Patent GB-797083-A (GB)
Title: Improvements in or relating to electrical instrument movements of moving magnet type
Inventor: JEFFERY GLYNN BENNETT
IPC: G01R5/16
Filed: 1955-04-23
Abstract: 797,083. Electromagnetic ammeters and voltmeters. NATIONAL RESEARCH DEVELOPMENT CORPORATION. April 18, 1956 [April 23, 1955], No. 11799/55. Class 37. In a moving magnet instrument the control magnet and/or coil and the deflecting coils are positively located in a one-piece frame and the sensitivity is adjustable to match a scale or other fixed reference attached to the frame by variation of the strength, but not the direction of the control field. As a ratiometer, a quadrature coil is provided in addition to the normal deflection coils, but a weaker control magnet of reversed polarity is preferably included to indicate the absence of current in one coil by deflecting the pointer off the scale. Fig. 1 shows a one-piece die-cast frame 15 having a transverse part 15a with spaces to accommodate the deflection coils 41, 42 and the spindle 12 carrying the moving magnet 11. The spindle 12 which is pivoted in jewel bearing screws 13, 14 also carries a toothed quadrant 16 which provides a 1: 3 gear ratio with a pinion 18 carried by the pointer spindle 19 which is pivoted in jewel bearing screws 20, 21. The channel section pointer 26 has two right angle bonds to bring it through a hole in the dial 22 to the raised scale 22a. An L-shaped member 27 attached to the pointer 26 limits rotation to about 270 degrees by engagement with opposite sides of the lug 15d. The spindle 12 has adjustable balance weights as at 17 ; spindle 19 is balanced by deforming the lower end of the pointer or the quadrature member 28. The dial 22 is attached to three bushes in the top of the frame by screws passing through arcuate slots which permit dial rotation for zero adjustment. The lower end of the transverse frame part 15e is formed into a boss having a square upper section 15m to take the diametrically magnetized control magnet 29 which is retained by burring over the square comers. Assembly. The two formerless deflection coils 41, 42 are each cemented to a copper plate 31, 32, Fig. 4, having a rectangle cut out to clear the moving magnet 11 leaving two tongues 35, 36 to support the coil. The plates 31, 32 also have central grooves 37, 38 to clear the spindle 12. The plates are brought together face to face from opposite sides of the spindle 12 with the quadrant 16 and moving magnet 11 already fast thereon, to form a sub-assembly which is slid (downwards in Fig. 5) between a groove 39 in a lateral extension 15a and another in 15e. The lower bearing screw 14 is inserted and tightened until the shoulder at the lower end of its reduced diameter 14a bears firmly against the lower edges of the plates 31, 32. The upper bearing screw 13 is now inserted and adjusted until the jewel engages correctly with the pointed end of the spindle 12. The upper circular magnetic screen 43 having a radial slot to allow it to pass the spindle 12 is fitted on the top of the transverse part 15a. A cylindrical magnetic screen 44 having an insulating liner 45 is next fitted and secured by bending tags formed in the upper screen 43 into holes in the cylindrical screen 44. A non-magnetic guide disc 48 fits over the cylindrical part of the boss 15n and is retained by a spring 49 compressed by a nut 51 which also carries the adjustable magnetic screen 50. The quadrature coil 53, Fig. 4, for use in a ratiometer is held between an accurately formed face 15p of the transverse frame part and the insulating liner 45 by a resilient rectangular ring 54. Connections are made to a tag post riveted under the top of the frame. Sensitivity adjustment and accuracy. It is stated that provided the control and deflection fields in a 0-1 mA movement are spaced by 135Œ1.6 degrees and the sensitivity is adjusted by movement of the magnetic screen 50 to agree with a printed scale at 0.831 mA, all other readings will be within 1 per cent of f.s.d. A ratiometer, which has control and deflection fields spaced by 90 degrees has its control current adjusted to give 135 degrees point deflection in either direction when the deflection current is at its maximum value. Alternative preprinted scales can be provided for instruments having the angle between the control and deflection fields outside the above-mentioned tolerance. Specification 660,894 is referred to.