A vacuum‐tube voltmeter using the Type 954 acorn pentode for use in the measurement of the root‐mean‐square value of alternating voltages is described. A feature of the meter is the method whereby the tube response is made to fit a pre‐marked square‐law meter scale by the simultaneous adjustment of the screen voltage and the microammeter shunt. Operation from power lines and relative freedom from changes in calibration due to line voltage variations make the meter a useful laboratory instrument.