1881.] and Strain on the Action of Physical Forces. 



43 



(1.) That, in all metals, provided the wire has not been kept heavily 

 loaded for some time before testing, permanent extension produces 

 decrease of elasticity, if the strain be not carried beyond a certain 

 limit. 



(2.) That, if the extension be carried beyond the above-mentioned 

 limit, further permanent increase of length causes increase of 

 elasticity. 



(3.) That, in the case of iron, heavy loading for some time so in- 

 creases the elasticity that, even when the extension would have 

 caused diminution of elasticity without such continued loading, the 

 latter will, if sufficient time be allowed, change this diminution into an 

 increase ; in the case of copper this is not so. 



It was also observed that, with iron which has been very heavily 

 loaded for some time, the ratio of the temporary elongation to the 

 load producing it becomes less as the load employed becomes greater, 

 until a certain limit, depending upon the extent of the previous heavy 

 loading, has been reached ; whereas, with the other metals, and with 

 iron which has suffered permanent extension without allowing the 

 load producing the extension to remain for any appreciable time on 

 the wire, the elongation increases in a greater proportion than the 

 load. 



The behaviour of iron in this respect, as well as the fact that this 

 metal does not, until many hours have elapsed, attain its maximum 

 elasticity, after having undergone permanent elongation, is probably 

 to be attributed to the great coercive force of the metal. 



To a similar cause may also be assigned the well-known increase of 

 portative power of a magnet which can be produced by gradual load- 

 ing ; and the great difference between the tenacity of iron when 

 loaded by slow degrees, and when loaded quickly.* 



One of the above-mentioned indirect methods of determining the 

 effect of permanent extension on the elasticity consisted in combining 

 torsion with traction. The wire to be examined, some 95 centims. in 

 length, passed through a small hole in a stout table, and was clamped 

 at its upper extremity into a brass block, the latter resting on the 

 table, and being sufficiently secured by a heavy weight placed on the 

 top of it. Near the lower extremity, which was looped to receive 

 a scale-pan, was clamped a second small brass block, to one end of 

 which was attached a light mirror ; the latter used in connexion with 

 a scale and lamp enabled the observer to detect very small changes of 

 torsion. The wire having previously received a certain amount of 

 permanent torsion was loaded and unloaded several times with 

 different weights, and the amount of temporary twist or untwist pro- 

 duced thereby determined. 



* See J. T. Bottomlej's experiments, " Proc. Roy. Soc," vol. 29, p. 221. 



