PROCEEDINGS 
OF THE 
Cambridge Philosophical Society. 
Experiment on the harmonic motion of a rigid body*. By 
G. F. C. Searzez, Sc.D., F.RS., University Lecturer in Experi- 
mental Physics, Fellow of Peterhouse. 
[Read 24 November 1913.] 
1. Introduction. One of the problems which confront a 
‘demonstrator of experimental physics is to teach the principles 
‘of harmonic motion in such a way that they shall be absorbed by 
the students and shall be incorporated in their mental equipment. 
For some reason, not easy to understand, harmonic motion does 
present real difficulties to students, but every effort should be 
made to clear away these difficulties, since many important methods 
of determining physical quantities depend upon measurements 
‘made on bodies which vibrate harmonically. The experiment 
‘described below is designed to illustrate the principles of har- 
| monic motion in the case of a rigid body vibrating about a vertical 
laxis under the influence of the torsion of a wire. 
2. Theory of experiment. Let a rigid body be suspended 
| from a fixed support by a vertical torsion wire and let the moment 
jof inertia of the body about the axis of the wire be M gm. cm.’. 
‘Let the couple required to turn the rigid body through one radian 
jagainst the torsion of the wire be » dyne-centimetres. Then, 
‘when the angle is @ radians, the couple is w@ dyne-cm., and, when 
the body is free to move, the angular acceleration of the body 
* The description of this experiment has been reproduced in a small manual on 
| Experimental Harmonic Motion which is now (Nov. 2, 1914) in the press. The 
manual will be published by the Cambridge University Press and is similar in 
character to the author’s Experimental Elasticity. 
VOL. XVIII. PT. I, 3 
