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XLTI. Increased Accuracy in the Use of Bifilar Suspensions. 

 By A. Norman Shaw, M.Sc, B. 0. King Research Fellow, 

 and Lecturer in Physics, McGill University, Montreal*. 



IN the case of single suspensions it is well known that 

 many materials exhibit complicated movements after 

 the wire or fibre has been twisted first in one direction and 

 then in the other for a considerable number of times. As it 

 is often necessary in practice to employ suspensions whose 

 direct elastic properties are constant, the choice of a material 

 which shall also have a minimum elastic after-effect becomes 

 a difficult matter. A thorough search for a suitable suspen- 

 sion was recently made by K. E. Guethe, at the Bureau of 

 Standards in Washington, in connexion with his determina- 

 tion of the electromotive force of standard cells, but he states 

 that the attempt was only " partially successful w f. A large 

 number of investigators have examined this phenomenon of 

 elastic af er-effect, and its behaviour has often been pointed 

 out as being analogous to that of electric absorption in con- 

 densers +. The theory of the subject was, indeed, developed 

 first for this " elastiche Nachv^ irkung " by Weber, Kohlrausch, 

 Meyer, Boltzmann, Natanson, and others, and then applied 

 to the analogous case of absorption §. Schweidler gives a 

 very complete list of papers on this subject in the Ann. der 

 Physik, vol. xxiv. 1907, on page 711. 



In the choice of bifilar suspensions, it is desirable that the 

 wire should have, in addition to a minimum after-effect, an 

 especial constancy of length under strain, and an absence of 

 appreciable fiexural rigidity at the extremities of the sus- 

 pensions. Although the controlling couple in the bifilar 

 suspension is affected by the elastic after-effect to a much 

 less degree for a given substance than the single suspension, 

 yet the satisfying of all the requirements has rendered it a 

 difficult matter to obtain a suitable material. The constancy 

 of the main factor in the controlling couple suggests the 

 possible attainment of great accuracy in the use of bifilar 



* Communicated bv Prof. H. T. Barnes, F.R.S. 



t Guethe, Bull. Bur. of Stan. vol. ii. 1906, p. 33 ; McCollom, Bull. 

 Bur. Stan. vol. vi. 1910, p. 503. 



X Hopkiuson, Phil. Trans, vol. clxvi. 187G, p. 489, and Proe. Roy. Soc. 

 vol. xxviii. 1878, p. 148; Trouton and Russ, Phil. Mag vol. xiii. 1907, 

 p. 578. 



§ Weber and Kohlrausch, Pogg, Annal. Bd. liv. rp. 119 & 128; 

 Kohlrausch, Zeitschr. Gesammt. JS'aturiviss, vol. xiii. 187G, p. 368 : Meyer, 

 Annal. Phys. Chem. vol. iv. 1878, p. 249 ; Boltzmann, Sitzungsbenehte 

 tier k. Akad. der Wiss. zu Wien, Bd. xl. 1874, p. 275 ; Natanson, Bull, 

 de V Acad, des Sciences de Cracoic, Oct. 1902, p. 512. 



