PRACTICAL OPERATION OF DYNAMOS. 



203 



is usually calculated from ammeter and voltmeter readings. This 

 method is not however applicable in case of alternating current. 

 A method for measuring power which is always applicable is by 

 means of the wattmeter. The wattmeter is a special form of 

 electro-dynamometer, the connections of which are shown in Fig- 

 121. A coil of fine wire, A, is connected to the mains in series 



with a non-inductive resistance R. 

 This fine wire coil A is delicately 

 suspended inside of a stationary 

 coarse wire coil, B, through which 

 flows the current that is delivered 

 to the lamps or other receiving 

 units L. The force action between 

 the two coils A and B is definite 

 for a definite value of power de- 

 livered to L, be the voltage high and the current small, or the 

 voltage low and the current large. Therefore the deflection of 

 the movable coil, A, depends only upon the power delivered, so 

 that the scale of the instrument may be arranged to indicate 

 watts directly. The wattmeter is sometimes called the indicating 

 wattmeter to distinguish it from the watt-hour meter which is 

 sometimes improperly called a recording wattmeter. 



A wattmeter connected as shown in Fig. 121, indicates the 



0000 



z 



Fig. 121. 



Fig. 122. 



Fig. 123. 



power delivered to L plus the power lost in heating the coil B. 

 On the other hand, a wattmeter connected as shown in Fig. 122 

 indicates the power delivered to L plus the power delivered to the 



