Earth inductor compass

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 Air is continuously withdrawn from the casing 1 through a pipe coupling 15, air at atmospheric pressure entering through a screen 16 and passing through an axial aperture 17 in the stud 11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, 461 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 c