32 Dr Searle, Eapervment on the harmome motion 
is “6/K radians per sec. per sec. towards the equilibrium position 
The motion is therefore harmonic and the periodic time is given by 
[P= Dig a) . seconds |... 06 feces eee (1). 
In the experiment, K is found by weighing and measuring | 
the rigid body and yp is found from a series of measurements of | 
the angle through which the lower end of the wire is turned by) 
a series of couples applied statically. The periodic time is / 
calculated by (1) and this time is compared with that which is: 
observed when the body is allowed to vibrate. The agreement’ 
between the observed and the calculated values of the periodic} 
time forms an experimental test of the accuracy of the dynamical | 
principles employed in. the calculation. | 
3. The wbrating system. It is essential that the torsion wire’ 
should be properly secured (1) to the fixed support, and (2) to the: 
vibrating body. This result is best obtained by soldering each | 
end of the wire into a hole drilled along the axis of a cylindrical | 
rod a few centimetres in length and about 0°5 cm. in diameter, | 
} 
\ 
Fig. 1. 
One of these rods is secured by a set screw to the fixed support ' 
and the other is secured by a set screw in a hole drilled in any | 
body which is to be suspended by the wire. These rods are so | 
much stiffer than the torsion wire that small variations in the | 
positions of the points at which the set screws press upon — 
them make little difference in the couple required to turn the q 
2 Ste 
suspended body through one radian against the torsion of the: wires | 
Care should, however, be taken that the set screws, which, ix 
(1) the vibrating system and (2) the cylinder shown in Fig. 2, | 
make contact with the rod at nearly the same point. The torsion | 
wires used in the author’s practical class at the Cavendish / 
Laboratory are of steel and are about 32 cm. in length, and | 
0-175 cm. in diameter. | | 
A convenient rigid body is a rectangular bar (Fig. 1) of length: | 
