Patent GB-752709-A (GB)
Title: Improvements in magnetic compasses and similar apparatus provided with magnetic correcting means such as quadrantal and semi-circular correctors
IPC: G01C17/38
Filed: 1954-05-28
Abstract: 752,709. Magnetic compasses. BELLON, F. B. May 28,1954 [May 30,1953], No. 15867/54. Class 97 (2). In a magnetic compass with correctors made of magnetic material the apparent magnetic permeability of which is lower in the direction parallel to the axis of rotation of the compass card than in directions perpendicular to this axis and the common mean horizontal plane of which is different from that of the magnets of the compass card, the distance between these two planes being determined as a function of the distance of the correctors from the vertical axis passing through the centre of rotation of the compass card, in such a manner that the deviations due to the induction effects of the magnets of the compass card, first in the horizontal direction in the plane of the magnetic symmetry of each corrector which passes through the centre of the compass, secondly in the horizontal direction perpendicular to the last-mentioned direction, and thirdly in the vertical direction, substantially compensate one another. The compass card may carry an arrangement of two or more magnets in known manner. The correctors are flat bodies, preferably discs, and have an apparent permeability lower in the vertical direction than in any horizontal direction. They may consist of flat discs of ferromagnetic material separated by plat s of non-magnetic material such as copper, brass or aluminium, or an insulator. This laminated construction avoids errors due to hysteresis effects when the craft supporting the compass turns in opposite directions. The theory of the correctors is fully discussed, and it is shown that the amplitude of quadrantal correction may be varied: (a) by positioning differently dimensioned correctors in a given plane, (b) by varying the distance between the plane of the correctors and that of the compass card, (c) by varying the plane of the correctors and their distance from the axis together, (d) by rotating the correctors relatively to each other about the axis of the compass card, and (e) by varying the angular displacement and the distance between the planes together. In the quadrantal corrector shown in Fig. 12, the disc-shaped correctors 5 and 6 are carried by non-magnetic supports 13, 14 of copper, brass of aluminium, which are pivoted on a non-magnetic spindle 12 which coaxially supports the compass card. Links 15, 16 (also non-magnetic) are pivoted to supports 13, 14 and to a knob 8 slidable in a slot 9 in an arm 10. This enables the angle # subtended by the centres of the correctors at the axis of the spindle 12 to be varied. A bar 7 of magnetic material (preferably, like the correctors, of laminated form) is carried by bar 10 and so positioned that the correction is reduced to zero when # = 90 degrees. By sliding knob 8 in its slot, is varied to give the required correction. The whole apparatus is also rotatable about the axis 12. In another form of corrector, Fig. 15, the compass bowl 18 is gimballed in a fixed casing 17. Non-magnetic plates 3, 4, fixed symmetrically to the casing so as to be adjustable around a vertical axis, are inclined to the vertical at an angle of 47 degrees 30 minutes. Disc-shaped correctors 1, 2 of radius rare slidably mounted on plates 3, 4 so that their centres Oa, Ob are adjustable to vary the correction along generators of a cone with its apex at Sc. If 0 is the centre of the compass card, Sc lies on the vertical axis OX at a distance of about 0.5r above O. Correctors of varying sizes may be used, the brackets being fixed, or also movable if desired. When the brackets are fixed, a semicircular corrector 11 may be positioned at R on OX, the points O, R being symmetrical with respect to the horizontal mean plane of the correctors, without affecting the quadrantal correction. The semicircular corrector 11 comprises similar magnets 25, 26, Fig. 17, fast with gear wheels 27, 28 rotatable in opposite directions by a gear wheel 29. In this way the amount of semicircular correction may be varied. Semicircular correction may also be effected by two Flinders bars subtending a right-angle at the axis OX. In a further alternative, Fig. 19, discs 33, 34 are magnetized uniformly and with equal magnetic moments, along their diameters. The discs are mounted on toothed wheels 35, 36 in a casing 39, the disc 34 being adjustable axially on wheel 36. The toothed wheels mesh with a worm 37, by rotation of which the resultant field of the two discs is varied. The direction of the field is varied by rotation of casing 39 in its support 38, which may be the glass plate covering the compass. The semicircular corrector is placed on the side of the compass card opposite the quadrantal correctors, and if the radius, of the discs is small, close to the point O, as shown in Fig. 15.