42 Professors Ayrton and Perry's Experiments 



remainder may be utilized in producing useful work. For 

 example, if a time-curve be drawn for 44*25 AV when charging 

 a Faure accumulator, the area of the curve will measure the 

 total energy put into the accumulator in foot-pounds ; but 

 of this some portion has been wasted in heating the cell, due 

 to the charging having been more rapid than was absolutely 

 necessary. It was, of course, of considerable importance in 

 our experiments to ascertain what portion of the energy put 

 into the cell was really thus wasted; and to measure this the 

 following experiments were made. 



Occasionally the main current was stopped, the shunt cur- 

 rent through the voltmeter being left completed. The reading 

 now on the voltmeter gives the difference of potentials pro- 

 duced by the cell itself, whereas the previous reading was the 

 combined difference of potentials produced by the cell and the 

 dynamo-machine charging it. If now a new time-curve be 

 drawn in which the ordinates represent the product of 

 44 - 25 AV', where V is the electromotive force of the cell 

 measured on the circuit being broken, and A is the mean 

 value of the current flowing just before breaking and just after 

 closing the circuit, the area of the new curve will represent 

 that portion of the energy put into the cell which is usefully 

 employed in chemical decomposition. The difference between 

 the areas of these two curves represents, then, the amount of 

 energy wasted in heating the cell in foot-pounds. 



Again, on discharging the cell, experiments of a similar 

 nature have to be made. The product 44*25 AV represents 

 the number of foot-pounds of work per minute the cell is pro- 

 ducing in the external circuit, V being the difference of poten- 

 tials between the two poles of the cell while it is discharging; 

 but, in addition, there is a certain amount of energy which is 

 being expended in heating the cell itself during discharge. 



This, as before, may be ascertained by breaking the main 

 circuit, leaving the shunt-voltmeter circuit completed. The 

 reading on the voltmeter V now indicates the real electro- 

 motive force of the accumulator during discharge; whereas the 

 previous reading, obtained just before breaking the circuit, 

 represents merely the fraction of the total electromotive force 

 employed in sending the current through the external resist- 

 ance. If a time-curve be drawn with its ordinates propor- 

 tional to 44*25 AV', where A is the mean value of the current 

 just before breaking and just after closing the circuit, its area 

 will represent the total number of foot-pounds of energy per 

 minute being given out by the cell; and the difference between 

 the areas of the last two curves will represent the number of 

 foot-pounds of energy employed in heating the cell itself. It 



