24 ELEMENTS OF ELECTRICAL ENGINEERING. 



rate at which work is delivered by the generator is El, and the 

 rate of generation of heat in the circuit is RP. When the whole 



o 



of El is used in the generation of heat we have El = RP or 



=RI (160) 



or 



7=| (166) 



This relation was established by Ohm in 1827 and it is called 

 Ohm's law. 



The volt. When R in equations ( 1 6) is expressed in ohms 

 and / in amperes then E is expressed in terms of a unit called 

 the volt. That is, one volt is an electromotive force sufficient to 

 maintain one ampere of current through a wire having a resistance 

 of one ohm. When, in equation (15), electromotive force is ex- 

 pressed in volts, and current in amperes, then the power P which 

 is delivered to a circuit is expressed in watts. 



When, in equations (16), current is expressed in abamperes 

 and resistance in abohms, then electromotive force is expressed 

 in c.g.s. units. The c.g.s. unit of electromotive force is called 

 the abvolt. When, in equation (15), electromotive force is ex- 

 pressed in abvolts and current in abamperes then the power P is 

 expressed in ergs per second. 



It is to be particularly noted that equations (14), (15), (16*2), 

 and (i6) apply to the whole or to any portion of an electrical 

 circuit. Thus if R is the resistance of a transmission line deliver- 

 ing a current / to a receiving circuit such as an electric motor, 

 then RP is the power lost in the line in watts, and RI is the 

 electromotive force lost in the line. This electromotive force lost 

 in the transmission line on account of line resistance is called line 

 drop of electromotive force or pressure, or simply line drop. Simi- 

 larly the electromotive force RI used to overcome the resistance 

 of the armature of an electric generator, is called armature drop. 



18. Series and parallel connections. When two portions of an 

 electric circuit are so connected that the entire current in the cir- 



