422 ELECTRICAL MACHINERY 



Various types of frequency meters have been designed, 

 one of the more common types being the vibrating reed 

 frequency meter. A series of steel reeds, of different natural 

 periods of vibration, are so mounted that they are all 

 attracted by an electro-magnet, which is connected across 

 a line the frequency of which is required. Although the 

 alternating magnetic field acts on all of the reeds only that 

 one whose natural period of vibration is the same as that of 



FIG 278. A Watt-hour Meter for Use on a-c. Circuits. Westing- 

 house Elec. and Mfg. Co. 



the magnetic field will vibrate appreciably. The natural 

 frequency of the different reeds is marked on the face of 

 the meter; by noticing which reed is vibrating the operator 

 can tell at a glance the frequency of the line. 



Another type of frequency meter consists of a set of 

 coils connected to the line through inductances and resist- 

 ances and a movable iron vane which carries a pointer. 

 The combination of coils is such that the iron vane assumes 



