40 



Mi; .1 Ml IK ON THE RECOVERY OF IKON FROM OVERSTRAIN. 



a temporary check in the rate of extension. Time readings of a few minutes 

 duration taken while the bar was stretching, detected no change in the rate of 

 extrusion when tin- current was allowed to pass for some time, and so the bar for that 

 time kept magnetised. 



When the stretching at the yield-point had practically ceased, the specimen was 

 re-measured and the curve showing semi-plasticity obtained. The specimen was then 

 allowed to rest for two hours, and the recovery effected was recorded by a curve. 

 The current was next passed through the coil for periods of from 10 to 15 minutes, 

 and was reversed all the time rapidly by hand, so that the bar was subjected to con- 

 siderable magnetic agitation. Such treatment was found to have no appreciable 

 effect on the recovery of the specimen, the curve obtained on re-testing being almost 

 exactly the same as that obtained after the 2 hours' rest. 



Compression Experiments. 



The experiments which are now about to be described illustrate the recovery of iron 

 from tensile overstrain by means of compression tests. These tests were carried out 

 on small cylindric blocks, l inches diameter by 1-J- inches long, compression being 

 applied by means of the 50-ton testing machine. The small compressional strains 

 obtained were measured by an instrument specially designed by Professor EWING. 

 This instrument resembles in principle Professor EWING'S exteusometer, especially a 

 more recent form of that instrument, and like it is self-contained, and is entirely 

 supj)orted by the specimen under test. A detailed description of this instrument, 

 which is shown attached to a compression specimen in the following illustration, need 



not be given here, but it may be stated that with a mechanical multiplication of 10, 

 and further optical magnification, a contraction of u^nnF^ of < inch can be 

 measured. This corresponds to a corapressional strain of srAooth, since the length 

 of specimen actually tested is only 1 inches. It may be recalled that the unit of 

 t In- raulings of Professor EWING'S extensometer represented an elongation of ^oWoth, 

 tin- length of specimen tested V-ing 8 inches. 



The following two series of readings, obtained with the new compression instru- 

 ment, clearly show by comparison the semi-plasticity which is induced in iron by 



