1913] on Gyrostats and Gyrostatic Action 035 



duces the astronomical phenomenon called the precession of the 

 equinoxes. Tlie turning action, or couple, as I shall now call it, may 

 be said to cause the flywheel to " precess " towards the couple-axis. 

 This relation of directions is very important, and should be kept 

 always in mind. 



If this turning response of the body, about an axis which we 

 shall call (3), is prevented when turning about an axis (2), at right 

 angles to (3), is changing the direction of the axis of a rotor — an 

 axis (1), say, at right angles to (2) and (3) — a preventing couple, 

 usually called gyrostatic. about the axis (3), must be applied by the 

 bearings to the axle of the rotor, and therefore an equal and opposite 

 couple by the axle to the bearings. This couple, it is easy to prove, 

 is equal to the product of the spin-momentum and the angular speed 

 at which the direction of the axis of the rotor is being changed. 

 Thus the greater the moment of inertia of the rotor, or its angular 

 speed, or the angular speed of the change of direction of the axis, 

 the greater is the gyrostatic couple. 



For example, the rotor of a dynamo, mounted on one of the decks 

 with its rotor-axis athwart ship, applies, when the ship rolls, a couple 

 to the bearings, the plane of which is parallel to the deck, and which 

 consists of a forward force on one bearing and a sternward force on 

 the other. These forces are reversed with reversal of the direction 

 of rolling, so that an alternating force is applied to each bearing 

 tending to shear it off the deck. Thus if the bearings are at all 

 loose, the axle will knock alternately on the front and back of each 

 bearing. 



Similarly the axle of the rotor of a fore-and-aft turbine, when the 

 ship pitches, applies a force to port to the bearing at one end, and a 

 force to starboard at the other end, which forces are reversed when 

 the direction of the pitching motion is reversed. When the course 

 is being changed the forces of the gyrostatic couple are applied to 

 the top of one bearing and the bottom of the other. 



Now, returning to the pillar-gyrostat, and putting the flywheel 

 in rapid rotation, I turn the pillar round on the table. I have 

 turned, as you see, the base round through one revolution, and 

 throughout the turning motion the axle of the flywheel has remained 

 pointing in the same direction. The friction at the axle about 

 which I have turned the pillar, which you will remember, was 

 sufficient to carry the gyrostat round when there was no spin, is now 

 quite insufficient to cause any serious change of position of the 

 gyrostat. Only a very small couple producing precession acted. 



This experiment illustrates the principle of permanence of direc- 

 tion of the axis of rotation, in the absence of a couple producing 

 precession, the principle on which depend the gyrostatic compass and 

 the self-directing torpedo. Carried within the body of the torpedo 

 is a fast-spinning gyrostat, and at the instant at which the torpedo 

 the impulse-tube this gyrostat is mounted freely with its axis 



