EFFICIENCY OF ELECTRIC GENERATORS. 89 



the algebraic sum of the quantities of heat which they absorb 

 and liberate internally. 



In absolute measure the electromotive force of a cell is equal 

 to the quantity of calories liberated per one electro-chemical 

 equivalent of the exciting liquid, multiplied by the mechanical 

 equivalent of heat. 



The formula E = 0434 H e volts (see p. 24) is appli- 

 cable to batteries as well as to electrolytic baths. Let us take, 

 for example, the Daniell cell and observe that the chemical 

 work of this cell consists in the formation of sulphate of zinc 

 and in the destruction of sulphate of copper. The first reaction 

 liberates -f- 53 5 calories and the second one 28 * 2 calories, 

 so that the available heat, electrically speaking, is He = 53*5 

 28 2 = 25 3 calories. The electromotive force of a Daniell 

 cell is therefore E = 0'0434 x 25 -3 = 1-09 volt. 



In practice, the electromotive force is essentially variable 

 with cells which polarise, and it is tolerably constant, during 

 the life of the active substances, in cells with depolarising agents. 



The following is the value of the electromotive forces of the 

 cells which we have described in Chapter V., the figures having 

 been obtained after the measurement of a great number of cells. 



Volt. 



Daniell cell and its derivatives .. .. from 0' 96 to 1*08 

 Bunsen cell and its derivatives .. .. 1' 90 to 2 '03 



Leclanche cell 1 * 48 



Lalande and Chaperon cell 0*8 to 0*9 



Smeecell 0'47 



Clamond thermo-electric battery . . . . 036 



MAXIMUM CAPACITY OF A CELL. The internal resistance of 

 batteries is dependent upon the nature of the liquids in which 

 the electrodes are immersed ; it also increases with the distance 

 of the electrodes and decreases when the immersed surfaces are 

 increased. 



The capacity varies with the external resistance. 



The capacity of a cell is maximum when the external resist- 

 ance is equal to the internal resistance. 



This formula, which is correct when the mode of working and 

 the consumption of active substances are not taken into con- 

 sideration, is easy of demonstration. 



