Ferrodynamic type frequency meter: (Electrical resonance type frequency meter): It consists of a fixed coil. The supply whose frequency is to be measured is connected
across it. This coil is also known as magnetizing coil. It is mounted on a laminated iron core. The core has a typical varying cross section. It varies along the length and is maximum at the end of core. The moving coil of it is pivoted over this iron core. The pointer is fixed to the spindle and the terminals of moving coil are connected to a suitable capacitor C .No controlling torque is required.
Working:- Current flowing through magnetizing coil produces flux in the iron core which will set up an emf in the moving coil .This emf lags the flux ø by almost 900 . This will cause current I to flow through capacitor C. If current is inductive it will lag induced emf and a torque will act on the coil. If current is capacitive then also the torque will act, but if the inductive reactance is equal to capacitive reactance two torques will act on the moving coil. The capacitive reactance is constant for given frequency but the inductive reactance depends upon the position of pivoted coil on the core. The nearer the coil approaches the magnetizing coil, the greater is it’s inductance .The moving coil is pulled towards the magnetizing coil until both the reactances are exactly equal. i.e. when torque is zero. The value of capacitor is so selected that the moving coil takes up a convenient position when frequency is of normal value.