254 NAVIGATIONAL COMPASSES
applicable only to a compass that spins in the same direction as the rotation of the earth.
8. Why are the spinning wheels of gyro-compasses always of great angular momentum?
8. Deduce an expression for the magnitude of the directive force acting upon a gyro-compass. Where, upon the earth's surface, does the gyro-compass possess its maximum and where its minimum directive force?
10. Suppose that, in assembling a gyro-compass, the gyro-wheel were mounted slightly too far toward the south. What would be the effect upon the direction of the settling position of the spin-axle of the gyro?
11. Show that if the spin-velocity of a Sperry gyro-compass diminishes considerably from the correct value, the compass will deflect westward of the meridian plane when the compass is in northern latitudes.
12. In going from the equator to latitude 60°, the spin-velocity of both the Anschutz and the Arma gyro-compass is reduced 50 per cent. Why? State the reason why this change is not accompanied by a deflection of the spinaxle out of the meridian plane.
13. Describe the cause of the " latitude error." Give the names of the gyro-compasses in which it exists and show how it is compensated.
14. Describe the cause of the " meridian-steaming " or " north-steaming " error and show how it is compensated.
15. Describe the cause of the " ballistic deflection " error and show that this error will not occur if the period of vibration of the spin-axle has the proper value.
16. Find the period of a gyro-compass that will have zero ballistic deflection error when the compass is at a given latitude.
17. Show what is meant by the " ballistic damping error " and state how it can be prevented.
18. Describe the cause of the " quadrantal " or "rolling " error and give two methods by which it is suppressed.
19. The mercury ballistic of the Sperry gyro-compass is supported by a " phantom element." Why could it not be attached directly to the gyrocasing?
20. The mercury ballistic of the Sperry gyro-compass is loosely connected to the gyro-casing by a pin that is a little to the east of the vertical line through the center of the gyro. Show what would be the effect upon the compass if this eccentric connection were offset to the west of the vertical axis rather than to the east.
21. Describe the methods used in three different makes of gyro-compass for causing the position in azimuth of the compass card of the master compass to remain in fixed relation with respect to the direction of the spin-axle of the gyro.
22. Name a natural error to which gyro-compasses are subject, that is avoided in each of the following types: (a) pendulous sensitive element, (b) non-pendulous element provided with a mercury ballistic, (c) two-gyro sensitive element. Explain how each type avoids the error named.