. The Bell System technical journal . 10, No. 2, April, 1922. THERMIONIC VACUUM TUBES 95 X. Miscellaneous Applications of Thermionic Vacuum Tubes 56. The Tube as a Voltmeter. The three-element tube may beused for the measurement of either or voltages. In the caseof voltages it is customary to apply the unknown voltage to theplate, counter-balancing this voltage with a known negative potentialapplied to the grid. Given the n of the tube, it is then possible tocalculate with a fair degree of accuracy the plate potential. Theusual procedure is to adjust the negative grid potential t
. The Bell System technical journal . 10, No. 2, April, 1922. THERMIONIC VACUUM TUBES 95 X. Miscellaneous Applications of Thermionic Vacuum Tubes 56. The Tube as a Voltmeter. The three-element tube may beused for the measurement of either or voltages. In the caseof voltages it is customary to apply the unknown voltage to theplate, counter-balancing this voltage with a known negative potentialapplied to the grid. Given the n of the tube, it is then possible tocalculate with a fair degree of accuracy the plate potential. Theusual procedure is to adjust the negative grid potential to such apoint that the plate current just becomes zero. The tube when usedin this manner becomes an electrostatic voltmeter, and it is evidentthat to give accurate readings the tube should have a well-definedcutoff (see Sec. 8, Fig. 10). In a somewhat similar fashion peak voltages may readily becompared with known voltages. A typical circuit is shown inFig. 69. In operation a fixed plate voltage is applied to the voltmeter.
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