194 Mr. G. F. C. Searle on the Elasticity of Wires. 



those pierced by the holes for the clamping-screws, the system 

 can be suspended by two parallel strings (about 1 metre in 

 length), the plane ABDC being horizontal. The fact that 

 the ends of the hooks, which engage in the strings, are about 

 4 cm. above the centres of gravity of the bars, ensures that 

 the horizontal position of the plane ABDC is thoroughly 

 stable. 



Fiflr. 1. 



■0 

 G' 



du 





»v 



'.'/ 



/ 



/ 



IP 



Fig. 2. 



\2<? / 



\ / 



\ / 



\ / 



» / 



If now the two ends B, D are made to approach each other, 

 the displacements of the two bars being equal, and if the bars 

 are then set free, the system will vibrate, each bar executing 

 harmonic vibrations in a horizontal plane. To the first order 

 of small quantities G and G' have no motion along the line 

 joining them; and when there is the additional limitation that 

 the mass of the wire is small compared with that of the bars, 

 the motion of G and G' at right angles to G G' may be 

 neglected. Thus to this order of accuracy the centres of 

 gravity of the bars are at rest, and consequently the action of 

 the wire on either bar is a pure couple. The stress across 

 any section of the wire is a couple of equal value, and thus the 

 wire is bent into a circular arc. 



If p be the radius of the arc, the bending-moment is EI/p *, 

 * Minchin, ' Treatise on Statics/ ed. 3, vol. ii. p. 424. 



