During the war, meter movements were extremely hard to come by, but a radio serviceman in need of an accurate measuring device could probably scrounge up a magic eye tube. This circuit on the cover of the September 1944 issue of Service magazine showed how to construct a very accurate vacuum tube voltmeter (VTVM) that didn’t actually require a meter. The switch was used to set the range, which could vary from 0-0.5 volts to 0-500 volts. Then, the 770 ohm potentiometer was adjusted to the point where the magic eye tube closed. The potentiometer was calibrated with the voltage, and the reading was obtained from that scale.

The magazine noted that the instrument was particularly good for tracking down transient or intermittent voltages that might not be seen because of the inertia of a standard meter movement. It noted that the instrument’s function was similar in many respects to an oscilloscope.