Patent GB-629299-A (GB)
Abstract: 629,299. Electromagnetic ammeters and voltmeters; frequency measurements. NALDER BROS. & THOMPSON, Ltd., and LIPMAN, C. L. Dec. 3, 1946, No. 35799. [Class 37] [Also in Group XXXVII] In a measuring-instrument or relay of the type comprising a moving magnet co-operating with one or more fixed coils, a high permeability flux return path rotates with the magnet. The invention is particularly concerned with ammeters, voltmeters and ratiometers of the midget type as required for example in automobiles and aircraft. Centre-zero ammeter-voltmeter shown, Figs. 1 and 2, has a ferromagnetic case H and a rotor magnet disc D. With zero current in fixed coil Cd which surrounds the disc D and two sides of spindle S the magnetized diameter NS (Fig. 2) of D is parallel with turns of coil Cd. A ferromagnetic ring R of high permeability, attached by arms F, F<SP>1</SP> to disc D, provides the latter's flux return path through air-gap A. Damping is preferably of the type described in Specification 574,894, consisting of conductor G fixed to spindle S so as to rotate in the field of a magnet M. The latter may be an electromagnet. Spring control at T may be utilized or the damping magnet M may be arranged to provide the control torque. In modifications, Figs. 3, 4 and 5 (not shown), the disc D may have shaped pole-pieces and the ring R may also be shaped according to the flux distribution required with the fixed coil or coils arranged to surround the ring R and disc D or the ring R only. The magnet rotor may be an axiallymagnetized cylinder, Figs. 16 and 17 (not shown), having the ring R as an extended polepiece ; with this construction the pointer travel is 250 degrees. A similar scale arc is obtained in Figs. 18 and 19 (not shown) but in this case the extended pole-piece terminates in two rings -an inner and outer ring in the same planewith a coil on each. Figs. 11 and 12 (not shown) illustrate a construction employing two rings, one above and one below a larger inner ring which surrounds an axially magnetized cylinder. The outer rings may be permanent magnets, Figs. 13 and 14 (not shown). Discs may be substituted for the outer rings. In Figs. 22 and 23 (not shown) a cylindrical rotor magnetized across a diameter co-operates with two rings which form part of an extended polepiece. Figs. 6 to 10 and Fig. 15 (not shown) indicate various dispositions of the fixed coils for the instrument shown in Fig. 1 to function as a ratiometer; in these constructions the damping magnet M is covered by a cup-shaped shield. In the ratiometer, Figs. 20 and 21, the disc D is mounted within and at right angles to the flux return ring R. In modification, Figs. 24 and 25 (not shown), one three-quarter and one whole ring are employed, the former disposed at right angles as in Fig. 20 and the latter in the same plane as the disc D as in Fig. 1. The form of ratiometer shown in Figs. 26 and 27 comprises two permanent-magnet movements common to spindle S. Each disc D<SP>1</SP>, D<SP>11</SP>, with or without pole-pieces P, co-operates with a ferromagnetic ring as in Fig. 20, but the latter are disposed at right angles to each other as shown, R<SP>1</SP>, R<SP>11</SP>. The rotor magnets may be moulded from particles of permanent-magnet alloys bound together by a suitable binding medium or sintered at high pressure. In constructions of ammeters having two fixed coils as exemplified in Figs. 4, 5, 12, 14 and 23 it may be arranged that the torques exerted are added or subtracted. It is stated in the Provisional Specification that a ratiometer according to the invention may be used to measure frequency by connecting one fixed coil to the A.C. source through a resistor and rectifier and the other to the source through a capacitor or inductance or both and a rectifier. If, for example, Fig. 1 is to be used'as a relay a contact-carrying arm is secured to spindle S.