I 76 



NATURE 



[April 17, 19 13 



I would now direct your attention to this motor- 

 gyrostat, which forms the bob of an ordinary com- 

 pound pendulum (Fig. 10). The tube carrying the 

 gyrostat is attached, by means of a universal joint, 



-Motor-gyrostat fitted up as a gyr 





to the apex of a triangular stand, made of telescope 

 tubing. The gyrostat is attached to the lower end of 

 its supporting tube by means of a special cap provided 

 with spring contact pieces to allow the current to be 

 led into the motor, and the flywheel is free to rotate 

 about an axis coincident with the rod. Screwed to 

 the lower sjde of the gyrostat is a 

 pen, which presses lightly on a card 

 placed below. 



We have now the pendulum rod in 

 the vertical position. I draw the pen- 

 dulum to one side and let go, when 

 you see that it vibrates to and fro, 

 and the pen traces out a straight line 

 on the paper. The flywheel has as 

 yet no spin. I start the flywheel 

 revolving, draw the pendulum to one 

 side, and let sjo, either from rest, 

 or with a certain amount of sidelong 

 motion, when you observe that the 

 pen describes a flower-shaped path 

 (Fig. 11). The path is shown for 

 different amounts of sidelong motion. 

 The peculiar appearance of these 

 curves is due to the rapid falling ofl 

 of amplitude produced by friction. 



When the flywheel is revolving there are, in general, 

 two couples acting on the pendulum, one due to 

 gravity, the other due to gyrostatic action. At an 

 instant at which the axis of the gyrostat is vertical 



NO. 2 268, VOL. 91] 



the former couple is zero and the latter one is a maxi- 

 mum, for at that instant the angular velocity with 

 which the axis of the gyrostat is changing direction is 

 greatest. When the pendulum is at one extremity of 

 its swing the former couple is a maximum and the 

 latter one is zero. At that instant the deflection of 

 the bob from the vertical is a maximum, and it is at 

 rest, or is moving sideways, according to the mode 

 of starting, except in so far a^s the initial conditions 

 have been interfered with by friction. By this rela- 

 tion of the couples the form of the path can be 

 explained. 



Another mode of motion is possible which has a 

 very intimate connection with the theory of the vibra- 

 tions of light-emitting molecules in a magnetic field, 

 as indeed I pointed out here several years ago in a 

 Friday evening discourse (see Nature, April 13, 1899, 

 and August 24, 1899). The bob can be made to 

 move in a circle about the vertical through the point 

 of support either with or against the direction of 

 rotation of the flywheel. The two periods are 

 different, and the motions correspond to the circularly 

 polarised light of two distinct periods, which mole- 

 cules, situated in a magnetic field, are found to emit. 

 Thus the gyrostatic pendulum gives a dynamical 

 analogue of the cause of the Zeeman effect. 



In 1907 Herr Otto Schlick introduced a method of 

 employing a gyrostat to counteract the rolling of a 

 vessel at sea. The gyrostat is carried on bearings 

 placed athwart the ship. These bearings are in line 

 with the flywheel, and a weight is attached to the 

 frame of the gyrostat in a position in line with the 

 axis. It will be seen that when the ship is on 

 even keel the gyrostat rests with its axis vertical, 

 and with the weight vertically below the centre of 

 gravity of the flywheel. Heeling of the ship in one 

 direction causes the gyrostat to precess in one direction 

 on the bearings on which it is mounted ; heeling in 

 the other direction causes precession in the opposite 

 direction, and couples resisting the rolling motion are 

 brought to bear on the ship. The device may be 

 employed in two ways. In the first place, if the bear- 

 ings on which the frame of the gyrostat is carried 

 within the ship are smooth, the effect of the gyrostat 

 is to resist the rolling force of the waves, and to bring 

 about a lengthening of the free period of the ship, 

 according to a mathematical theory which, when put 

 in the proper way, is really very simple. Excessive 

 rolling of a ship is due to the cumulative action of 

 the waves, and such cumulative action is only possible 



11. — Some curves obtained with the gyrostatic pendulum. 



when' the period of the ship and that of the waves 

 are of about the same order. A large ship has a verv 

 long period, and synchronism of the ship and the 

 waves i- impossible. The effect of introducing a gyro- 



