Abstract
A vacuum-tube voltmeter circuit is described which is capable of measuring voltages within the range of from 0 to −5000 volts with an accuracy of 0.05 percent and a linear characteristic over the entire range. The input impedance is greater than 5×1010 ohms. The circuit was developed for the purpose of measuring accurately high voltage sources of extremely low current drain which were employed for cosmic-ray studies. It is based on a null method, using a vacuum tube having a high amplification factor as one arm of a balanced circuit and a constant current source as the other arm. The negative terminal of the measured voltage is applied to the plate of the tube, and the grid voltage is varied to maintain the same grid current which balances the current from the constant current source. Linearity results from the negligible varying space-charge effect at constant grid current, and high precision measurements are obtained by means of a special circuit for measuring the change in grid voltage. By setting the grid current equal to the value of 1/μ, where μ is the amplification factor of the tube, the meter can be made to read directly the measured voltage.
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