550 ELECTRICAL MEASUREMENTS 



In this case, the bank of reeds is supported from the base of the 

 machine in question. Owing to the lack of exact balance in the 

 moving parts of the machine, a vibration exists which is sufficient 

 to set the reeds in motion and thus indicate the speed. 



Induction Frequency Meter. The essential features of the 

 induction frequency meter made by the Westinghouse Company 

 are shown in Fig. 334. 



At A and C are two shaded-pole motor elements which tend to 

 rotate the movable element B in opposite directions. A is in 



FIG. 334. Westinghouse induction frequency meter. 



series with an inductance, C in series with a resistance. The 

 movable element is a flat plate of aluminum; its boundary above 

 the dotted diametral line is a semicircle with its center in the 

 axis of rotation, while below the same line it is practically a 

 semicircle with its center shifted a little upward along the line. 

 The torque exerted by each element is proportional to the fre- 

 quency and to the square of the current. If the voltage of the 

 circuit varies, both elements are affected and the movable sys- 

 tem is not disturbed, but if the frequency rises, less current will 

 flow through the inductance and the effect of the other element 

 will preponderate. The disc will begin to turn toward the left 

 but on account of the shape of the disc this brings less of it under 

 the influence of the element C, so that it takes up a new equilib- 



I FnCrEHTY OF ELECT; fORTl 



FACULTY'OF APPLIED SCIIHCE. 



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