ELECTROMOTIVE FORCES. 755 



ELECTROMOTIVE FORCES OF CONTACT. 



If ioo represents the difference of potential of the contact of 

 zinc and copper, the following are the values for the electromotive 

 force of contact of zinc with different metals ; from them can be 

 deduced the electromotive force of contact of the metals with each 

 other : 



Contact of Metals. 



Volta. Kohlrausch. Hankel. Ayrton and Perry. 



Zinc I Platinum .... 123 123 131 



,, Carbon 122 146 



,, Palladium .... 115 



,, Gold ..... 115 no 



,, Silver 109 109 118 



,, Copper ioo ioo ioo 



,, Iron 82 75 84 80 



,, Mercury .... 81 



,, Bismuth .... 72 



,, Antimony .... 69 



Tin 55 23 37 



,, Lead 45 44 28 



,, Cadmium .... 24 



,, Aluminum. ... -25 



According to Kohlrausch, the contact zinc-copper is about 

 half a Daniell, or 0-5 volt. M. Pellat* has found, on the con- 

 trary, 0*80 volt by direct electrostatic measures. It appears, on 

 the other hand, from an experiment of Sir W. Thomson,! that the 

 difference of potential of zinc and copper is not modified when 

 the direct contact is replaced by a drop of water; the contact 

 of zinc and copper would then be sufficient to give the total 

 electromotive force of a Volta's element, which appears to be 

 about 0*85 volt, in the first few minutes in which the fresh metals 

 are in contact with water. Professors Ayrton and Perry finally 

 estimate the electromotive force of the zinc -copper contact at 

 075 volts. 



The results obtained by various experimenters, for the con- 

 tact of metals with liquids, or of liquids with each other, do 

 not agree closely enough to enable us to state them in a 

 simple manner. 



* PELLAT. Journal de Pkys., Vol. ix., p. 123. 1880. 

 t JENKIN. Electricity and Magn., p. 46. 1873. 

 CCC 2 



