QUARTZ FIBERS. 



17 



(Fig. 2), which I recently obtained from Messrs. Nakler Brothers. It is 

 only a little over one-thousandth of an inch in diameter. Ordinary 

 spun glass, a most beautiful material, is about ouethousandtii of an 

 inch in diameter, and this would a})pear 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 

 raaterial in their endeavor to find a really delicate torsion thread. 1 

 have so turned only to bedist^^pointed. It has every good quality but 

 one, and that is its imperfect elasticity. For 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 ou one side, which however is 

 slowly changing, 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 unsuit- 

 able 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 fibers, 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 fibers must be separated from one another and 

 washed. Then each component will, according to the ex- 

 periment of Gray, carry nearly 60 grains before breaking, 

 and can be safely loaded with 15 grains. Silk is there- 

 fore very strong, carrying at the rate of from 10 to 20 tons 

 to the square inch. It is further valuable in that its tor- 

 sion is far less than that of a fiber of the same size of metal 

 or even of glass, if such could be i)roduced. The torsion 

 of silk, though exceedingly small, is quite sufficient to 

 upset the working of any debcate instrument, because it is 

 never constant. At one time the fiber twists one way, and 

 another time in another, and the evil effect can only be 

 mitigated by using large ai)paratus 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. 



The result then is this. The smallness, the length of fk;. 4. 

 period, and therefore (leli(;acy, of the instruments at the 



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