Mechanical resonance type frequency is a very simple instrument and has got advantage of giving reading free from errors due to change in temperature, wave form and magnitude of applied voltage.
It consists of a number of thin steel strips called reeds. These reeds are placed in a row alongside and close to an electromagnet. The electromagnet has a laminated iron core and its coil is connected in series with a resistance, across the supply whose frequency is to be measured. The reeds are approximately about 4mm wide and 0.5mm thick. The reeds are not similar to each other but differ either in their dimensions or weight or carry different flags at their tops. This is done to vary the natural frequency of vibration of each other.
The reeds are arranged in ascending order of natural frequency, the difference in frequency is usually 0.5Hz. Thus the natural frequency of first reed may be47Hz, of the second 47.5Hz, of the next 48Hz and soon.
The reeds are fixed at the bottom end and fixed at the top end. The flags are pointed white to afford maximum contrast against their black background.
When the frequency meter is connected across the supply whose frequency is to be measured, the coil of electromagnet carries a current i which alternate at the supply frequency. This produces a force of attraction on the reeds. This force of attraction is proportional to the square of current, therefore, it varies at twice the supply frequency. Hence force is exerted on the reeds varies every half cycle. All the reeds will tend to vibrate, but the reed whose natural frequency is equal to twice the frequency of the supply will be in resonance and will vibrate most. Mechanical resonance is obtained in the case of this reed. The tuning in these meters is so sharp that as the excitation frequency changes from resonant frequency the amplitude of vibration decreases rapidly becoming negligible for a frequency which is slightly away from resonant one.
The usual range of frequency in a frequency meter of this type is about six cycles say from 47 Hz to 53 Hz.