Patent GB-580656-A (GB)
Title: Improvements in or relating to the measurement of very small direct currents
IPC: G01R19/18
Filed: 1944-06-17
Abstract: 580,656. Thermionic valve ammeters. SANGAMO WESTON, Ltd. June 17, 1944, No. 11593. Convention date, Aug. 20, 1943. [Class 37] An electronic device for measuring small direct currents comprises a modulator, the input circuit of which is connected in series with the circuit in which the current to be measured flows, the modulator output after amplification and rectification being fed back in a degenerative sense to the input circuit through a resistor whereby a potential balance is established in the input circuit by the current to be measured and the degenerative current through the resistor, and the rectifier output voltage is a measure of the current input to the modulator. A plurality of degenerative resistors may be provided and adapted to be switched selectively into circuit to vary the current measuring range. As shown schematically in Fig. 4, the D.C. to be measured is derived from a source ES in series with a variable resistance such as a phototube P, this D.C. circuit being connected to the input terminals T, T<1> of a modulator tube 1. The second grid G<1> is connected to an alternating voltage source 2 to produce an alternating output modulated in accordance with the direct potential applied to the control grid G. The modulated output is amplified at 3 and rectified at 4. A biassing battery 5 or other D.C. source blocks all D.C. output from the rectifier in the absence of amplifier voltage output peaks less than a predetermined level thereby nullifying parasitic amplifier output components. The rectifier output is indicated by a voltmeter V shunted by a condenser C to smooth out ripples components and this voltage is fed back degeneratively to the modulator input grid G over a resistance R. Current balance in the D.C. circuit ES-P and the degeneration resistor R is obtained at any input current that maintains a definite relation between the grid G potential ei and the rectified output e which is a measure of the input current, so that the voltmeter V may be calibrated to read current direct in micro-amperes. Since current balance depends on the value of resistor R, a plurality of current ranges may be provided by using different resistors selectively switched into circuit. Fig. 5 shows the modulator tube 1, amplifier 3a, 3b and rectifier 4, the various electrode potentials for these tubes being suppied in known manner from a 60-cycle mains supply through a transformer 10-12 and rectifier 13. To eliminate spurious output voltages that may appear due to leakage the cathode K of the modulator 1 is connected to the grounded chassis over a potential divided 16 which is adjusted with the input contacts T, T<1> short-circuited until the output voltmeter V reads zero. The alternating 60-cycle voltage is applied to the grid G<1> over winding 10 and the biassing direct voltage for the rectifier 4 is derived from the voltage divider 21. Degenerating resistors R<1>-R<4> are adapted to be switched selectively into circuit to vary the measuring range by a switch 22 and each resistor has a condenser 23 in parallel therewith to eliminate the time lag that would otherwise ensue between changes in the rectifier output voltage and the application of these changes to the modulator grid over the corresponding degenerating resistor. Hunting of the voltmeter V pointer is thus prevented. Other types of modulator may be used in lieu of the tube, for example, a rectifier ring or vibrating contact modulator.