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LVI. Instruments for Measuring Small Strains in Bars 

 Subjected to Twist. By E. G. Coker, B.A., B.Sc, 

 M.I.M.E., Late Scholar of Peter house, Cambridge*. 



THE advances made within recent years in the scientific 

 testing of engineering materials have caused great 

 attention to be paid to the design of instruments for mea- 

 suring small strains. By far the greater number of such 

 instruments have been devised for measuring the small strains 

 of extension or compression in bars subjected to a direct pull 

 or push ; and but little attention has been paid to instruments 

 for the use of engineers in the measurement of the small 

 strains in a bar subjected to twist. 



The object of the present paper is to describe two arrange- 

 ments of apparatus intended for use in engineering labora- 

 tories and testing-houses for measuring such strains, and for 

 the determination of the Modulus of Rigidity. 



Each instrument is wholly supported by the test- bar, being 

 secured thereto by screws which grip the bar at two trans- 

 verse sections separated by a known interval, and the relative 

 angular displacement between these two sections is measured 

 directly. The instruments are adapted to measure both large 

 and small strains, and are self-contained, while they can be 

 used in a horizontal, vertical, or inclined position. 



One form of the apparatus is shown by fig. 1, and consists 

 of a graduated circle, A, mounted upon a chuck -plate, B, 

 provided with three centering-screws adjustable by hand. 

 A ring, C, secured to the test-bar by set screws at a known 

 distance away carries a swivel-arm, D, in which slides a tube, 

 E, so that the contact-ball, F, at its outer end can be brought 

 into position touching the centre of the faced end of a screw- 

 micrometer gauge, H, provided with a divided head. This 

 micrometer-screw is mounted upon a vernier-plate, J, of the 

 graduated circle, and can be clamped in any position, the 

 final adjustment being effected by a screw N. 



A silk-covered wire connects an insulated binding-screw, 

 K, upon the ring with the contact-ball, and this wire is 

 joined up in circuit with a simple form of galvanometer, L, 

 and cell, M, to a second uninsulated binding-screw on the 

 ring. If the contacting pieces are touching, a circuit is 

 completed through the test-bar, and the galvanometer-needle 

 is deflected. If now a twisting movement is applied to one 

 end of the test-bar, the contacting pieces are separated, and 

 the micrometer-screw must be advanced until the circuit is 



* Communicated by the Author, having been read at the Bristol 

 Meeting of the British Association, 1898. 



