The Tin- Chromic Chloride Cell. 



445 



current ol any magnitude at low temperatures, but at high 

 temperatures it does. 



+ pole. 



ft 



Platinum electrode. 







// / / / 



Chromic chloride solution. 



Tin amalgam. 



If the E.M.F. of the cell be measured on a potentiometer, 

 or by means of a ballistic galvanometer and condenser, by 

 either of which methods the cell is not called on to produce a 

 current, it is found that the E.M.F. at all temperatures is 

 approximately \ volt, and that the E.M.F. is slightly less at 

 the boiling-point of water than at the ordinary temperature. 



The explanation of the results is not difficult. The cell 

 polarizes very rapidly at low temperatures, and the opposing 

 E.M.F. of polarization increases so rapidly and so largely 

 that almost at the moment of connexion the effective E.M.F. 

 becomes zero. The cell behaves like a condenser which is 

 connected to a battery through a very high resistance. Such 

 a condenser would discharge itself when its plates were con- 

 nected, but when they were isolated again it would slowly 

 become charged by the battery through the high resistance. 



At the high temperature the polarization is very largely 

 reduced, and the cell will produce a continuous current. 



Another interesting feature of the cell is that it may be 

 used as the mechanism of a heat-engine for the production of 

 work, and we can trace in it the complete cycle of Carnot. 

 Let the cell be placed in a hot chamber, and work may be 

 derived from it until all the Cr 2 Cl 6 or the tin is used. Then 

 let the cell be placed in a cold chamber, it will give up heat, 

 and becomes restored to its original chemical condition. 



Measurements of the electromotive force of two cells are 



