464 Mr. C. V. Boys on the 



and it is in sucli cases that the stability of the fastening is 

 most severely tried. 



The first thing I found was that it was a mistake to solder 

 the fibre to the torsion-rod and to the suspension directly. 

 The difficulty of the manipulation is great and a change of 

 fibre is very troublesome. The preferable plan is to solder 

 the ends of the fibre to little tags of metal so small and 

 light that they may be picked up by the fibre from any- 

 thing on which they rest without risk of snapping the fibre 

 at the point of junction. These tags, which are conveniently 

 made of copper-foil, five millimetres long and one millimetre 

 wide about at the wide end, tapering nearly to a point, can 

 afterwards be fastened to the torsion support and the sus- 

 pension by shellac varnish or by melted shellac, and now the 

 enormous surface and the stiffness of the foil is sufficient to 

 prevent any trouble from the causes to which reference has 

 already been made. 



These tags might also for some purposes, either or both of 

 them, be made of T-form to hang in a pair of Y's, and so 

 dispense with cement altogether, and allow of the easy inter- 

 change of suspensions or of fibres, but 1 have not myself 

 employed such a form. 



The following operations are those which I have found to 

 answer : — 



1 . Select a fibre of the right diameter to give the desired 

 torsion. Since the torsion depends on the fourth power of 

 the diameter, a small change in the diameter makes a four- 

 fold change in the torsion, and great accuracy of measurement 

 is needed where an exact torsional rigidity is required. Cut 

 off a piece from two to three centimetres longer than will 

 ultimately be required. 



2. Fasten to the extreme ends of the fibre, with melted 

 shellac, little weights of gold or platinum heavy enough to 

 pierce a liquid surface. 



3. Hang the fibre over a fixed round horizontal rod of 

 wood, 1 centimetre in diameter or thereabouts, so that the 

 little weights hang side by side, and lift up from below a little 

 glass of strong nitric acid, so as to wet and clean the fibre 

 well above the final points of attachment. The vessel must 

 be wide enough to prevent capillarity from drawing the 

 fibres to one side, or it must be brimful so that the surface 

 is convex, which with nitric acid is objectionable. The 

 vessel must be moved both upwards and downwards past 

 the place at which the weights pass through the surface very 

 rapidly, practically w T ith a jerk; otherwise the weights will be 

 drawn together by capillarity, and the fibres will get twisted, 



