Patent GB-533907-A (GB)
Title: Improvements relating to electric measuring apparatus
IPC: G01R27/02
Filed: 1939-12-15
Abstract: 533,907. Measuring physical quantities and dimensions electrically. BRITISH THOMSONHOUSTON CO., Ltd. Dec. 15, 1939. No. 32282. Convention date, Dec. 17, 1938. [Class 40 (1)] [Also in Group XXXVI] In unbalanced A.C. bridge apparatus for measuring quantities or qualities such as temperature, pressure, tension, compression, strain, weight dimensions, humidity, resistance, inductance, or capacity, the unbalance of the bridge is balanced out in the measuring circuit for the zero or reference position so that the indicating instrument reads zero at the reference position and indicates directly the deviation from the original unbalance. The sensitivity control and zero adjustment are arranged to be independent. Fig. 2 shows a gauging apparatus in which a magnetic armature 19 is moved by a rod 14 having its lower end bearing against the work 27 to be gauged. The armature 19 is positioned between E-shaped cores 22, 23 on which are wound coils 24, 25. A cam 28 between the armature 19 and the upper end of the rod 14 is rotatable by a worm 29 to provide a zero adjustment. The armature 19 is pressed downwards by a compression spring 30. The windings 24, 25 form two arms of an A.C. bridge, of which the other two arms are formed by windings 31, 32 of a transformer 33. The bridge is supplied by means of a saturating transformer 35 and a secondary winding 36 on a second transformer 37. The output of the bridge is amplified by a pentode amplifier 40. The amplified current is rectified and produces a unidirectional voltage across the resistance 66. For zero reference point the armature 19 is displaced from the central position by an amount OE, Fig. 3, producing a voltage output OG from the bridge. A centre-zero milliammeter M1, Fig. 2, is connected in series with a high resistance 68 so as to read the difference between the voltage across the resistance 66 and that across the resistances 60, 61 in the amplifier anode circuit. The resistance 60 is then adjusted so that these voltages are equal and the meter M1 indicates zero for armature displacement, OE, Fig. 3, and gives a positive or negative reading for displacements to one side or the other of this position. The sensitivity is adjusted by placing in the gauge an object of known deviation from the standard dimension and adjusting a shunt resistance 72 until the desired deflection is obtained on the instrument M1. A recording meter M2 may be connected in the diagonal of a bridge having a resistance 73 in one arm, resistances 74, 75 in a second arm, resistances 76, 77 in a third arm, and a pentode amplifier 78 in the fourth arm. The control electrode of the valve 78 is connected to a tap 86 on the resistance 68, which is adjustable to vary the sensitivity of the recording instrument M2. In a modification, Fig. 5, the coils 24, 25 on cores 22, 23 are replaced by condensers 24<SP>1</SP>, 25<SP>1</SP>. The transformer 33, Fig. 2, may be replaced by a transformer 33<SP>1</SP> having a secondary winding with terminals 38, 39. The capacity of the condenser 24<SP>1</SP> is varied in accordance with variations in the quantity or quality being measured either by allowing the material under test to pass between the plates of the condenser or by moving one of the plates. The condensers 24<SP>1</SP>, 25<SP>1</SP> may be replaced by resistances, one of which responds to temperature, humidity &c. to be measured. Specification 396,246, [Group XXII], is referred to.