GB375316A - Improvements in and relating to measuring instruments - Google Patents

Abstract

375,316. Measuring instruments. JEFFREE, J. H., University College, Oxford. March 16, 1931, No. 7967. [Class 118 (ii).] In instruments of the type in which the deflection of the moving part is detected by the displacement of a beam of light or other suitable radiation reflected from a mirror fixed thereto, the displacement is relayed by directing the reflected beam on to electrically opposed thermojunctions c, Fig. 1, in circuit with coils a rotatably suspended in a magnetic field. Normally the radiation falls symmetrically on the opposed junctions c, no current flowing in the coils a. Deflection of the beam causes unequal heating of the thermojunctions and consequent rotation of the coils a in a direction corresponding to the beam deflection. The amount of rotation is limited (1) by balancing the electromagnetic torque against a suitable restoring torque the median line m of the thermojunctions in this case being disposed in the suspension axis b, or (2) by disposing the median line m, Fig. 3, in front of the suspension axis b, the coils a rotating until the beam d again falls symmetrically on the junctions c. In (1) the rotation is, for small displacements, approximately proportional to the displacement of the beam d and to its illumination intensity and in (2) proportional to displacement of the beam d but independent of the illumination intensity of the latter. The amount of rotation may be observed by any suitable means as a pointer and microscope but preferably the beam d is partly received on a mirror fixedly mounted with the coils, the reflected beam therefrom forming an optical pointer for the moving system. An instrument for case (2) above comprises a copper wire loop a, Figs. 4 and 5, suspended between magnetic poles N, S and core R, from a quartz fibre b fixed with shellac to the back of a mirror f to which the prolongations p of coil a are similarly fixed. The opposed thermojunctions are formed by a suitably blackened constantan disc c to which bent copper wires e are soldered and connected at their other ends to the loop a at g, h, thereby positioning the thermojunctions in front of the suspension axis b, Fig. 5.