460,244. Gyroscopic compasses. RAWLINGS, A. L. July 25, 1935, No. 21100. [Class 97 (iii)] [Also in Groups XXXVIII and XL] In a gyroscopic compass the driving means for spinning the rotor is capable of movement relatively to the spin axis of the rotor and is caused by a followup device to follow the movements of the rotor both in azimuth and elevation. Movement of the driving means in elevation causes it to be offset in azimuth relatively to the rotor so as to make the gyro meridian seeking. In the form shown in Fig. 1, the ball rotor 1 is spun in a cup 2 by an air jet 3 and is supported solely by an air film. The cup 2 is carried by a yoke 5 supported on trunnions 6, 6 by a fork 7. The latter can turn in azimuth on a stud 10 on the gimbal mounted main frame 11 and carries an azimuth gear 8 engaged by the azimuth follow-up motor 13 and an elevation follow-up motor 14 which engages a toothed sector 15 on the yoke 5. A ring 16.pivoted about a slightly inclined axis 17, 18 in the yoke 5 carries two pairs of electrically heated coils 21, 22. The coils of the pair 21 are mounted vertically with their mid-points level with the spin-axis and are slightly spaced apart so that the air flung off at the equator of the rotor 1 normally passes between them. The coils 21 are arranged in two arms of a Wheatstone bridge of which one diagonal contains the field winding of the motor 13 so that when the coils 21 are not symmetrically arranged relatively, to the equatorial plane of the rotor they are differentially cooled by the equatorial air stream, and the consequent unbalancing of the bridge causes the motor 13 to restore the symmetry. The coils 22 lie horizontally above the rotor 1 and operate in a similar manner to control the elevation motor 14. A pendulum 25 pivoted to the yoke 5 at 24 and damped by a dashpot is connected by a link 30 to the ring 16 and, when the spin axis of the rotor 1 and the follow-up yoke 5 are inclined causes the ring 16 to rotate and adjust the coils 21, 22 so that the yoke 5 and driving jet 3 take up a position in which they are slightly offset both in azimuth and elevation relatively to the rotor. The offset of the driving means 3 in azimuth serves to process the gyro towards the meridian and the offset in elevation to provide damping. The follow-up elements may also be offset in azimuth by an amount equal to the latitude error for a particular latitude so that a compass card 19 on the fork 7 indicates true azimuths against a lubbers mark 20 which requires only slight adjustment for different latitudes. The coils 21, 22, may be carried by separate frames separately controlled by adjustable pin-andslot connections from a single pendulum. In a further modification the coils 22 are secured to the yoke 5 and to provide damping the pendulum 25 controlling the coils 21 is influenced by a spring causing it to deflect through a fraction of the inclination of the yoke 5 and carries a pair of liquid bottles with a N-S connecting pipe which is constricted so that the liquid can only pass slowly when the arrangement is tilted. An electromagnetically operated valve stops the flow when the supporting craft is accelerating. In a further modification both pairs of coils 21, 22 are fixed to the yoke 5 and the offset of the follow-up system is effected, for example in azimuth, by providing the bridge controlling the motor 13 with unequal resistance arms R (not shown) opposite the arms containing the coils 21 so that the motor stops only when the resistances of coils 21 are correspondingly unequal. This occurs when the yoke 5 is offset so that one coil 21 is cooled by the equatorial air flow more than the other. In order that the resistances R may be varied in accordance with the inclination of the yoke 5, they may comprise the parts into which a resistance is divided by a pendulum controlled slider or loops of wire 49, 50, Fig. 9, inserted into a glass tube 46 and partially immersed in and short circuited by mercury 52, the tube 46 being secured to the yoke 5 so that when the latter is inclined mercury flows from one side of the constriction 48 to the other shortening one loop and lengthening the other. Alternatively the resistance R may be replaced by light sensitive cells which are differentially illuminated by a light source in accordance with the position of the shadow of a pendulum or of mercury in a tube such as 46, Fig. 9. In a modification shown in plan in Fig. 12, the ball rotor 1 is spun on a self-generated air film in a cup (not shown) by a poly-phase stator 57 secured to the yoke 5. The cup is fixed to the main frame of the compass. The rotor has two flat polished surfaces 58, 58<1> at the ends of, and perpendicular to, the spin axis. The coils 21 are replaced by a pair of photoelectric cells 61, 62, and a beam of light from a source 63 is reflected from a mirror 60, surface 58, back to mirror 60 and falls between the cells 61, 62 when the yoke 5 and rotor 1 are in alignment. A similar arrangement turned through a rightangle and co-operating with the surface 58<1> or a second pair of cells placed above and below the azimuth cells 61, 62 and co-operating with the same light beam control the elevation motor. The follow-up yoke 5 and stator 57 are given the required offset by a pendulum 23, 25 which is hung from the yoke 5 by a leaf spring and has an arm 68 engaging and slightly adjusting the mirror 60 on its spring support 66 when the rotor axis and yoke 5 are inclined. When the responses of the cells 61, 62 are to be amplified dull black sectors are painted on the surface 58 so that the light falling on the cells is continuously interrupted and the amplification thereby simplified. In a further modification the follow-up motors may each be controlled by the difference between the amplified outputs of the secondary windings of a pair of transformers with their poles close to the edge of a recess in the rotor. The required offset may be effected by moving the transformers relatively to the yoke 5 by means of a pendulum or by varying the bias of the amplifier in accordance with the inclination of the yoke. Specification 11028 /11, [Class 97 (iii)], is referred to in the Provisional Specification.