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.