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V. On Silk v. Wire Suspensions in Galvanometers, and on the 

 Rigidity of Silk Fibre. By Thomas Gray, B.Sc, F.R.S.E.* 



IN the last Number of the Philosophical Magazine there is 

 a short article by R. EL M. Bosanquet drawing attention 

 to some eccentricities of a galvanometer used by him. A 

 determination of the rigidity of the suspending " fibre " of the 

 galvanometer-needle would have been interesting, as it would 

 have thrown considerable light on the probability or improba- 

 bility of the explanation offered. It must have caused no little 

 surprise to many of the readers of the Philosophical Magazine 

 to find that Mr. Bosanquet based his condemnation of silk- 

 fibre suspensions on the trouble he experienced with an instru- 

 ment the suspending fibre in which was "left just stout 

 enough to carry the weight/' and which was of such a nature 

 that it could possibly twist or untwist with stretching or 

 with hygrometric changes in the atmosphere. Surely Mr. 

 Bosanquet is scarcely in earnest when he writes about sus- 

 pending the needles of a sensitive galvanometer with a hoisted 

 silk thread, or when he proposes to go back something like 

 half a century in the history of this subjectf, and adopt galva- 

 nometers with needles seven inches long made of stout knit- 

 ting needles and suspended by a wire five feet long. 



A galvanometer-needle should never be so heavy that it 

 cannot be suspended by a single fibre of silk (that is* half an 

 ordinary cocoon fibre) , because such a fibre will bear easily, 

 leaving a good margin of safety, two grammes ; and it is an 

 easy matter to so arrange such a mass that the period of vibra- 

 tion will be not only so much as thirty seconds but even several 

 minutes. With an astatic arrangement, especially if it be 

 only " nearly astatic," there will be changes of zero cer- 

 tainly, but I can hardly see any thing comparable to a 

 " ghost " in what could occur. 



About a year ago I made, in the Physical Laboratory of 

 Glasgow University, a number of experiments on silk fibres, 

 which included among other things some determinations of 

 their rigidity. Mr. Bosanquet's paper has suggested to me 

 that possibly a few of the results may be worth publication. 

 Some of the results of these experiments are in type in 

 vol. iii. of the Reprint of Sir W. Thomson's Mathematical 

 and Physical Papers now in the press. 



Two methods were used for the determination of the rigidity. 



* Communicated by the Author. 



f Some interesting- experiments u On the Suspension of the Magnetic 

 Needle by Spiders 1 Fibre " are described by the Rev. A. Bennet, F.R.S., 

 in the R. S. Trans, vol. lxxxii. 1792. 



