1889.] on Quartz Fibres. 549 



familiar object, and one that is generally spoken of as if it were 

 ratber fine. Miicli finer than this is the specimen of copper wire now 

 on the screen (Fig. 2), which I recently obtained from Messrs. Nalder 

 Brothers. It is only a little over one- thousandth of an inch in 

 diameter. Ordinary spun glass, a most beautiful material, is about 

 one-thousandth of an inch in diameter, and this would apj)ear to be 

 an ideal torsion thread (Fig. 3). Owing to its fineness, its torsion 

 would be extremely small, and the more so because glass is more 

 easily deformed than metals. Owing to its very great strength, it can 

 carry heavier loads than would be expected of it. I imagine many 

 physicists must have turned to this material in their endeavour to 

 find a really delicate torsion thread. I have so turned, only to be 

 disappointed. It has every good quality but one, and that is its 

 imperfect elasticity. I'or instance, a mirror hung by a piece of spun 

 glass is casting an image of a spot of light on the scale. If I turn 

 the mirror, by means of a fork, twice to the right, and then turn it 

 back again, the light does not come back to its old point of rest, but 

 oscillates about a point on one side, which, however, is slowly chang- 

 ing, so that it is impossible to say what the point of rest 

 really is. Further, if the glass is twisted one way first, and ^^^"- ^' 

 then the other way, the point of rest moves in a manner 

 which shows that it is not influenced by the last deflection 

 alone : the glass remembers what was done to it previously. 

 For this reason spun glass is quite unsuitable as a torsion 

 thread ; it is impossible to say what the twist is at any time, 

 and, therefore, what is the force developed. 



So great has the difficulty been in finding a fine torsion 

 thread, that the attempt has been given up, and in all the 

 most exact instruments silk has been used. The natural 

 cocoon fibres, as shown on the screen (Fig. 4), consist of 

 two irregular lines gummed together, each about one two- 

 thousandth of an inch in diameter. These fibres must be 

 separated from one another and washed. Then each com- 

 ponent will, according to the experiments of Gray, carry 

 nearly 60 grains before breaking, and can be safely loaded 

 with 15 grains. Silk is, therefore, very strong, carrying at 

 the rate of from 10 to 20 tons to the square inch. It is 

 further valuable in that its torsion is far less than that of a 

 fibre of the same size of metal or even of glass, if such could 

 be produced. The torsion of silk, though exceedingly small, 

 is quite sufficient to upset the working of any delicate in- 

 strument, because it is never constant. At one time the 

 fibre twists one way, and at another time another, and the 

 evil effect can only be mitigated by using large apparatus in which 

 strong forces are developed. Any attempt that may be made to 

 increase the delicacy of apparatus by reducing their dimensions is at 

 once prevented by the relatively great importance of the vagaries of 

 the silk suspension. 



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