Section III., 1906. [159] Trans. R. S. C. 



X. — Effect of an Electric Current on the Modulus of EJasticiiy. 

 By E. W. Boyle, M.Sc, 



Demonstrator of Physics, McGill University. 

 (Communicated by Dr. H. T. Barnes). 



Read May L'3rd. 1906. 



For a long time there has been an idea that the passage of 

 electricity in a strained wire had some effect on its longitudinal elas- 

 ticity, enough at least, to alter its modulus. 



Werthein, in 1844, considered that he had shown that the longi- 

 tudinal elasticity of metals was temporarily diminished by the passage 

 of an electric current, independently of the alteration which would 

 result from the elevation of temperature produced by the current. 

 (Werthein, Ann. de Chimie, 1844, torn XII.) 



Tomlinson, in his work on '•' Influence of Stress and Strain on 

 the Action of Physical Forces" (Phil. Trans., Vol. 1T4, 1883), says 

 that in Werthein's work on the coefficient of elasticity there must have 

 been errors due to his method of proceeding when determining the 

 elasticity by static extension. He (Werthein) put on a weight, took 

 a reading with the measuring microscope, and, after removing the 

 weight], took a second reading; from the difference in these two read- 

 ings he determined the value for the modulus. 



Tomlinson says that if after considerable extension had taken 

 place Werthein had, repeated his trials with each of the previous weights, 

 he would have obtained appreciably different values, and the general 

 result would have been to give him a greater mean value for the 

 modulus. 



Every experimenter on stress and strain by tension must notice 

 this fact. It is very rarely that any material so tested immediately 

 returns to exactly its original dimensions, even though the stress may 

 be considerably below the elastic limit; but usually the difference is 

 small. This, perhaps, can l>e accounted for by the want of uniformity 

 of the material, the heat developed by the straining, and the liysteresis 

 effect. I.e., the lagging of the relation of stress to strain, an effect 

 analogous to magnetic hysteresis though proportionately not so large. 



The work of Werthein probably was the cause of the suspicion 

 tliat the passage of an electric current in a wire altered its modulus 

 cf elasticity. 



Sylvanus Thompson in his '' Electricity and Magnetism," 1902 

 edition, under the heading of " Physical Effects of an Electric Current" 



