372 
ME. W. CEOOKES ON KEPULSION RESULTING EEOM RADIATION. 
This is a nearly perfect lubricant, allowing very free movement and preventing any access 
of air. 
210. The torsion-fibre must be selected with great care. Ten threads were drawn 
(103) and were suspended from a horizontal beam. Weights were then gradually hung 
on to the lower ends. Only two were found strong enough, the others having broken 
before 450 grains had been added. The one selected stood 450 grains without breaking. 
Its diameter is less than ’001 inch. 
The torsion of the glass fibre was taken by the method given in par. 186. With the 
weight of 15’46 grains hanging on to it half an oscillation took 30 seconds. 
A fellow thread, selected of as nearly as possible the same strength and tension, and 
having a weight of about 100 grains at the end, broke when it was twisted 36 times. 
The actual thread used in the balance has been tested up to 30 turns without breaking. 
211. A flat oblong piece of soft iron, weighing accurately O01 grain, is put loose into 
the cross tube under the pith surface (see z, fig. 17, vertical section of cross tube). This 
weight can be picked up by a horseshoe magnet outside the tube, and dropped on any 
part of the pith. A mark is made at the exact centre of the pith surface, and by moving 
the magnet about, it is easy to place the iron weight accurately on this mark. 
212. A silvered glass mirror is supported at an angle of 45° over the pith surface, and 
so that the centre of the mirror is 2 inches from the pith when the beam is in equili- 
brium. The whole is enclosed in a blackened box, in such a manner that when a 
candle is placed a few inches from the mirror in a horizontal line 
with its centre, no direct ray, but only the reflected ray, falls on 
the pith. Fig. 18 shows the arrangement: ad is the mirror; 
e e e e is the wooden frame with the aperture ( g h) in front to 
allow the flame of the candle (/) to fall on the mirror and thence be 
reflected on to the pith without illuminating the pith with direct rays. 
b c is the cross tube, j the pith, i the beam, and z the iron weight. 
213. A ray of light from a lamp is directed on the small central mirror ( Tc , fig. 17) of 
the beam, whence it is reflected back to a millimetre scale 4 feet off, forming a sharply 
defined image, and making evident by its movement the slightest angular motion of the 
beam. When the reflected ray points to zero on the scale, it is evident that a turn 
of the torsion-handle ( [v ) in one or the other direction will raise or depress the pith end 
of the beam, and thus cause the index ray to travel along the scale to the right or to 
the left. It is also evident that if a small weight is placed on one end of the beam so 
as to depress it, and the torsion-handle be then turned, the tendency of the glass fibre 
to untwist itself will ultimately balance the downward pressure of the weight, and 
(provided the glass torsion-fibre does not break) will bring the beam to a horizontal 
position, the index ray again pointing to zero. The object of the spring p is to keep 
the torsion-thread always stretched ; and the silk fibre (m o) connecting the torsion-fibre 
and the spring is to allow the whole of the torsion of the fibre to be utilized in moving 
the beam, as the filament of silk may be considered practically free from torsion. 
Eig. 18. 
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