Patent GB-556073-A (GB)
Title: Improvements in and relating to electrical measuring instruments
IPC: G01B7/24
Filed: 1942-01-16
Abstract: 556,073. Determining dimensions ; inductance measurements. MAPP, A. G. Jan. 16, 1942, No. 704. [Class 37] A measuring instrument comprises an inductance coil completely shrouded in magnetic material and having a core movable in it to vary an air-gap in the magnetic circuit whereby movements of the core relatively to the coil cause changes in its inductance value. Fig. 1 shows an instrument for measuring changes in a linear dimension, such as an extensometer. In a casing 10 are mounted a number of spaced laminated or solid rings 11 of magnetic material. These are rigidly secured together by end blocks 12, 13 and intervening spacing pieces 14 which are also of magnetic material. A central rod 15 is provided with cylindrical shoulders 17 of magnetic material between which are mounted coils 16. The shoulders 17 are slightly staggered with respect to the rings 11 so that endwise movement of the rod causes the cross-sectional areas of the air-gaps between them to be varied. One gauge point, 18, is fixed to the casing, whilst the other, 19, is mounted on the rod 15. In a modification, Fig. 2, the length of the air-gap is altered instead of its cross-sectional area. A coil 21 wound on a bobbin 22 is enclosed in a tubular member 23 of magnetic material closed by end members 24, 25 also of magnetic material. A magnetic core 26 passes through a central hole in the member 24 and is secured at the end 27 to a foot 28. The coil and its shrouding are mounted on another foot 29, the feet 28, 29 being secured to the member to be tested. A non-magnetic spacer 30 is provided between the feet 28, 29 and serves as an enclosure for the core 26. Adjustment of the air-gap between the core 26 and the member 25 is effected by shims between the foot 28 and the part 30. The measuring inductance is arranged as one arm of a Wheatstone bridge of which a balancing inductance forms another arm, the other two arms being resistances. The bridge is supplied with a high-frequency current generated by a tetrode oscillator. The indicating instrument may be a rectifier voltmeter or a cathode ray oscillograph. An internal micrometer is shown in Fig. 3. A coil 41 in a magnetic pot 42 is mounted in a carrier 43 by means of a spring 45 which permits transverse movement. Its position is adjusted by means of a screwed pin 46 with a tapered end 50 engaging an axial bore in a block 52 carried by the pot, so that the block is moved transversely against the action of a compression spring 53 by adjustment of the pin 46. The core 47 of the coil 41 is mounted on a light spring 48 tending to move it outwards, its motion being limited by a set-screw 54. The upper end of the core is provided with a contact member formed by a hardened steel ball 49 or a jewel. The body 43 is formed with parts 55, Fig. 4, which are pressed against the wall of the hole being gauged by means of springs 56. Any variation in the diameter of the hole causes movement of the core 49 which is shown as a reading on the instrument. For the measurement of fluid pressure, the core of the measuring coil may be connected to the centre of a diaphragm exposed to the fluid.