36 Dr Searle, Experiment on the harmonic motion ' 
instrument, all that is necessary is that the mirror should be nearly 
vertical and that the rays from the wire, after passing through the 
lens, should fall upon the mirror. No other adjustments are re- 
quired, and the centre of the spherical pivot need not lie on the 
vertical axis about which the body turns. . 
The goniometer is placed so that its lens is three or four centi- 
metres from the mirror carried by the suspended system. _ 
In the present experiment, a plane mirror is attached to the 
suspended system in the manner shown in Fig. 2, by means Of, 
soft wax, or, more conveniently, by means of the simple device: 
shown in Fig. 4, in which the mirror is attached to a horizontal) 
| 
| 
v 
Fig. 4. 
axis and is thus capable of easy adjustment. The mirror is 
adjusted so that it is possible to see the inverted image of the. 
small scale attached to the goniometer arm crossing the vertical. 
wire of the goniometer. The goniometer is securely fixed so as to. 
be free from shake and from liability to accidental displacement. | 
The base is adjusted so that when the arm is in its central position, 
the image of the wire coincides as nearly as possible with the wire 
itself. The arm is then adjusted so that the image of the wire © 
exactly coincides with the wire itself, and the reading of. the | 
indicating wire on the edge of the scale on the cross-bar of the | 
goniometer is taken. 
The thread is then attached to the cylinder and is passed over | 
the pulleys, carrying the pans alone. The goniometer arm is then — 
moved until the goniometer wire again coincides with its own — 
image, and the reading on the scale is taken. The load on each | 
end of the thread is then increased by steps of 10 gm. and the i 
observations are repeated for each load. 
It is easily seen that the goniometer arm turns through the | 
same angles as the cylinder, when it is properly adjusted at each | 
stage. 
If the reading on the scale of the goniometer for any position | 
of the arm differs from the reading when the arm is in the central | 
position by #cm., and if the distance from the centre of the 
spherical pivot to the edge of the scale be pcm., then the angular 
displacement of the arm is @ radians, where tan 0 = a/p. 
° 
