Working of Weston Type Frequency Meter:
The mounting and connections are shown in the figure. Inductor L damps the harmonics in the current. When connected across the supply, coils A and B draw currents to produce magnetic fields that act on the soft iron needle to deflect it. The position of the needle depends on these currents. Under normal frequency (due to proper selection of RA, RB, LA, LB, ) two forces make the pointer to show normal frequency. When the frequency is other than normal, the reactances of LA and LB will be different with resistances unchanged, leading to deflections in either direction depending on the currents there in, due to changed impedances.