i 
| 
| seen of a rigid body - 33. 
OL em., of width 2.4 cm., and of depth 2Bcm. A hole, into which 
the cylindrical rods attached to the torsion wire fit, is drilled through 
the bar at right angles to the plane of the edges 22, 2A, as in 
‘Rig. 1. The effective mass of the bar, JJ grammes, should: be 
marked upon the bar. This is the mass of the bar before the 
hole was drilled through it or the set screw was fitted to 1t*. 
_ The moment of inertia of the bar is calculated by the formula 
| [C= SIUC (TEP TS ae) (oar, C00 aaa papeeoae cocoon (2). 
‘The mass of the metal taken out of the hole and the mass of the 
set screw are appreciable in comparison with the mass of the bar 
itself, and hence, if the actual mass of the bar after it has been 
drilled and fitted with a set screw were used in formula (2) an 
appreciable error in K would result. But the hole and the set 
screw are so close to the axis of vibration that the moments of 
ie of the metal taken from the hole and of the set screw 
about the axis are quite inappreciable compared with that of the 
bar itself, and thus the moment of inertia of the bar as actually 
used does not differ appreciably from that given by (2), provided 
that by M is understood the mass of the bar. before the hole was 
drilled through it or the set screw was fitted to it. a) ous 
The moment of inertia of the .cylindrical rod soldered to the 
wire is quite negligible in comparison with that of the bar. 
One of the heavy compound laboratory stands supplied by. 
W. G. Pye and Co., of Cambridge, forms a convenient support for 
the upper end of the torsion wire. Whatever support is used 
should be rigid and free from shake. 
The periodic time of the inertia bar should be deduced from 
two or three observations of the time occupied by at least 100 
complete vibrations, and the time-piece should, if necessary, be 
compared with a reliable clock. 
— 4. Determination of the relation between couple and angle. 
In the determination of the couple required to twist the lower 
‘end of the wire through one radian, the inertia bar is removed 
from the torsion wire and a cylinder is substituted, as shown in 
Fig. 2. The couple is applied by means of a thread passing over 
two ball-bearing pulleys and supporting two small scale pans; 
it is convenient to adjust the mass of each pan to be 10 gm. 
A loop is made in the thread and this loop is passed over the set 
‘screw securing the cylinder to the rod at the end of the torsion 
wire. 
Care must be taken that the parts of the thread between the 
eylinder and the pulleys are parallel and horizontal so that, when 
the two loads are equal, the threads may exert a pure couple on 
* Searle, Experimental Elasticity, Note VIL, 
3—2 
