A High-Impedance d.c. Valve Voltmeter
I.A.D. Lewis, K Firth · Journal of Scientific Instruments · 1948
A general purpose electronic d.c. voltmeter is described. Full-scale deflexion is obtained with inputs of ± 25 or ± 250 mV., and the input impedance and grid current have values of the order of 1010 Ω and 10-11 A. respectively. Currents down to the order of 10-9 A. (full-scale deflexion) can be measured by externally connecting an input resistor of suitable value. A backing-off circuit is included, and the instrument measures small voltage changes about any value in the range ± 9 V. An accuracy of within 5% is obtained, apart from drifts in zero setting; the latter may have a value of up to 10 mV.