46 



NATURE 



[March ii, 1920 



nibtion of the pendulum persists. When the compass 

 is carried round in a horizontal circular path without 

 wobble, the plate still goes round, or tries to go round, 

 with a circular movement. This should be of interest 

 to mathematicians. 



Before leaving- the instrument I will set it spinning 

 so as to demonstrate the frictionlessness of the vertical 

 axis. It is rotating now entirely by means of the 

 energy of the motion of the plate, and I think you 

 will find at the end of the lecture that it is still 

 revolving, but, of course, not so fast as at present. 



The magnetic compass is a simple piece of ap- 

 paratus, but it is complicated in its readings and cor- 

 rections, and points to the magnetic north. The m'ro- 

 compass is a complicated instrument, but simple in 

 its readings, and it points to the true north. 



Before proceeding to describe the gyro-compass I 

 wish to direct attention to the equipment here dis- 

 played. A gvro-compass is in full operation, and at 

 the present moment is recording its movement upon 

 a travelling strip of paper. About half an hour before 

 the lecture started the compass was deflected from 

 the north position, and it has since been left to itself. 

 The record shows that it is engaged in swinging back 



again to the north, recording a curve upon the paper 

 strip, and this record can be followed during the whole 

 of the lecture. 



The compass is working two repeaters, which 

 truly copy the reading of the master compass. 

 Of course, any number of repeaters could be 

 used on board ship if it were necessary. The 

 steering repeater (Fig. i) has a card that revolves four 

 times to one of the master, and the divisions are, 

 therefore, very much enlarged. The other is a cor- 

 rection repeater; it moves backwards and forwards 

 very slightly, and this motion we term the "hunt." 

 In the steering repeater the "hunt" has been cut out 

 by providing the mechanism within the case with a 

 requisite amount of slackness. 



About sixty-eight years ago Foucault did what was 

 thought a wonderful thing at the time ; he gave a 

 lecture-room proof that the earth was rotating on its 

 axis — he looked through a microscope at a gyrostat. 

 He could not get a frictionless, free, vertical axis, so 

 that the experiment could not last for long. I shall 

 be able to show you a piece of apparatus which carries 

 out Foucault 's idea in a perfect way, and will be 

 visible to this audience. 



NO. 2628, VOL. 105] 



A gyrostat consists of an accurately balanced 

 spinning wheel, mounted with as little friction as 

 possible, and in such a way that the axis of the 

 wheel may point in any direction in space. Mere 

 translation in space has no action on the instrument; 

 carrying it about, for instance, does not alter the 

 direction of the axis. On the other hand, the gyrostat 

 is acted upon by any force that tends to tilt the axis 

 or to give the axis a new direction in space. 



The wheel (Fig. 2) spins round its axis ; call the 

 direction of this oa. If we impress a force upon the 

 wheel tending to tilt or rotate it round another axis oh, 

 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 oh coincide, and the direction of spin 

 of the wheel to coincide with the new direction of rota- 

 tion that we are trying to produce by the applied force. 



An electric circuit has similar mathematical laws 

 to those of the gyrostat, and may be used as an 

 illustration. The circuit here used (Fig. 3) cpnsists 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 cf 

 the wheel. If a direct current indicated by h is sent 

 round the outer coil, then the central coil will move 

 in such a direction as to make not only the axes of 

 the magnetic fields of the two coils, but also the 

 direction of the two currents, coincide. 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. 



Tins 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 

 momentum 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 instrument, 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 



