38 



ME. LEONAED BAIESTOW ON THE ELASTIC LIMITS OF 



By combining the use of either of these scale-pan weights with that of the jockey, the 

 machine was made suitable for tests in which the stress was never wholly removed. 



The scale-pan weights were applied at regular intervals by means of hydraulic 

 pressure operating through the specimen and raising and lowering each .end of the 



10,000 



REPETITIONS 



3O.OOO 2O 



10 O 1 



3TRESS . TONS m SQ IN 



Fig. 1. 



beam iu turn. The valve controlling the water supply and exhaust was moved by 

 link-gear driven from an electric motor, whilst the running of the motor itself was 

 regulated by the movements of the beam and by a commutator attached to the 

 link-gear. 



The number of cycles was registered automatically. 



Observations on the change of length of the specimen were made by means of the 

 delicate extensometer designed by Prof. MARTENS. In this instrument, two knife- 

 edge rhombs, each of which carries a mirror, are held against the back and front of 

 the specimen by spring clips. The rotation of the mirrors is measured by observing 

 the reflection of a fixed scale. 



To render rapid simultaneous observations possible, an arrangement was adopted 

 by means of which only a single telescope was necessary for making the readings 

 corresponding to the movements of both mirrors. 



Materials used in the Research. Three samples of commercial iron and steel were 

 obtained in the form of round bars, and complete analyses and tensile-test results are 

 given in Tables I. and II. 



The Swedish iron is that described by Dr. STANTON and the author in a paper on 

 " The Resistance of Iron and Steel to Reversals of Stress."* A bar of Bessemer 



* 'Min. Proc. Inst.C.E.,' vol. clxvi., 1905-6, Part iv. 



