same temperature^ a finite 



