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XII. On the Elasticity of Wires. By G. F. C. Searle, M.A. 



Demonstrator in Experimental Physics at the Cavendish 

 Laboratory, Cambridge*. 



TVffR. L. R. WILBERFORCE f, following a suggestion 

 JAJL due to Profs. Ayrton and Perry £, has described a 

 method of using a spiral spring in the determination of the 

 Young's modulus and the simple rigidity for the material of 

 the wire forming the spring. The great improvement intro- 

 duced by Mr. Wilberforce consisted in determining the 

 periodic times of the longitudinal and angular vibrations of a 

 mass hung at the end of the spring, instead of measuring the 

 longitudinal and the angular displacements of the end of the 

 spring due respectively to a measured force and a measured 

 oouple. Some inquiries from my pupils as to the theory of 

 the method led me to endeavour to simplify the conditions of 

 the experiment,, while retaining the principles laid down by 

 Mr. Wilberforce. The apparatus and the method described in 

 this paper are the outcome of that endeavour. 



It should be noted at the outset that the assumption is 

 made, in accordance with general practice, that the wires 

 under test may be regarded as formed of isotropic metal. But 

 it is very unlikely that it is possible to produce such a wire 

 except by cutting it out of a very large block of the metal, 

 The results given below show that in some cases isotropy 

 must be very largely departed from ; and in fact the method 

 described in this paper may be more valuable in giving an 

 indication of the extent of aeolotropy than in giving values of 

 so-called " elastic constants.'" 



I have modified Mr. Wilberforce's method by the substi- 

 tution of a straight wire for a helical wire, and thus not only 

 is mathematical simplicity gained, but the determination of 

 the radius and the slope of the helix is avoided. Since the 

 w r ire under test need not be more than a few centimetres long, 

 there is great economy of material; and since the wire is 

 straight, the method is available for materials which may be 

 too brittle to be formed into a helix. 



In fig.. 1, A B, CD are two equal brass bars of square 

 section. Through the centres of these bars at G, G' holes are 

 drilled, and by means -of the clamping arrangement, of which 

 a section is shown in detail in fig. 2, the wire under test is 

 clamped securely to the two bars. By means of two hooks 

 screwed into the bars at G, G' into the faces perpendicular to 



* Communicated by the Author, 

 t Phil. Mag. Oct. 1894. 

 } Proc. Hoy. Soc. vol. xxxvi. p. 311. 

 Phil, Mag. S. 5. Vol. 49. No. 297. Feb. 1900. 



