290 THE SPERRY GYRO-COMPASS IN SERVICE. 



meridian; the sensitive element of the gyro is peculiarly suspended with 

 three degrees of freedom independent of the compass card, so this inclination 

 of the axis will take place independently from the compass card, which is 

 always in a horizontal plane by reason of its gimbal ring suspension. These 

 facts give us a means for finding the meridian independently of the compass 

 card. The device used is a level, attached to the gyro casing or sensitive 

 element, and capable of adjustment for latitude in such manner that the 

 bubble of the level will always be in exact center when the compass is on 

 the meridian. Any deviation from the meridian can be noticed by a change 

 in the position of the bubble almost before the deviation occurs, and in 

 finding the meridian after starting up the compass it is only necessary to 

 level the compass by impressing forces by the hand about the vertical axis, 

 and as it inclines from the horizontal count the number of divisions which 

 the bubble passes in one minute; this multiplied by a known factor gives 

 approximately the number of degrees which the axis of the gyro is from the 

 meridian. Knowing this, the compass may be placed on the meridian by 

 impressing a force downward on the end of the axis which is inclined up from 

 the horizontal. 



An alarm is supplied which sounds when current is removed from the 

 main system supplying the gyro motor, when current is removed from the low 

 voltage system supplying the azimuth motor and repeaters and when any 

 serious oscillation of the compass occurs. 



Recent severe tests of the Sperry gyro-compass have shown that the 

 speed of the wheel need not be kept constant to hold the axis of the gyro 

 on the meridian. By numerous tests it was found that power could be 

 removed from the gyro wheel and that absolutely no variation would occur 

 until the speed of the wheel dropped from 8,600 to as low as 5,000 revolu- 

 tions per minute. This drop of speed would cover a period of approximately 

 one and one-half hours. This development is very important to the use 

 of the gyro-compass inasmuch as it obviates the necessity of keeping the 

 voltage of the supply constant and makes it certain that the compass will 

 be useful for at least one hour in case the current supply is removed by 

 damage to the dynamos or by any other cause. In case of failure of the 

 direct current supply, the 20-volt system is automatically energized by 

 storage battery so that the follow up, and repeating compass systems will 

 continue to operate for a period of at least ten hours. 



It can be seen from Fig. i , Plate 1 01 , that the construction of the compass 

 is strong and durable and that there are no parts requiring expert supervision or 

 attendance. In fact the only parts of the master compass which require any 

 degree of care are the silver trolleys and segments on which they make con- 



