84 



ELECTRICAL ENGINEERING 



The drop of voltage across the resistance RI is 



E l = IR l 



where I is the current in the circuit. 

 Similarly 



E 2 = IR 2 , 

 and 



but 



E = E! + # 2 + E 3 



and the resistance of the whole circuit is 



Tjl 



R = - = RI + Rz 



(118) 



62. Resistances in Parallel. Determine the resistance of 

 a circuit consisting of a number of resistances RI, Rz and R* 

 (Fig. 67) connected in parallel. 



FIG. 67. Resistances in parallel. 



If an e.m.f. E is applied to the circuit a current 7 will flow 

 which will divide up into branch currents 7i, I 2 and h. By 

 Ohm's law 



E 



and 



h = lk' 



T E 

 7 = s' 



where 72 is the resistance of the whole circuit, 



