Patent GB-677604-A (GB)
Abstract: 677,604. Electrostatic instruments. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. March 8, 1951 [March 28, 1950], No. 5652/51. Class 37. [Also in Group XL (b)] A mirror electrometer has as its moving element a movable vane which comprises a thin sheet such as of mica having a lightreflecting metallized surface serving as the mirror of the electrometer. The parts are mounted on a metal base 1, which may be provided with a metal cover (not shown) the top of which is provided with a glass window, treated to be electrically conducting, directly over the movable vane 2 of the electrometer which also serves as the mirror. The vane 2 is attached to a filament 3 stretched between the ends of a bow-shaped member 4, which is held by a clamp 5 attached to the centre of a flat spring 6, which is slotted at one end and fastened to the base by screws 7, 8. By means of a screw 9, Fig. 4, the vane 2 and its support may be moved up or down. On opposite sides of the vane 2 are fixed electrometer plates 10, 11. The plate 10 is a flat metal sheet having a three-tined shape as shown in Fig. 1. An outer portion of the centre tine, adjacent to the vane 2, forms the active portion of the plate 10, the outer end being bent upward as shown in Fig. 4. The ends of the outer tines are supported by pillars 12 extending from an insulated mounting disc 13, which is supported within a hole through the block 1 and insulated from the block by a pair of insulating washers 15, Fig. 4. These are held in place by flanges on the block 1 and disc 13 and the assembly is held together by a ring 16. An adjusting screw 14 presses against an inner portion of the centre tine and enables the outer portion of the tine to be adjusted parallel to the plane of its original position. The lower plate 11 is supported by a rod 17 passing through an insulating bushing 18. This fits a hole through the disc 13 and is held therein by a retaining ring 19. The rod 17 and ring 19 extend below the disc 13 to provide a coupling for a coaxial cable. Damping members 20, 21 are positioned close to the vane 2 on the opposite side of the filament 3 from the fixed plates 10, 11. Damping of the vane is effected by air confined in the small space between it and the damping members. The damping members are supported by pillars 22, 23 and are adjusted by screws 24, 25. The bow 4 may be a rod of quartz glass bent into the shape shown. The filament 3- is a quartz fibre stretched between the ends of the bow 4, and may be attached thereto by fusing the quartz. Included in the filament 3 is a closed loop 26 of quartz fibre, shaped as shown in Fig. 1, which acts as a spring to maintain the tension of the filament. The filament is coated with platinum to provide conductive connection to the vane 2. The vane 2 is a thin sheet of platinized mica held in position on the platinized filament 3 by silver paste applied. at two points only, to prevent warping of the mica. In a modification both the fixed electrometer plates are situated below the vane and on opposite sides of the filament, the damping members both being above the vane and on opposite sides of the filament. As shown in Fig. 10 one fixed electrometer plate 47 is connected to a terminal 45, between which and a second terminal 46 the voltage to be measured is applied. A source of adjustable potential, such as a battery 48 and potential divider 49, is connected to the other fixed plate 50. A voltmeter 52 is connected to measure the potential applied to the plate 50. The position of the vane 51 is indicated by reflection of a light beam from its surface. The voltage divider 49 is adjusted to bring the vane 51 to its normal or balance position, in which the torque applied to the vane 51 by the electrostatic attraction between the vane and the plate 47 is equal to the torque applied by the electrostatic attraction between the vane and the plate 50. The potential of the plate 50 is measured by the voltmeter 52 and is a measure of the unknown potential. In a modification the vane 51 is connected through a source of bias voltage to the fixed plate 50, in order to maintain constant sensitivity. Fig. 13 shows the circuit diagram of an electrometer for measuring the charge on " pencil type " pocket ionization chambers used as radiation exposure meters. The " pencil " is inserted into a well comprising an outer annular member 72 and an inner rod-shaped member 73, these members making contact with the outer and inner plates of the " pencil " capacitor. The member 72 is connected to earth and the member 73 to a fixed electrometer plate 74. Connected across batteries 75, 76 is a potential divider 77, 78, 79, 80, 81. Connected across the D.C. supply is a series circuit comprising a resistor 82, two photo-tube sections 83, 84, and a resistor 85. A lamp 86 and condensing lens 87 direct a beam of light on to the movable vane 88, from which it is reflected on to the photo-tube sections 83, 84, so that any rotation of the vane 88 changes the potential at their junction. This junction is connected to the input of a D.C. amplifier 89 comprising a triode section 90, a plate load resistor 91, and a cathode resistor 92 in series across the D.C. voltage source. A resistor 93 is connected in series between the resistors 82, 92 to provide bias at the cathode of the section 90. The output of the amplifier 89 is connected through a resistor 94 to the input of a cathode follower comprising a triode section 95 and a cathode resistor 96 connected across the D.C. supply. The cathode of the section 95 is connected to the other fixed plate 97 of the electrometer. Bias for the movable vane 88 is provided by a neon lamp 98 and a resistor 99. When an unknown potential is applied to the member 73 the vane 88 rotates and varies the relative illuminations of the photo-tube sections and thus changes the potential applied to the grid of the amplifier section 90. An amplified change of potential results at the cathode of the section 95, which changes the potential of the fixed plate 97 and thus that of the vane 88, this action continuing until the forces acting on the vane 88 are balanced and the vane comes to rest in its normal position. The potential of the plate 97 is measured by a voltmeter circuit comprising a galvanometer 100 and a resistance network 101. A switch 102 when closed increases the sensitivity of the voltmeter by reducing the resistance of the network 101, thus changing the range. The voltmeter circuit is connected between the cathode of the section 95 and the tap on a voltage divider 104 in parallel with the resistor 80, which is adjusted to the standard voltage to which pocket ionization chambers are initially charged. The galvanometer 100 then indicates the amount by which the charge has decreased. An adjustable tap on the resistor 80 is connected through a resistor 105 to a switch 106 and, when this switch is closed, to the member 73 so as to recharge the capacitor of the pocket ionization chamber.