GB724903A - Improvements in or relating to measuring devices for low electric currents - Google Patents

Abstract

724,903. Electric tests. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Jan. 7, 1953 [Jan. 10, 1952], No. 511/53. Class 37. [Also in Groups XXXVII and XL (b)] In apparatus for measuring a weak direct current from, e.g., an ionization chamber by a dynamic electrometer, when the pointer of the indicating instrument touches an adjustably fixed contact a further valve causes the meter current to increase so that the contact pressure is increased and the contacts are maintained closed. As shown, the current from an ionization chamber 1 produces a voltage drop across a resistor 4 which is converted to A.C. by electrodynamic vibrator 6 and amplified at 9, the output current after rectification passing through the coil 16 of a meter 17. At a given level the pointer 22 touches a fixed contact to apply negative bias to a valve 28, whereupon the anode current falls, the anode voltage rises, and an increased current is sent through the meter winding 16 from the junction of resistors 34, 35 to increase the pressure of the pointer 22 on the stationary contact, the resulting reduction in contact resistance increasing the negative bias on valve 28 and further reducing the anode current until a relay winding 31 in the anode circuit releases to close contact 36 thereby connecting the junction 34, 35 to H.T. positive at terminal 37. A further contact 38 of relay 31 operates a signalling device 39 or a radiation-control circuit.