GB748415A - Improvements in or relating to circuits for current measurements - Google Patents

Abstract

748,415. Current measurement; resistance measurement; determining physical qualities of materials. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 14, 1954 [June 18, 1953], No. 16929/53. Class 37. [Also in Group XL (b)] In a circuit for measuring logarithmically the current flow in a high impedance, the impedance is connected between a high potential source and the control grid of a thermionic valve, whereby the current to be measured flows through the control grid cathode space of such valve which has a cathode and anode circuit comprising a feedback loop stabilizing the anode current therein so that the feedback voltage is a logarithmic function of the impedance current, the circuit being also applicable to the measurement of insulation resistance and the resistance of timber as a function of its moisture content, and to the measurement and control of a nuclear reactor whose neutron sensitive ionization chamber constitutes the high impedance. In Fig. 1 the boron filled neutron sensitive ionization chamber 1 of a nuclear reactor constitutes the high impedance and has its central electrode 2 connected to a 1000 v. positive source 3 and its outer electrode 4 connected to control grid 5 of an electrometer pentode 6 having its cathode earthed and its anode 8 connected to a 150 v. positive source 10 over a load resistance 9. A D.C. amplifier without phase inversion is connected between anode 8 and screen 12 of the pentode, to stabilize the anode current, which would otherwise increase with increase in current of the ionization chamber with rising reactor power. The feedback voltage measured by voltmeter 13 (which may be calibrated in reactor power) is logarithmically connected with the ionization current (Fig. 2, not shown) and this feedback voltage may be applied to operate the automatic control system of the reactor. The pentode may be replaced by a triode having a cathode load resistance and the D.C. feedback amplifier connected between the cathode and anode with phase inversion; the voltmeter being connected to indicate the anode voltage which is the D.C. output of the amplifier.