216 Prof. E. F. Herroun on the Divergence of 



Thus if the cell be one giving an excess, or E— E;i= +6? 

 there should be an absorption of heat when the cell is worked 

 forwards and an evolution when worked backwards against 

 the E.M.F. of the cell. Exactly the reverse would hold 

 with cells giving a deficit from the theoretical value. 



Since the heat due to resistance will be evolved in both 

 cases, we have for the net heat evolved 



T-r c^ r t + e c t 

 ^= J ' 



and where the value of e ct is negative we have a negative 

 total if it exceed the value of c^ r t. 



In order therefore that the absorption of heat may be a 

 maximum, with cells giving an excess of E.M.F. when 

 worked forwards, or with cells giving a deficiency when 

 worked backwards, the current-strength must be adjusted in 

 accordance with the determined value of the internal re- 

 sistance of the particular cell. 



Since the heat absorbed will be 



TT ect—(?rt 

 -Hi = J , 



or ~Hi Qcec — G^ry 



and -T-=e—2cr; 



etc 



equating to maximum value we have 



_ e 



'^~^r 



as the current-strength giving the greatest absorption of heat 

 in the case of a cell in which the computed and observed 

 E.M.F.s differ by e volts and internal resistance is r ohms. 



It is thus a matter of importance to determine the internal 

 resistance, not only in order to be able to calculate the heat 

 generated against it, but also to calculate the best current- 

 strength for observing the eff'ects. In many cells, however, 

 the resistance is so unavoidably high that c would be too 

 small for any thermal changes to be detected ; and in other 

 cases the current has to be increased above the best theoretical 

 to a best practical strength, time being an important factor in 

 such delicate thermal measurement. 



It was found experimentally that the resistance in many 

 cells is subject to considerable and, in some cases, enormous 

 variations when currents are passing through the cell, being 

 partly dependent upon the direction of the current. This 



