Patent GB-506458-A (GB)
Title: Improvements in or relating to electrical earth-inductor navigating or flight instruments
IPC: G01C17/30
Filed: 1938-05-18
Abstract: 506,458. Magnetic tests and measurements. SPERRY GYROSCOPE CO., Inc. May 18, 1938, No. 14768. Convention date, May 18, 1937. [Class 37] [Also in Group XXXV] An earth-inductor compass consists of a coil rotatable in the earth's magnetic field and mounted in such a manner that its rotational axis can be freely maintained in a vertical position, and a compass card rotatable with the coil and illuminated by a stroboscopic flashing lamp controlled from the electrical output of the coil. The instrument shown in Figs. 2 and 3 is mounted in a closed casing 1 having a window 2. A gyroscope rotor 3 is mounted on a vertical shaft 4 journalled in bearings 5, 6 within a rotor casing 7. This casing is mounted on horizontal trunnions 8, 9 in a semicircular gimbal ring 10 which is journalled at its rear for oscillation about a horizontal axis at right-angles to the axis 8, 9 by means of a long stud 11 journalled in bearings 12, 13 3 in the rear wall of the casing 1. Air is continuously withdrawn from the casing 1 through a pipe coupling 15, air at atmospheric pressure entering through a screen 16 and passing through an axial aperture 17 in the stud 11 and a channel 18 in the gimbal ring 10 to jets 19 so as to spin the rotor 3 by impinging against buckets 20 thereon. The rotor 3 is of brass or other non-magnetic material and has a coil 21 embedded in its face. Over this is shrunk a non-magnetic ring 22, in the lower part of which the buckets 20 are cut. The upper part is graduated in degrees or compass graduations, or these may be on a separate thin ring 23. A small portion of the card is visible through a window 24 in the rotor case 7. The rotor is maintained horizontal by the means described in Specification 393,695, [Group XX]. The ends of the coil 21 are connected to sliprings 31 on which bear brushes 30. A neon or other gas-filled quick-flashing lamp 32 is arranged to be momentarily illuminated at a selected point in each revolution of the rotor, or each predetermined number of revolutions, so as to render the compass reading visible. On the rotor casing 7 or an extension 71 arereference lines 45 which co-operate with fixed pointers 46, 46<SP>1</SP> on the casing 1 so as to indicate roll and pitch. The coil 21 is connected through a transformer 33, amplifier 34, and transformer 35 to one of the grids 36 of the gas-filled thermionic valve 32. A second grid 38 is so biassed that the valve will not pass current until the grid 36 passes through zero. A variable resistance 39 and condenser 40 are arranged in the plate circuit of the valve 32 and are so adjusted that the time taken to build up an operating plate potential on the condenser 40 is greater than the period of a half-cycle of the current from the coil 21, but usually equal to or less than a full cycle. The valve discharges when the potential from the coil 21 applied to the grid 36 passes through zero at the beginning of its positive half-cycle. The compass may be corrected for magnetic variation by changing the phase position of the output of the coil 21 by an amount equal and opposite to the magnetic variation. This may be effected by a condenser 43 and resistance 42 in shunt to the coil 21, either or both being variable.