212 ELECTRICAL MACHINERY 



Resistance Reaction. The resistance reaction, of course, 

 opposes the flow of current so when the current is positive 

 the resistance reaction is negative and vice versa. As 

 this reaction is always equal to iR (i being the instanta- 

 neous value of the current) it must be a sine curve similar 

 to the current. But if it is a sine curve it may be properly 

 represented by a rotating vector, in phase opposite to the 

 vector representing the current in the circuit. In Fig. 127, 

 01 represents the current in a non-inductive circuit and OA 

 the resistance reaction. As the impressed force must always 

 be equal and opposite to the sum of the reactions in the 

 circuit it is properly shown in Fig. 127 by the vector OE. 



Resistance reaction 



A 



Current ^ 



FIG. 127. Vector Diagram of Resistance Reaction. 



\ A, 



This construction brings the current in phase with the 

 impressed force as we know it should be in such a circuit. 



52. Inductance. Suppose a circuit is made up of 1000 

 feet of No. 10 copper wire running out 500 feet and back, 

 and that there is no other resistance in the circuit except 

 that of the wire, which will be approximately one ohm. 

 If 100 volts of alternating e.m.f. of frequency 60 cycles 

 were impressed on this circuit the current which would 

 flow would be about 100 amperes; the only reaction which 

 the circuit would offer to the impressed force would be a 

 resistance reaction. 



If now this same piece of wire is wrapped up in the form 

 of a coil, say about one foot in diameter, and 100 volts 

 (alternating and of the same frequency as before) is 



