&| 
34 Dr Searle, Experiment on the harmome motion j 
3 
the cylinder. Special care must be taken to ensure that the 
threads are tangential to the cylinder. 
If the diameter of the cylinder be D cm. and that of the thread | 
be dem., the distance between the axes of the two threads is 
Fig. 2. 
D+dcm. and hence, when ine load on each thread is m gm, 
the couple is 
= mg(D+d)dyne-cm. ........ eee (3). 
The angle 0, ae which the couple G causes the cylinder ~ 
to revolve, is determined by aid of the simple goniometer shown in | 
Fig. 3. 
This instrument was designed in conjunction with W. G. Pye 
and Co. to provide a means of eas Urn, angles up to about 
4 radian (about 14°) with an accuracy of z)55 radian. 
Fig. 3. 
The base is formed of a strip of wood furnished at one end with 
a spherical pivot and at the other with a cross-bar carrying a scale. 
Angles are measured by means of a movable arm which turns at one 
end about the pivot, while the other end moves over the scale on 
