Patent GB-416241-A (GB)
Title: Compensated compass especially for aerial vehicles
IPC: G01C17/38
Filed: 1934-01-12
Abstract: 416,241. Magnetic compasses. ROBERT, G. 0., 11, Rue Tronchet, Paris. Jan. 12, 1934, Nos. 1202, 1203, and 1204. Convention dates, Jan. 12, 1933, May 26, 1933, and Nov. 23, 1933. [Class 97 (ii).] In a compass having a liquid-filled transparent bell surrounding the float carrying the card and directing magnets and compensating devices beneath the bell, all parts in contact with the liquid are shaped as perfect surfaces of revolution presenting no irregularities able to disturb relative movements of the parts and the liquid. In the form shown, the bell 1 has a cylindrical or frustro-conical part 90 merging into a part 92 formed by a segment of a sphere and having a diminishing thickness towards the centre so as to provide an optical magnifying effect. The bell is closed by a corrugated, flexible cap 11 adapted to accommodate the thermal expansion of the liquid. A second cap 13 has a dome shaped central part into which air bubbles may be passed through holes in the cap 13 and retained by a baffle 20. The movable element comprises an aluminium float and card 8, the parts being pressed together without rivets, bolts, or soldering. The directing magnets are arcuate in form and the graduations are marked on a frustro-conical rim 71 of the card. The movable element is supported with its centre of gravity at the pivot 7 by a pillar 2 screwed in the cup 13. A threaded plug 75 in the cap 11 provides access to the pillar 2 for adjustment. The lubber line may be marked on the bell 1 or may be provided by the virtual image 55 by reflection in the top of the bell of a black line drawn on an opaque ground 53. The latter is illuminated at night by two lamps 52, the unwanted light from which is masked by a hood 56. In daytime the hood 56 is reversed so that daylight is admitted through the window 57. The movable element may be lifted from the pivot support and applied to the internal surface of the bell when desired by a magnet fastened on an arm hinged to the aeroplane or fastened to the lid of the box used for transporting the instrument. The means for compensating for the quadrantal deviation comprises soft iron bars on carriers 39, 40, which are moved towards or away from each other along the surface of the supporting plate 32 by threaded spindle 38. The plate 32 is inserted in a chamber 81 between the two bottoms 19 and 21 of a casing 18 in which the bell 1 is mounted. A pinion 35 spring urged out of a recess in the bottom 19 serves to retain the plate 32 in the chamber 81 and also enables it to be turned about the vertical axis of the compass by the thumb wheel 36. The means for compensating for semi-circular deviation are mounted under the bottom 19 and enclosed in a casing 29. A spindle 25 carrying a pair of parallel, but opposite, magnets is adapted to be rotated to bring one pair of poles, e.g. S, N<1>, nearer to or further from a soft iron bar 28 serving to shunt the magnetic flux. A similar spindle 25<1> with magnets and a soft iron bar 28<1> is arranged in a direction at right angles. The spindles 25, 25<1> may be turned by worm gearing which also drives a small drum indicating the position of the magnets. The casing of the instrument may be made from a moulded synthetic resin.