GB920097A - Improvements in or relating to temperature measuring or temperature responsive apparatus - Google Patents

Abstract

920,097. Measuring electrically. WAYNE KERR LABORATORIES Ltd., and R. CALVERT. July 28, 1961 [Aug. 3, 1960], No. 26936/60. Class 40 (1). Temperature measuring or temperature responsive apparatus comprises an electrically conductive expansible liquid in a non conductive tubular container, said liquid forming one electrode of a capacitive arrangement of which a second electrode is disposed outside said container adjacent a part thereof, a conductive screen extending axially in both directions beyond the second electrode, means for applying a reference A.C. voltage between one of the electrodes and the screen, a current responsive device to which is fed the current between the other electrode and the screen resulting therefrom, and a balancing circuit arranged to feed a balancing current to the current responsive device in opposition to the said resulting current so that the current responsive device presents a low impedance to said resulting current. As shown, Fig. 1, mercury in a tube 10 of glass or quartz forms a first electrode; a second electrode 12 around the tube 10 may be axially movable. Electrode 12 and a screen 15 are embedded in a resin casting 16 and are connected to the inner and outer conductors respectively of a coaxial cable 14. A.C. is applied between the screen and the mercury; as shown, the mercury is capacitively coupled to an earthed electrode 17; alternatively a direct connection between mercury and electrode 17 may pass through the container. The device may be calibrated in the same way as an ordinary mercury thermometer. In the measuring circuit of Fig. 2 the output of a secondary half-winding 23 of an input transformer 25 is supplied between electrode 17 and screen 15, electrode 12 and screen 15 being connected to the primary winding 20 of a transformer 21 whose secondary 31 is connected to a detector 32 with a null balance indicator 33. The primary 20 is also fed via an adjustable capacitor 28 with the output of a second secondary half-winding 24 of transformer 25, a null output from transformer 21 being obtained by adjustment of variable capacitor 28 or a tap 29 on primary 20. To prevent the mercury sticking to the tube the latter may be vibrated as by a moving-coil vibrator 60. In a modification, Fig. 3, the output from electrode 12 and screen 15 is fed to a high-gain amplifier 40 having a negative feed-back circuit 41, the output voltage of the amplifier 40 being measured by a meter 46; this output may also be used to control the temperature of a body as through a servo-system. The input to the amplifier may be partially balanced by a signal of known magnitude derived from a secondary half-winding 49 of the input transformer feeding a standard impedance 50. If visual access to the mercury is required electrode 12 and screen 15 have aligned axial slits, or are made of transparent material, e.g. conductive glass.