86 ELECTRICAL ENGINEERING 



and 



and 



bj = //t3 + IzRz 

 (~T i T \ P 



= V,-* i \ * 2y -ts ~ 



but Iz = /i ^> 



therefore, 



and 



Ii = n 



and 



The power lost in the potentiometer is 

 the power output is 



7} T 2E> 



the efficiency of the potentiometer is 



output output 



T) = -T - X 100 per cent = - - X 100 per 



input output + losses 



P 7i 2 /2i 



= 5- - X 100 per cent = y-r^ ^ ^ X 100 per cent; 



r + p li-JKi + Iz til + l*tXk 



but 7 2 = 1 1 7^ and 7 = 



and therefore 



7" 2p 



X 100 per cent 



100 per cent. 



This method of varying the voltage supplied to a receiver circuit 

 is very inefficient. 



