GB828459A - Improvements in or relating to electric ratiometers - Google Patents

Abstract

828,459. Moving-coil instruments. SMITH & SONS (ENGLAND) Ltd., S. Jan. 22, 1958 [Jan. 22, 1957], No. 2351/57. Class 37. A cross-coil ratiometer has a magnetic system comprising a cylindrical magnet having N and S poles on opposite sides of its axis, and an encircling yoke of such shape as to define a gap of varying width round the magnet. The yoke 48, Fig. 7, may have an internal surface comprising two part-cylindrical surfaces 49, 50 formed about spaced, parallel axes 51, 52, each surface subtending an angle of less than 180 degrees at its axis. The yoke is mounted coaxially about a permanent magnet 11, Fig. 1, which may be anisotropic and may have flat faces on each side of its axis, on which are placed polepieces 14, 15 of segmental crosssection, forming a cylindrical core. The crossed coils 21, 22, Fig. 3, are mounted on a carrier 18 which may be of aluminium to provide eddy current damping, and which is free to rotate, in the gap 20, through an angle which is less than 100 degrees, about pivots 30 which engage jewel bearings 31. The gap 20 is a minimum at the ends of one diameter passing through the core, for example at the centres of polepieces 14, 15, and a maximum at the ends of a second diameter at right-angles to the first. The core is supported in the yoke 48 by brass distance pieces which extend along the inside of the yoke in the spaces not traversed by the coils. The coils are connected together and to a hair spring 23 at one end, and have separate, flexible, silver gilt leads at the other end. An arcuate toothed rack 34 is fixed to a carrier trunnion 28 which transmits the movement of the coils to a pointer 47 by way of a pinion 43. Two arms 35, Fig. 3, are formed integrally with the toothed rack 34 and are threaded to receive weights (not shown) to counter-balance the rack.