PRINCIPLES OF ALTERNATING CURRENT 209 



measurement, care must be exercised in the relative arrange- 

 ment of the meters. In the connection diagram in Fig. 126, 

 for example, the wattmeter, W, is recording not only the 

 amount of power used in the circuit being tested but also 

 the power used up in the voltmeter V and the ammeter A. 

 The power used in each of these instruments will generally 

 be less than 5 watts, hence if the circuit B is using several 

 hundred watts, no appreciable error is introduced by the 

 power consumption of the meters. A more complete dis- 

 cussion of this point may be obtained from books specializ- 

 ing in a-c. measurements. 



The power factor of circuit B is obtained at once by the 

 ratio of the wattmeter reading to the product of the read- 

 ings of the voltmeter and ammeter. 



50. Resistance of an Alternating Current Circuit. When 

 an alternating e.m.f. is impressed upon a circuit con- 

 sisting of incandescent lamps, water rheostats, straight 

 wires, etc., the amount of current which flows is just the 

 same as would flow if a continuous e.m.f. of the same 

 voltage were impressed. It is said that such a circuit 

 possesses only ohmic resistance; this is a poor term but 

 is used by some writers and, therefore, is here given. It 

 is used to designate a circuit in which the only opposition 

 to the flow of the current is offered by the conductor of which 

 the circuit is composed; we shall call this the conductor 

 resistance. In many circuits this is not the only resist- 

 ance which the circuit offers, as will be explained hereafter. 



Effective Resistance. We shall use the term effective 

 resistance of an a-c. circuit in general; it may (as in the 

 case cited above) be the same as the conductor resistance 

 and in other cases it may be much more. The effective 

 resistance can be less than the conductor resistance, only 

 in very special cases. 



For any a-c. circuit we may put 



Watts =I 2 R, (44) 



