PHASE METERS 547 



nate at 68 cycles, the other at 52 cycles; each circuit is connected 

 to one of the crossed coils which tend to turn the system in oppo- 

 site directions. The fixed coil carries the total current. 



If the frequency is high, in the neighborhood of 68 cycles, 

 the 68-cycle member of the system will carry a large current 

 while the current in the 52-cycle coil will be small. The effect 

 of the 68-cycle coil will preponderate and it will turn toward the 

 left, carrying the 52-cycle coil with it. As it turns, the 68-cycle 

 coil moves to a less advantageous position while the 52-cycle 

 coil is moved toward the position where its effect will be a maxi- 

 mum. The movable element thus arrives at an equilibrium 



C R Jk 



-=] WSAA/V 1/ >}- 



52-^^- \i C/ 



68 **- 

 YIG. 331. Diagram for General Electric resonating frequency meter. 



position which depends on the frequency. A high degree of 

 sensitiveness may be attained, so that the full scale of a 60-cycle 

 instrument extends from 55 to 65 cycles. At abnormally low 

 frequencies, the currents in both coils would be very small and 

 the pointer might drift back on the scale and thus give rise 

 to errors; for this reason a circuit tuned to 38 cycles is added, its 

 effect being to keep the index off the scale at low frequencies. 



The inductances are wound on laminated iron cores provided 

 with air gaps ; the gaps are necessary, for in order that' the tuning 

 may be accomplished the power factors must be low and wave 

 distortion must be avoided. The iron must be worked at a low 

 flux density and the hysteresis reduced to a minimum. These 

 instruments are made both in the indicating and in the curve 

 drawing forms. 



In 1888 Ayrton suggested that it was possible to determine the 

 frequency of an alternating current by employing the principle 

 of mechanical resonance, and of late years this suggestion has 

 been developed into commercial forms of frequency meters. In 



