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XI. On the Modulus of Tontioita/ Rigidity of Quartz Fibres and its 



Temperature Coefficient. 



By FRANK HORTON, D.Sc., B.A., St. Johns College, Cambridge, 

 Macirinnon Student of the Royal Society. 



Communicated by Professor J. J. THOMSON, F.K.X. 

 Received Deceinljer 15, 1904, Read January 26, 1905. 



THE author recently had the honour of reading to the Society the retmltH of some 

 experiments in which a new method of timing by means of " coincidences "- a 

 method devised by Professor POYNTING was applied to an investigation of the 

 effects of changes of temperature on the modulus of torsional rigidity of metal wires. 

 It seemed desirable to repeat similar experiments with quartz fibres, seeing that now 

 they are almost universally employed as suspensions in torsion instruments where 

 accuracy is required. 



The modulus of torsional rigidity of quartz fibres was first investigated by 

 C. V. BOYS ('Journal Soc. of Arts,' vol. 37, p. 827, 1889), who found considerable 

 variations between different specimens. The mean value of the modulus obtained by 

 him was = 2 '38 x 10" dynes per sq. centim. 



An independent study of the subject was made a little later by THRELFAU, (' Phil. 

 Mag.,' V., vol. 30, p. 99, 1890), who also investigated the effect on the rigidity of 

 heating a fibre to 100 C. It was found that the rigidity increased with the 

 temperature, and, assuming that the change of rigidity with temperature obeyed a 

 linear law, the value obtained for the temperature coefficient of the modulus was 

 + '000128. The mean value of the modulus of rigidity was found to be 2'88x 10" 

 dynes per sq. centim., but the different determinations were not in very good 

 agreement. 



BARNETT (' Phya Rev.,' vol. VI., p. 114, 1898) has also investigated both the 

 modulus of rigidity and its temperature variation. From observations on six fibres 

 he obtained a mean value n = 1 '46x10" dynes per sq. centim., or about half th- 

 value obtained by BOYS and THRELFALL. The relation between the rigidity and the 

 temperature, for temperatures ranging from 20 C. to 100 C., ww shown to be linear 

 by timing the torsional oscillations of a fibre at several temperatures l>etween these 



(382.) 10.5.05 



