198   NAVIGATIONAL COMPASSES

of the " gun-fire control " grade the error must not exceed 0.2 degree.

A gyro-compass equipment includes the master compass, an electric motor-generator, storage battery and switchboard, together with repeater compasses. Many installations include a course recorder, a radio direction finder, and an automatic pilot operated by the master compass. By means of a gyro-compass controlled automatic pilot, the ship may be kept in a straight course as long as may be desired without the aid of a helmsman. The master compass and electric plant are installed between decks. Then the ship is steered from a regular repeater compass, and bearings are taken from two other repeater compasses at the ends of the officers' bridge.

The magnetic compass is a simple and relatively cheap instrument, but the errors to which it is subject can be but partially eliminated or allowed for. On the other hand, the gyro-compass is a complicated and expensive outfit, but the errors to which it is subject can be completely eliminated automatically. A standard magnetic compass for a large ship costs about $500. The cost of a master gyro-compass such as is found on merchant ships and the smaller naval vessel costs about $3000. The cost of a gyro-compass equipment as used on merchant ships, consisting of the master compass, electric plant, three repeaters and course recorder, costs about $5500. An automatic pilot adds about $2200 to the cost of the compass equipment. A single gyro-compass equipment of the highest grade, such as is used for gun-fire control on large battle ships, costs from three to four times as much as one of the " navigational " grade used on merchant ships. It is usual to have two complete " gun-fire control " gyro-compass equipments on a large battle ship.

§3. The Sperry Gyro-Compass

119. The Principal Parts of the Master Compass. - Since 1920 all gyro-compasses made by the Sperry Gyroscope Company* have been single-wheel instruments of the liquid-controlled nonpendulous type (Art. 107). The various models differ in directive torque, degree of precision, and in details of design, but they are similar in all essential respects. The models commonly employed on mercantile vesselss are designated Mark VI and Mark VIII. The gyro-wheel of each is 10 inches in diameter, weighs 54 pounds,

* Made in Brooklyn, N. Y., in London, England, and in Tokio, Japan.

THE SPERRY GYRO-COMPASS   199

and rotates in the open air at a speed of 6000 revolutions per minute. The two models differ in that the gyro-wheel of Mark VI constitutes the armature of a direct current motor and the field coils are imbedded in the gyro-casing, whereas the gyro-wheel of Mark VIII constitutes the rotor of a three-phase alternating current motor and the stator coils are imbedded in the gyro-casing. Mark V and Mark X are designed for use on naval vessels. The gyro-wheel of the Mark V is 12 inches in diameter, weighs 45 pounds and rotates at 8600 revolutions per minute. The gyro-wheel of the Mark X Gun-Fire Control Gyro-Compass* is 138 inches in

diameter, weighs 122 pounds and rotates at 10,000 revolutions per minute. The gyro-wheels of Mark V and Mark X rotate in a partial vacuum of about 28 inches of mercury below atmospheric pressure.

A simplified diagram showing the fundamental parts of the Sperry gyro-compass, as seen when looking toward the compass from the south, is represented in Fig. 152. The gyro-wheel, spinning about a nearly horizontal axis perpendicular to the plane of the diagram, is enclosed by the gyro-casing G. The gyrocasing is supported on horizontal bearings by a vertical ring R which, in turn, is suspended by a system of vertical wires from the top of a tube extending upward from an outer ring P called a " phantom." The phantom is supported by ball bearings on a " spider " S supported on gimbal rings carried by the binnacle XX. The phantom carries the compass card C, and the " azi

* See Frontispi