An Improvement in Vacuum Tube Voltmeters

A vacuum tube voltmeter for the measurement of audio frequencies, which depends for its operation almost entirely upon the amplification factor of the tube, is described. This voltmeter combines the marked advantages of a single voltage source for filament, grid, and plate supply with the absence of zero adjustment requirements on the indicator. It has a further advantage over the more common type of vacuum tube voltmeters in that it can usually be made direct reading in volts on an 0-2 milliampere thermocouple type of indicating instrument.