15° 



NATURE 



[April io, 1913 



point toward your left. I push down the side of the 

 gyrostat nearest me, and you see that the spin-axis 

 turns towards the left. Again, I turn the couple-axis 

 so as to point to your right. When so placed it repre- 

 sents a turning action tending to depress the end of 

 the axle of the flywheel that is nearest you. I apply 

 such an action and the spin-axis turns towards your 

 right. In both cases the spin-axis turned towards 

 the instantaneous position of the couple-axis. 



Now I set the couple-axis vertical, pointing- up. It 



Fig. 2.— -Motor-gyrostat 



d to demonstrate the principle of the dirigible torpedo 



represents a turning action tending to produce hori- 

 zontal turning in the counter-clock direction as seen 

 from above. I apply such an action to the fork, when 

 you see that the gyrostat turns the spin-axis towards 

 the upward direction. Finally, I set the couple-axis 

 vertical but pointing down, as in Fig. i. It now 

 represents a turning action tending to produce clock- 

 wise rotation as viewed from above, counter-clock 

 rotation as seen from below. I apply the action re- 

 presented and the gyrostat turns the spin-axis towards 

 the downward direction. 



These experiments may be summed up as follows : — 

 The flywheel is spinning about axis (i). Any attempt 

 to tilt the gyrostat about axis (2) produces turning 

 about (3) ; an attempt to tilt it about (3) produces 

 turning about (2). This response of the body seems 

 paradoxical, but in point of fact, and this is the secret 

 of the whole affair, this turning of the body as a 

 whole amounts to the production of spin-momentum 

 about the couple-axis at exactly the proper rate. It 

 is quite easy to prove this by the consideration, in the 

 most elementary way, of the accelerations of the 

 different particles composing the wheel. 



The turning of the spin-axis towards the couple- 

 axis is called a precessional motion, from a similar 

 motion of the earth which produces the astronomical 

 phenomenon called the precession of the equinoxes. 

 The 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 bearing-s, 

 the plane of which is parallel tn the deck, and which 

 consists of a forward force on one bearing and a 

 no. 2267, VOL - 9 1 ] 



slernward force on the other. These forces are re- 

 versed 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 

 al the oilier end, which forces are reversed when the 

 direction of the pitching motion is re- 

 versed. 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 By- 

 wheel has remained pointing in the same 

 direction. The friction at the axle about 

 which I have turned the pillar, which, you 

 will remember, w^as sufficient to carry the gyrostat 

 round when there was no spin, is now quite insuffi- 

 cient to cause any serious change of position of the 

 gyrostat. Only a very small couple producing pre- 

 cession acted. 



This experiment illustrates the principle of perman- 

 ence of direction 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 tor- 

 pedo is a fast- 

 spinning gyrostat, 

 and at the instant 

 at which the tor- 

 pedo leaves the 

 impulse-tube this 

 gyrostat is 

 mounted freely 

 with its axis coin- 

 cident with that 

 of the torpedo — 

 that is pointed, so 

 to speak, exactly 

 along the "cigar." 

 Any turning of 

 the torpedo body 

 sideways brings 

 about a relative 

 shift between the 

 gyrostat and tor- 

 pedo axes, and 

 this shift brings 

 into operation a 

 vertical rudder at 

 the stern of the 

 torpedo. If the 

 nose of the tor- 

 p e d o turns to 

 port, the rudder 

 steers the craft to 

 starboard, and 

 vice versd. 



Here (Fig. 2) is a skeleton frame representing a 

 torpedo. It is mounted on a vertical axle, and carried 

 on pivots within the structure is one of our motor- 

 gyrostats. At the stern of the frame is a small rudder, 

 and this is connected by means of cords to the gyro- 

 stat. I set the flywheel in rotation. When, as I do 

 now, I turn the nose of the torpedo to port, the rudder 



