40 



ELEMENTS OF ELECTRICITY AND MAGNETISM. 



of a generator is called the electromotive force drop or voltage drop 

 in the generator. It is analogous to the decrease of pressure- 

 difference between the terminals of a fan blower due to the resist- 

 ance which is encountered by the stream of air in passing through 

 the fan chamber. 



Voltage drop in a transmission line. A current of 7 amperes 

 is delivered to a distant lamp or motor over a pair of wires the 

 combined resistance of which is R ohms. Let E Q be the elec- 

 tromotive force across the terminals of the generator, and let E l 

 be the electromotive force across the terminals of the distant lamp. 



o 



reference axis 



Q' 



large return pipe 



Fig. 17ai. 



The difference between the voltage across the terminals of the 

 generator and the voltage across the terminals of the lamp, namely, 

 E E l is equal to the electromotive force which is used to over- 

 come the resistance of both wires, namely, RI volts. This loss 

 of electromotive force over a transmission line is called the volt- 

 age drop over the line. 



Example. The electromotive force across the terminals of a 

 generator is 115 volts. The generator supplies 100 amperes of 

 current to a motor at a distance of 1,000 feet, and the wire (2,000 

 feet) used for the transmission has a total resistance of 0.05 ohm. 

 The voltage drop over the line is 100 amperes x 0.05 ohm, or 5 

 volts, and therefore the voltage across the terminals of the motor 

 is 1 1 5 volts 5 volts = 1 10 volts. 



Hydraulic analogue of voltage drop. Definition of potential 



