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of its true position relative to the vertical through the gyro-axis. The N.-S. component causes the bail-weight to exert an alternating pressure on the case at the eccentric pivot, first N. then S. These two alterations, the E.-W. swing of the pivot and the N.-S. pressure at the pivot, keep step, and so a torque of invariable sign in a horizontal plane is produced. This causes the gyro-axis to tilt and wander and so introduces rolling error. To get rid of this error Sperry aimed at eliminating the E.-W. swing of the pivot by making use of a small gyro pendulum, called the floating ballistic, to form the connexion between the bail weight and the case. This fitting gave excellent results except in bad weather; a further modification, ob- tained by the addition of a frame, carry-ing a lead weight, to the stem of the pendulum, in order to raise its centre of gravity and so increase its period, produced a great improvement. Even so the compass was not quite reliable in really bad weather, and it was not until the mercury control was fitted that the intercardinal rolling error was finally overcome. In this device the period of the liquid pendulum is so arranged that the forced oscillations of the mercury due to the rolling of the ship are approximately 90 out of phase with the roll, and so complete compensation is obtained.

VERTICAL AXIS UPPER GUIDE LUBBER RING

FIG. I. Thwartship section of frame. South view of compass.

Mechanical Operation of the Compass. In the Sperry compass a follow-up system is essential to the correct functioning of the master compass; in other types, such as the Brown and Anschutz, this is not the case. The reason for this lies in the different methods of suspension, all of which must be as nearly free from friction as possible, as has been previously stated.

LU88ER PING PHANTOM GEAR

FlG. 2. Aft view of frame. North view of compass.

The gyro and case in its vertical ring, known as the sensitive ele- ment (see figs. I, 2 and 3), is supported from the top of the phantom or follower ring by a wire suspension. The vertical ring is also pro- vided with upper and lower guide bearings in the phantom. These bearings do not support any of the weight, the whole of which is taken by the suspension. Suitable trolley contacts (not shown in

the diagram) are carried on the trolley posts and work over fixed contactors situated opposite them on the upper part of the phantom. The compass card is fixed to the top of the phantom ring and the rack just below it gears into the azimuth motor which is fixed to the frame or spider. The trolley and fixed contactors are suitably con- nected to the relay of the azimuth motor.

VERTICAL PHANTOM

FIG. 3. Starboard view of frame. West view of compass

This electrical follow-up system operates so as always to keep the phantom co-planar with the vertical ring. If the gyro precesses in azimuth the trolley wheel is carried to one side or other of the fixed contactor which is insulated in the middle; this operates the azimuth motor through the relay and moves the phantom which carries the compass card, to follow the gyro. On the other hand, when an al- teration of course takes place the phantom is at first carried round by the ship until the contacts cause the azimuth motor to drive it back to its normal position in relation to the vertical ring.

In addition to the master compass, which is usually placed more or less centrally in the ship and near the water line, repeater com- passes form part of an equipment and may be placed in any con- venient position.

BRACKET CARRYING MERCURY BOXES

LEO CARRYING HOLLER

CASING TBACK

CONNECTING TUBE

ROLLER

FIG. 4. General view of mercury control.

They are worked through a transmitter which is operated by a pinion working into the phantom rack just under the compass card. This pinion forms the head of a camshaft which carries three double-- faced cams set 60 apart. These cams operate contacts which, using a common return, work the repeater motor. The speed and latitude dials in conjunction with the cosine ring provide an automatic correction for both speed error and damping error, by turning round the lubber ring through an angle equal *o the sum of these errors.

As seen from the S. side the direction of rotation of the wheel is clockwise, but this must be reversed when the mercury control is fitted, because this form of gravity control is top-heavy.

References. For a detailed description of the whole equipment and instructions as to the care and maintenance of the Sperry compass, the Sperry Handbook, 2nd ed. Feb. 1919, gives full in- formation. This, and a handbook on their Twin Compass, may be