The instruments have 2 independent moving coil movements to measure the frequency for example between 2 generators or one generator and the net.
These instruments are made of a moving coil movement with electronic transducer. The meter movements are jewelled and shock-proofed by sprung storage of jewels.
The indication is mainly independent of curves, form errors and fluctuations of the measuring voltage.
Admissible change in rating voltage: ± 20 %
External magnet field: 0.5 mT