88 ELECTROLYSIS. 



CHAPTEE VII. 



CAPACITY AND EFFICIENCY OF ELECTRIC GENERATORS. 



Definitions Electromotive Force of Batteries Maximum Yield of a Cell 

 Electrical Efficiency of a Battery Maximum Capacity of a Battery com- 

 posed of a certain number of Cells Electromotive Force of Dynamo- 

 electric Machines Maximum Capacity of Dynamo-electric Machines. 



DEFINITIONS. The capacity of an electric generator is the 

 quantity of useful work which it produces. It is equal to the 

 total work generated by the apparatus, less the work absorbed 

 by its internal resistance. 



The electrical efficiency is the proportion of the capacity to 

 the total work. For example, a machine having an internal 

 resistance of 1 ohm and producing a current of 25 amperes with 

 a fall of potential equal to 100 volts at the terminals has a capa- 



25 x 100 

 city of - --^ = 250 kilogrammetres. The external resist- 



1 00 

 ance consequently is -gg- = 4 ohms ; the total work generated is 



_ 2 



C 2 (r -4- r'} 25 x 5 



-~ ' = ^~ = 312*5 kilogrammetres and the electrical 



250 

 efficiency is = 08. 



The mechanical efficiency of a machine is the proportion 

 between its capacity and the initial work expended for keeping 

 it in motion. If we suppose that in the preceding example 

 the work expended was 500 kilogrammetres, the mechanical 



250 

 efficiency becomes ^r = 50. 



ELECTROMOTIVE FORCE OF BATTERIES. The electromotive 

 force of batteries is not dependent on their sizes, but solely on 



