246   NAVIGATIONAL COMPASSES

THE FLORENTIA GYRO-COMPASS   247

siderably altered while the spin-axle is tilted from the horizontal, the spin-axle will deflect in azimuth. The method used in both the Anschiitz and in the Arma gyro-compasses for avoiding the ballistic deflection error by changing the spin-velocity of the gyros as the latitude of the ship changes must result in a certain azimuthal deflection. This deflection is reduced to a negligible amount, however, if the proper change in spin-velocity is made corresponding to each small change in latitude. At latitudes in the neighborhood of 40° the azimuthal deflection is about 0.2 degree for changes of spin-velocity corresponding to 5-degree changes in latitude.

146. Avoidance of the Ballistic Damping Error. - When the meridian component of the velocity of the ship changes either in direction or in magnitude, the inertia of the oil in the damping tanks causes oil to move in the direction opposite that of the acceleration of the ship. For example, if a ship while steaming northward either suddenly stops or makes a quick turn, there is an acceleration of the ship toward the south and oil will move from the south damping tank to the northern one. If the oil could flow freely from one tank to the other, the oil would become at the same level in the two tanks when the acceleration ceased. The constricted passage connecting the two tanks retards equalization however, and develops a precession of the sensitive element away from the normal resting position. This results in a ballistic damping or ballistic turning error (Art. 114).

This error is avoided by preventing damping of vibration of the sensitive element during the time that the velocity of the ship is changing. The greatest acceleration produced by any practical change of speed of a ship on a straight course is so small that the ballistic damping error is negligible so long as the course of the ship is unchanged. The acceleration may be so great when a ship steaming at considerable speed makes a sudden change of course that the turning of a ship may produce a large ballistic damping error. All devices for avoiding the ballistic damping error are designed to prevent damping throughout the time that the ship is making a turn.

The Arma damping cut-out device consists of an electric solenoid-operated valve in the oil line between the two damping tanks. The solenoid is energized by a unidirectional intermittent current that is controlled by a contact switch mounted on the shaft of the phantom element. This cut-out contact switch consists of a con

tact stud attached to the compass frame and a fork attached to a collar that can turn with a certain amount of friction about the phantom shaft. When the ship starts to turn in either direction, the friction between the cut-out switch and the phantom shaft carries the switch around and brings one prong or the other into contact with the fixed contact stud. This completes the electric circuit through the solenoid and causes the cut-out valve to close. If the ship had turned in the opposite direction, the other prong would have made contact and the valve would be closed as before. The distance between the prongs is such that contact is always made before the ship has turned more than about 10 degrees. After the ship has ceased turning, the yawing of the ship back and forth quickly breaks the switch contact, and normal damping is resumed.

147. Avoidance of the Quadrantal or Rolling Error. - A compass with a single gyro is subject to an error when on a rolling ship on an intercardinal course (Arts. 115, 116). This error is avoided in the Arma, as in the AnschUtz gyro-compass, by the use of two gyros with the spin-axles inclined to one another (Art. 140).

§7. The Florentia Gyro-Compass

148. Arrangement of the Principal Parts of the Florentia Master Compass. - The sensitive element of this instrument,* made by the Officine Galileo, Florence, Italy, consists of a single meridian-seeking gyro-wheel in a case, G, Fig. 197, hanging from a hollow ring F that floats in an annular trough of mercury T. The gyro has a mass of about 5 kg. and a moment of inertia of about 70,000 gm. em'. It rotates at a speed of 20,000 revolutions per minute.

The mercury trough T hangs from a vertical spindle carried by a ball-bearing on the horizontal frame or spider D. The spider is suspended by gimbal rings, R,R2, from the top of the binnacle B. The lubber line is engraved on an arm L fastened to the spider. The compass card C is fastened on top of the spindle. The rigidly connected system from the mercury trough to the compass card constitutes the phantom element, P. The phantom element is stabilized by a second gyro SG within a casing directly below the compass card. By means of a reversible motor M, pinioned to a

' U. S. Patent. Martienssen, No. 1493213, 1924.