In this instrument one amplifier first amplifies the input signal. Then it fads the amplified signal into the directly heated filament of a vacuum diode. The filament should have small thermal time constant to respond the rapid change in measuring voltages level.
In the true R.M.S voltmeter there must be a cathode follower to match the impedance between the amplifier and the low impedance filament. The input current rises the temperature of the filament. Therefore there will be an anode current to rise. Therefore anode to cathode voltage falls. Then this D.C amplifier amplifies this drop in the voltage. The output of this amplifier supplies the feedback current to the filament. The feedback circuit is so adjusted that the decrease in If2Rf equals to the input signal power Is2Rf in the filament. Where If is the feedback current and Is is the R.M.S current of input signal.
The motor connected in the true R.M.S voltmeter reads the output voltage of the amplifier. Again output voltage of the amplifier directly proportional to the feedback current Iv. Again Iv is directly proportional to the R.M.S signal current. Therefore ultimately the true R.M.S voltmeter reads the R.M.S signal current.