64 Dr Searle, A bifilar method of measuring the rigidity of wires 



the method of using it for experiments of this type will be found in 

 Proc. Catnb. Phil. Soc, xviii, p. 31, or in the author's Experimental 

 Harmonic Motion, p. 35. The goniometer measures the tangents 



of angles. 



The motion of the suspended system, as so far described, being 

 only slightly damped, it is consequently not easy to reduce the 

 system to rest, and the vibrations of the building add to the diffi- 

 culty. A simple damping device is therefore used. An annulus of 

 thin sheet metal is carried by the bar GH, which is clamped to the 

 rod Q. The annulus is immersed in motor lubricating oil or other 

 highly viscous liquid contained in the annular trough U, which 

 rests on the table. The rod Q passes through a hole in the table. 

 By adjusting the height of GH, the annulus can be brought close 

 to the bottom of the trough, and then the motion is so highly 

 damped that tte system is practically immune to vibrations of 

 the floor or the table. 



If the wires are overstrained by turning the heads through toO' 

 large angles, the wires will no longer be vertical when the heads read 

 zero, and it will be necessary to readjust the screws in the bar EF, 

 To prevent overstrain, and at the same time to allow the heads to 

 be turned through tt in either direction from their zeros, a movable 

 safety device is used. A metal disk, about 1 cm. in diameter, 

 can turn freely about its centre on a screw by which it is attached 

 to the board XY (Fig. 2). A vertical pin is fixed excentrically in 

 the disk, the greatest distance from the pin to the axis of the head 

 being small enough to prevent the steel wire, which forms the 

 index of the head, from passing the pin. The torsion head can then 

 be turned only a little more than tt in either direction from zero. 



Care must be taken not to bend the wires near the soldered 

 joints. A bend at 5 or Z) will alter the effective value of a^. If the 

 wire AB is bent near A, the effect, when the torsion head is turned, 

 will be the same as if the point A describes a small horizontal 

 circle. This causes changes in a-^ as the head is turned, and, what 

 is more serious, causes the bar EF to turn through angles which are 

 by no means neghgible, in addition to the angles directly due to 

 the torsion of the wires. For this reason, annealed wires are more 

 suitable for the experiment than hard drawn wires, as they are 

 more easily straightened. 



The torsion heads are read on circles divided at intervals of 45°, 

 the dividing lines being scribed on the board XY. 



§ 4. Theory of the method. If each torsion head is turned from 

 its zero through </> radians in either direction, the bar EF will turn in 

 the same direction until the bifilar and torsional couples are equal. 

 If EF turns through 6, the whole twist of each mre is (f> — 6. 



Let the radius and the length of each wire be r cm. and I cm.. 



