OF ARTS AND SCIENCES. 259 



XVIII. 



NOTE ON THE PRESSURE COEFFICIENT OF THE 

 VOLTAIC CELL. 



By Caul Barus. 



Presented January 14, 1891. 



Sdbjecting cells of Zn/Pt and of Zn/Cu, in dilute sulphuric acid 

 and Daiiicll's cells, to pressures between 10 and 1800 atmospheres, 

 at temperatures between 15*^ and SOO*^, I obtained certain results 

 bearing on polarization, the temperature coefficient, and the pressure 

 coefficient of the galvanic couple. For the purposes of the present 

 note I need only give the pressure coefficient, k, of the Daniell's cell 

 at lo"^, K = — 5/10^' (volts per atmosphere), whereas the ordinary 

 temperature coefficient is — 200 10' (volts per degree). 



The feature of these results, corroborated by my other data, is 

 obvious : the pressure coefficient (volume contraction) and the tem- 

 perature coefficient (volume expansion) have the same sign. Hence, 

 quite apart from changes of volume, quite apart from what in a liquid 

 corresponds to the mean free path of the molecules of a gas, the 

 observed change of electromotive force is to be associated with the 

 change of the stability, or the vibration status of the galvanic system 

 as a whole. I infer from this, that the secret of the relation of the 

 Volta contact to the Peltier contact, will probably fall into the posses- 

 sion of him who devises means for carrying a suitable galvanic cell, 

 suitably compressed, through a large range of temperature (I mean 

 fully into red heat). For it is thus possible to discriminate between 

 the metallic contacts and the other contacts, by exposing any one or 

 all of them in the thermal field. 



An allied electrical result, indicating a specific effect of temperature 

 even on the metallic molecule, I observed some time ago by com- 

 pressing mercury and a solution of zinc sulphate. I deducted the 

 isopiestic resistance decrement per unit of volume decrement, from 

 the corresponding isothermal resistance decrement per unit of volume 

 decrement, of the same substance. Thus I found that the purely 

 thermal efEect of rise of temperature (i. e. the effect apart from change 



