38 LABORATORY GUIDE IN PHYSIOLOGY. 



Join up 16 cells in series. Experiment has shown 

 that when cells are joined in series the internal resistance 

 increases in proportion to the number of cells, for the 

 current must pass through all of the cells ; further, the 

 electromotive force is reinforced as it passes through each 

 cell so that it also increases in proportion to the number of 

 cells. Our formula then would be : 



C ' = ifai+re > but ri = ' therefore, C'=^; C=16C. 



Therefore the current will increase in proportion to the 

 number of cells joined in series, when the external resist- 

 ance is incomparably greater than the internal resistance. 



CASE IV. 



Let the internal resistance be incomparably greater 

 than the external and join 16 cells in series, then : 



C '~ is^i+re > but re = ; therefore C'= ~~ = ~. 

 In this case, however, Cr=^; therefore there is no ad- 

 vantage gained by increasing the number of cells in series 

 when the external resistance is very small. 



CASE V. 



Practically, however, one deals with cases where 

 neither the external nor the internal resistance is so 

 small as to be ignored. Let us suppose that we have a 

 battery of a cells, that the internal resistance of each cell 

 is r and that the total external resistance is R. It 

 has been shown experimentally that the current is great- 

 est when the external resistance is equal to the internal 

 resistance; i. e., when -^j- = R; s being the number of 

 cells in series and m the number in multiple arc. 



