Ml; .1 Mi III ON THE RECOVERY OF IRON FROM OVI l;MI;.\IN 39 



plastic condition produced by overstrain, and Curve No. 3 the condition of the material 

 jitter half-un-hour's rest. After this test the specimen was taken out of the testing 

 machine and vigorously tapped with a hammer. On re-testing Curve No. 4 was 

 obtained, which shows that not only has the effect of the half-hour's rest been 

 annulled by the vibration, but that the material was rather more plastic than it had 

 been immediately after overstrain. The specimen was next allowed to rest for 

 H'i.1 In mrs and was then re-tested, Curve No. 5 showing the progress made towards 

 recovery. The specimen was then taken out of the testing machine, and once more 

 struck with the hammer so as to make it ring. On again testing, the elasticity was 

 found just as before to have been made more imperfect, Curve No. 6, which illustrates 

 this test, lying below No. 5. The specimen was then put in boiling water for a little, 

 and Curve No. 7 shows that recovery was complete. Hammering was found to have 

 no appreciable effect on the elastic condition of material whose elasticity had been 

 thus restored. 



In concluding this section it may be of interest to state that the effect of turning 

 down the diameter of a recently overstrained specimen was to produce partial recovery 

 of elasticity. This was in all probability due to the warming which accompanied the 

 cutting action the bar being heated by conduction, and only the surface subjected to 

 severe mechanical vibration. 



The Influence of Magnetic Agitation in Hastening or Retarding the Recovery 



of Elasticity. 



The experiment which is now about to be described was made with the object of finding 

 the effect on recovery, of magnetising and de-magnetising an overstrained specimen. 



A coil (Ij inch diameter X 7^ inches long) was made which gave a field strength at 

 the centre of about 140 C.G.S. units, when a current of 10 amperes was passing. 

 This coil was put round a specimen and supported at the 8-iiich length, to which the 

 extensometer was to be applied. 



The material used was the same as that of Diagrams IV. and VII. ; the specimen, 

 however, was not in its virgin condition, it had been largely overstrained and had 

 recovered its elasticity again, so that a yield-point was not expected till a stress of 

 about 40 tons was attained. During the loading of the specimen, a current of 10 

 amperes was passed at intervals through the coil, and it was found that the extenso- 

 meter could clearly detect (when the current was passed) the slight elongation due to 

 magnetisation. This elongation occurred only at the lower loads ; at the higher ones 

 the slight contraction, which is known to occur, was quite readily observed.* 



At a stress of 40^ tons per square inch a yield-point was obtained, and while the bar 

 \\as stretching rapidly at this load the current was put on and off several times, its 

 direction being constantly reversed. The contraction, which had been noticed just 

 In fore the yield-point had been reached, was still clearly shown at each "make," by 



* For tho chnnge in length caused by magnetisation, when iron is under various stresses, see papers 

 l,y Sin i IMKI. Uii.wKi.i, 'Phil. Trans.,' A, 1888, and 'Roy. Soc. Proc.,' 1890. 



