Prof. Franz Exner on Contact Electricity. 281 



motive force produced, and the heats of combustion of the 

 metals. 



This latter has been already published in the old experi- 

 ments of Pavre, Favre and Silberinann, and, in later times, 

 in the comprehensive thermo-chemical researches of J. Thom- 

 sen. The data of this last author may well claim credit 

 for the greatest exactitude and correctness ; therefore in all 

 the following calculations his figures only are taken. 



Far less agreement exists between the works of various 

 authors with regard to the electromotive force at contact 

 of two metals ; here, no doubt, the great difficulty of 

 the experiments causes considerable differences. Never- 

 theless it follows, from the evidence of all known experiments, 

 that the tension series is identical with the series of oxida- 

 bility. 



Even the first experiments of Volta * show this. Volta 

 finds the tension series, with dry contact of the metals, Zn, 

 Pb, Sn, Fe, Cu, Ag, Au, C, thus always descending from the 

 easily oxidizable metals to the unoxidizable. 



These, although only qualitative experiments, must yet draw 

 attention to the part played by oxidation. For quantitative 

 results without reference to any known unit of electromotive 

 force, we have to thank Hankel |» He finds when the tension 

 between Zn and Cu is 100 : — 



Al. Zn. Pb. Hg. Fe. Cu. Au. Ag. C. Pt. 

 220 200 156 119 116 100 90 82 78 77 



These numbers (at least with the exception of mercury) 

 correspond with those mentioned above. Gerland % ob- 

 tained similar results. He finds, for instance, that, repre- 

 senting the difference of potential between Zn and Cu as 

 Zn|Cu, Zn|Cu = 100, Zn|Ag = 109, Zn|Au = 115. 



When, however, we wish to have a distinct view of the cause 

 of these differences of potential, it is certainly of the greatest 

 importance to know, not only their relative, but also their abso- 

 lute value — that is, the ratio they bear to a known electromotive 

 force, e. g. that of a Daniell cell. This important determina- 

 tion was, so far as I know, first made by R. Kohlrausch; and 

 since then unfortunately only isolated and unreliable observa- 

 tions have appeared. 



Kohlrausch§ originated the following method : — He formed 

 a condenser of the metals to be examined, connected it with 

 the poles of a Daniell cell, first in one direction, then in the 



* Ann. de Chcm. ct Phys. xl. p. 225. 



t Pogg. Ann. cxxvi. p. 440, 1865 ; cxxxi. p. 007. 



\ Pogg. Ann. cxxxiii. p. 513, 1868. 



§ Pogg. Ann. lxxxii. p. 407, 1851. 



Phil. Mag. S. 5. Vol. 10. No. 62. Oct. 1880. X 



