PRELIMINARY NOTIONS. 13 



E 



C the intensity ; E the resistance ; C is equal to ^ , from which 



E 



E = C E and K = ^ or, in other terms : the intensity of a 

 C 



current is obtained by dividing the electromotive force by 

 the resistance ; the electromotive force is equal to the product 

 of the intensity by the resistance ; and the resistance is equal 

 to the quotient of the electromotive force divided by the 

 intensity. These formulae are of common use in laboratories 

 and in the electrical industries. 



ELECTRICAL WORK. The quantity of energy of a current 

 is equal, in kilogrammetres, to the product of the amperes 

 multiplied by the volts and divided by g (9*81). 



_, CE CE . .. 

 W = = ~ Q= kilogrammetres. 

 g yl 



By combining this equation with those derived from Ohm's 

 law, the work in relation to resistance can be expressed as 

 follows : 



172 T> f12 



W = \ and W = . 

 9& g 



Thus the electrical energy or work which can be obtained 

 from a given current can be calculated by either multiplying 

 the ohms by the square of the amperes and dividing by 9' 81 ; 

 or by multiplying the amperes by the volts, and dividing by 



9 81 ; or again, by dividing the square of the volts by the 

 product of the ohms multiplied by 9 '81. 



A sufficient approximation is obtained by substituting 



10 to 9* 81 ; this simplifies the calculations, which, however, do 

 not present any practical difficulties. 



For example, if a machine gives a current of 750 volts and 

 10 amperes, it can be at once asserted that it develops a work 



equal to * = 750 kilogrammetres, or 10 horse-power; 



and should the current be sent through another dynamo having 

 a known efficiency of 80 per 100, Prony's brake indicates a 

 work of 8 horse-power really produced on the spindle of that 

 second machine. 



