GB773369A - Improvements in or relating to magnetic compasses - Google Patents

Abstract

773,369. Magnetic compasses. COOK, H. June 30, 1955 [July 19, 1954], No. 20912/54. Class 97(2) In a magnetic compass, the magnetic needle is mounted in a compass body by means of a vertical spindle rotatable in axially spaced bearings which locate the needle and spindle against tipping relative to the compass body, and the needle and spindle have a period of vibration of less than one second. As shown, the motion assembly consists of a tubular barrel 26 into which the top and bottom plates 28, 27 fit tightly, being separated by a spacing tube 29. A spindle 30 is supported in a thrust bearing 31 in the lower plate 27 and by a bearing 32 in the upper plate 28. A magnetic needle 37 of flat lozenge shape is secured by an adhesive to a flange 35 connected to the spindle 30. The needle may be made of exceptionally hard magnetic material, such as is known under the Registered Trade Marks "Alnico" or "Alcomax IV", and an indicator 17, which may be shaped for example as a pointer, and is made of thin light, but stiff sheet material, is fixed to the needle by means of an adhesive. The bearings 31, 32 are spaced apart by a difference at least as great as the length of the needle, and the dimensions are so chosen that the periodicity of the needle is well under one second. The motion assembly is seated in a recess 24 in a cylindrical body 10, having a base 20 with bores 21, 22 receiving adjustable compensating magnets 23. The body 10 is supported by screws 11, 12 inserted in a plate 13 fixed to the side of the body 10. An annular washer 38 is placed in the body 10 over the motion assembly, and supports a scale plate 15 which lies in the plane of the indicating member 17. A transparent plastic window 14, separated from the scale plate 15 by a wire spacing ring 39, closes the assembly. In a modification, the motion assembly is supported in gimbals within the body 10, and the space between the motion assembly and the body 10 is filled with a transparent damping liquid. When the compass is to be used as a hand compass, the spindle is shortened so that the difference between the bearings is only about equal to the length of the needle, and the body 10 has only a thin bottom.