To measure the frequency of the circuit, it is compulsory to connect the frequency meter to a supply. The electromagnet is connected to the supply for which frequency is to be measured. The magnetism of the electromagnet alternates with the same frequency and the electro-magnet exerts theon each reed once every half cycle. All reeds start vibrating but the reed whose frequency is double, vibrates with maximum amplitude due to mechanical resonance. The vibration of the other reeds is so small and these are unobservable.