AVAILABLE WORK OF A VOLTAIC BATTERY. 685 



so that the expenditure reduces itself to that of the corresponding 

 sulphuric acid. 



We may estimate the price of these various substances at ^/^d. 

 each for the zinc and nitric acid, and id. for the sulphuric acid; but 

 the cost of amalgamating the zinc, so as to protect it from waste, will 

 double the price. We have then 



COST OF THE HORSE-POWER-HOUR. 



For Daniell's Cell 



Bunsen's 



It is important to observe that only a fraction of this energy can 

 be utilised. For the conditions of maximum work, all the preceding 

 numbers should be doubled. 



The comparison of this expenditure with that of steam engines 

 shows how illusory it would be to seek for any economy in the use 

 of batteries as sources of energy. Moreover, the liquids are only 

 useful when they have the proper concentration, or when they are 

 not charged with other bodies in solution ; and, lastly, local actions 

 are always produced oi\ the electrodes, which represent an unprofit- 

 able expenditure of energy.. All these causes contribute to increase 

 the cost of this source of electrical energy. 



1243. AVAILABLE WORK. The work required from the current 

 consists in overcoming either a resistance R', or an electromotive 

 force E', or both together; if the intensity is I, the useful work 

 is R'l 2 , E'l, or R'I 2 + E1, according to circumstances. Moreover, 

 wires offering a resistance p are necessary for connecting the electro- 

 motor with the points where the work is done, so that the total 

 unutilised resistance is 



The work U really utilised is then U = EI-RI 2 , and the 

 expression for the efficiency 



_U_ _RI 

 ~W~ E ' 



