A Bridge for Precise Comparison of AC and DC Voltages
William James Cunningham, E.W. Vaughan · Review of Scientific Instruments · 1953
A simple bridge circuit is described in which one arm is a high vacuum incandescent lamp. Since the resistance of this lamp varies with the voltage across it, the bridge is in balance only for a single applied voltage. The elements of the bridge can be adjusted for balance with an unknown ac voltage applied. This voltage then can be replaced with a dc voltage, which is adjusted to restore balance. Subject to a known correction, this dc voltage and the rms ac voltage are the same. Precise comparison of ac and dc voltages is possible.