Let us discuss, rectifier type instruments
Alternating signals, such as alternating current, and voltage can be measured by electrodynamometer
But, the main problem is that, electrodynamometer is not much sensitive
The problem of low sensitivity associated with the electrodynamometer can be overcome
by rectifying the AC input and by measuring the signal by, D'Arseneval type instrument
Because, D'Arseneval movement is very sensitive to electrical signal. A diode bridge rectifier is used
The circuit and waveforms are shown. The bridge rectifier produces a pulsating unidirectional current
through the meter movement over the complete cycle of the input voltage
Because of the inertia of the moving coil, the meter will indicate a steady deflection
proportional to the average value of the current. AC voltages and currents are expressed in rms values
But the D'Arseneval deflection is proportional to the average value. rms value of current is 2 by pi times of dc value
Hence, the scale is to be calibrated in terms of the rms value of a sinusoidal waveform
Hope, you got the idea. Thank you