1920] 



on the Gyrostatic Compass 



25 



OA. If we impress a force upon the wheel, tending to tilt or rotate 

 it round another axis OB, then the rule is that the spinning wheel 

 will " precess " or move in such a direction as to try to make the two 

 axes OA and OB coincide, and the direction of spin of the wheel to 

 coincide with the new direction of rotation that we are trying to 

 produce by the applied force. 



On the right-hand side of Fig. 3 I have indicated an electric 

 circuit which has similar mathematical laws to that of the gyrostat. 

 It consists of an outer fixed coil and a central suspended coil. 



A strong direct current, indicated by a, is kept flowing in the 

 central coil ; this corresponds to the spin of the wheel. 



If a direct current, indicated by b, is sent round the outer coil, 

 then the central coil will move in such a direction as to make the 

 axes of the magnetic fields of the two coils coincide, and to make the 

 direction of the two currents also coincide. 



Fig. 3. — Gyrostat and Electrical Circuit Equivalent. 



In fact, the coils will move, or try to move, in such a way as to 

 make the self-induction of the whole circuit a maximum. 



This is very much like the gyrostat (or, in fact, any piece of 

 mechanism), which under impressed forces tends to move so as to 

 make the whole moment of inertia a maximum. 



Suppose, therefore, a gyrostat has its axis OA fixed parallel to the 

 earth's surface, but free to turn in " azimuth," as it is called, upon a 

 frictionless vertical spindle, the earth will act upon such an instru- 

 ment, and it would be a Gyro-Compass. 



The earth as it rotates is continually tilting the axis of the wheel 

 in space ; the wheel will, therefore, turn so as to set its axis of 

 rotation as nearly as possible parallel to the axis of the earth. It is 

 only when the two axes coincide that the wheel is free of any further 

 tilting action— that is, when it is pointing true north ; deviate the 



