April io, 19 13] 



NATURE 



'49 



1 



tiun of a small model to that of an important affair 

 of practical railway engineering, and that in the 

 gyrostatic compass gives a direction-pointer unaffected 

 by the iron of the ship, or the rolling and pitching of 

 the vessel. Its properties (summed up in what we 

 call gyrostatic action) have to be reckoned with in all 

 swift-running machinery, such as fast-speed tur- 

 bines, and rotary engines of all kinds, especially if 

 these drive flywheels or propellers. They affect very 

 seriously the stability of aeroplanes, and even of sub- 

 marines, and I am very doubtful if aviators have yet 

 become in sufficient degree instinctively alive to the 

 dangers of sudden turnings, such as those which are 

 encouraged by the promoters of aviation displays in 

 alighting competitions. 



The man who has spun and studied tops and gyro- 

 stats appreciates fs no one else can the extreme im- 

 portance of pro-'srly balancing rotating machinery, 

 and of avoiding gyrostatic action where such action is 

 likely to interfere with the running of the machine- 

 as a whole. 



The properties 

 of a top are best 

 studied in the 

 gyroscope, or 

 gyrostat, as it is 

 better called. Here 

 is a simple 

 gyrostat, of the 

 ordinary form sold 

 in the toyshops, 

 but with some 

 important modifi- 

 cations to enable 

 it to run for a 

 long time at a 

 high speed. It 

 consists, as you 

 s;e, of a heavy- 

 rimmed metal 

 disc, or flywheel, 

 capable of rotation 

 with but little fric- 

 tion on pivots 

 held in sockets 

 attached to a 

 metal frame. Thus 

 the flywheel may, 

 by the quick with- 

 drawal of a string 

 wound round its 

 axle, or in some 

 other way, be set 

 into rapid rotation 

 in the frame, which in turn is mounted in various 

 ways to show gyrostatic effects. But this 

 ordinary form, as well as some others of a more pre- 

 tentious character, suffers from the great disadvan- 

 tage of having no means of maintaining the spin, 

 and the continual renewal of the spin is a great 

 nuisance. 



I have here a gyrostat (Fig. i) in which this draw- 

 back has been overcome by the simple and 

 effective device of making the flywheel itself 

 the rotor of a high-speed continuous-current 

 electric motor. The ordinary gramme-ring arma- 

 ture is well adapted for this. It gives a wheel of 

 great moment of inertia, or, as I call it, "spin inertia " 

 (that is, the matter of the wheel is distributed so as 

 to be on the whole as distant from the axis as pos- 

 sible), which can be run at high speed for a long 

 time without trouble of any kind from bearings or 

 contacts. 



For my first experiments the motor gyrostat is set 



-Motor-gyrostat in " fork and pedestal ' 



NO. 2267, VOL. 91] 



up with the axis of the flywheel horizontal, in this 

 mounting, which consists, as you see, of a fork 

 perched on a pillar. Notice the possible motions, the 

 freedoms, 1 may call them, of the arrangement. The 

 flywheel can turn about its axle, the case can turn 

 about the line of the pivots which carry it in the 

 fork, and the fork about a vertical axis provided in 

 the pillar. These three axles, which we shall number 

 (0, ( 2 )> (3). are mutually at right angles and meet 

 at the centre of gravity of the movable system or 

 gyrostat proper. When thus set up the gyrostat is 

 said to be freely mounted. 



With the flywheel at rest I push down on one side 

 of the case, and immediately turning takes place, as 

 we should expect, about the axis (2). Pushing down 

 the other side of the case causes the instrument to 

 turn about the axis (2) in the opposite direction. I 

 grasp the fork in my hands and turn it about the 

 axle (3) in either direction. Nothing unexpected hap- 

 pens ; the gyrostat turns with the fork, its axis re- 

 maining horizontal throughout. Again, I grasp the 

 pillar in my hands and turn it on the table, and you 

 see that the friction of the axle (3) is sufficient to 

 cause the fork and gyrostat to move round with the 

 pillar. As before, the axis of the flywheel remains 

 horizontal. 



My assistant now causes a current of electricity to 

 flow in the coils which form part of the flywheel and 

 in the coils which surround the soft iron core of the 

 magnet which is stationary within the ring. So far 

 you can only tell that the flywheel is turning by the 

 faint hum which its motion sets up. But when I 

 repeat the operations which I have just performed on 

 the non-rotating gyrostat, the behaviour of the instru- 

 ment is quite startlingly different. I push down on 

 one side of the case as before; a resisting force is 

 experienced, and the gyrostat turns, not visibly 

 about the axle (2), but about (3), the vertical axis. 

 So long as 1 maintain the tilting force so long does 

 the resistance and this turning about the vertical 

 persist. I withdraw the tilting force, and the turning 

 motion ceases. 



Now I would direct attention to these rods with 

 arrow-heads, which are screwed to the gyro- 

 stat case. This curved one shows the direc- 

 tion in which the flywheel is spinning. The 

 straight rods are intended to represent the spin- 

 momentum and the tilting action respectively. Both 

 are completely known when their amounts and their 

 planes are known. The spin-momentum is got by 

 multiplying two numbers together, one representing 

 the spin-inertia of the wheel (which is greater the 

 more the mass is placed in the rim), the other the 

 speed of turning. The turning action or "couple" 

 is also got by multiplying the force with which I push 

 by the arm or leverage of the force about the axis. 

 So then we represent these two by lines drawn at 

 right angles to the two planes, making the lines of 

 lengths to represent the two products. Standing on 

 one side of the plane of the flywheel, you see it turning 

 against the hands- of a clock; standing on one side 

 oif the plane of the turning action I apply you observe 

 that action tending to turn the body also against the 

 hands of a clock. The two lines representing the 

 two products drawn towards you from the two planes 

 represent also the directions of the turning actions 

 of the couples. For example, the direction of rotation 

 of the flywheel being that shown by the curved rod, 

 the line representing the spin-momentum points out- 

 wards from the side of the gyrostat to which the rods 

 are attached. I call this the spin-axis. The other 

 line representing the turning action which I applied I 

 call the couple-axis. 



Now observe that I set the couple-axis so as to 



