GB561866A - - Google Patents

Abstract

561,866. Current meters. SMITH & SONS (MOTOR ACCESSORIES), Ltd., S., and KLEPP, E. C. Nov. 2, 1942, No. 15415. [Class 37] In a moving-coil instrument of the kind comprising two concentrically-arranged coils mounted to rotate in the same magnetic field and wound to produce opposing torques, the air gap is of constant width along the path of movement of the coils and the cross-section of the core or of the magnet pole is such that a part of the core or the magnet pole facing the gap approaches saturation, so that the field strength in the gap decreases along the path of movement away from this part. The core or the magnet poles or both may be formed from a metal alloy, the permeability of which varies in accordance with a predetermined law so as to be high at low flux densities and low at high flux densities. In the construction shown in Fig. 1, a core 12, in the form of a cylindrical annulus, is mounted coaxially with the pole faces 11 of a magnet 10 so as to provide a gap 13 of constant width. Encircling the core are two coils 14, 15 wound on a former 16 mounted to rotate about an axis coaxial with the core. The coils cross one another over the ends of the core and intersect on the axis. The cross-section of the core is such that the flux flowing through it from the magnet pole faces causes saturation at diametrically opposite points so that the flux in the air gap is graded from a maximum near the outer pole edges to a minimum at the centre of the poles. In a modification, Fig. 2, the core 18 comprises a gapped annulus and is in direct contact with one pole 19 of a horseshoe magnet 20. The other end of the magnet is either provided with a continuation or abuts against a disc-shaped polepiece 21 so that a gap 22 of constant width is formed. Two coils 23, 24 pass through the centre of the core 18 across the two flat faces and axially across the periphery. The cross-sectional area of the core or the polepiece 21 or both is chosen so that saturation takes place adjacent to the magnet proper and the flux density diminishes along the path of movement. Specifications 546,468 and 556,915 are referred to.