﻿Periodic Field of Force. 17 



teeth, and when a direct current is passed through the electro- 

 magnet, sets itself rigidly at rest with a pair of teeth at the 

 ends of a diameter opposite the two poles of the electro- 

 magnet. The equilibrium under such conditions is of course 

 thoroughly stable, and, in fact, the wheel possesses a fairly 

 high frequency of free angular oscillation for displacements 

 from this position of rest, and any motion set up by such 

 displacement rapidly dies out, apparently on account of 

 Foucault currents induced in the iron by the motion. This, 

 in general, is also true when an intermittent current supplied 

 by a fork-interrupter is used to excite the electromagnet, 

 except however in certain cases, wheu it is observed that the 

 equilibrium becomes unstable of its own accord and the 

 wheel settles down into a state of steady vigorous vibration 

 about the line of equilibrium : or that an oscillation of 

 sufficient amplitude once started maintains itself for an 

 indefinitely long period. 



An optical method can be conveniently used to study the 

 frequency and the phase of the oscillations of the armature- 

 wheel maintained in the manner described above. A narrow 

 pencil of light is used, which first suffers reflexion at the 

 surf ace of a small mirror attached normally to one of the prongs 

 of the fork-interrupter furnishing the intermittent current, 

 and then falls upon a second similar mirror attached to the 

 axle of the armature-wheel parallel to its axis of rotation. The 

 apparatus is so arranged that the angular deflexions produced 

 by the oscillations of the fork and the wheel are at right 

 angles to each other, and the pencil of light which falls upon 

 a distant screen, or which is focussed on the ground-glass of 

 the photographic camera, is seen to describe a Lissajous 

 figure from which the frequency, and the phase-relations 

 between the oscillations of the fork-interrupter and of the 

 armature-wheel, can be readily ascertained. It is then 

 observed that the period of the vibration of the armature- 

 wheel is equal to, or twice, or thrice, or four times, &c. the 

 period of the fork : in other words, the frequency is equal to 

 or -J of or J or ^ or J or J that of the fork. 



In making the experiments, the motor-wheel is relieved of 

 the large stroboscopic disk that is usually mounted upon it, 

 and in working down the series, the adjustment of frequency 

 is secured by suitably loading the wheel. The fine adjust- 

 ment for resonance is effected by altering the current passing 

 through the interrupter with the aid of a rheostat, and if 

 necessary by regulating the contact-maker on the fork. Any 

 oscillation ol the wheel, when started, dies away except in 

 the cases referred to above ; in other words, no Erequencies 



Phil Mag. S. 6. Vol. 29. No. 169. Jan. 1915. C 



