673 
1907-8.] Young’s Modulus under an Electric Current. 
the load was small, being in some cases '5 kilo, and in others 1 kilo, and 
the smaller of these is less than the total load used in any of my experi- 
ments. They found that in all cases there was an increase in the modulus. 
Since the loads were small, the results can be compared with my first set 
of experiments on each wire, and it is to be noted that there is agreement 
between them, viz. at the beginning of my experiments, and all through 
theirs, an increase in the modulus was obtained. 
In a second paper Stevens * examined the effect of a longitudinal field 
on rigidity, in which the same apparatus was used. In summing up his 
results he makes the following statements : — 
I. Magnetisation of an iron or steel rod increases the torsional elasticity. 
II. The effect is greater in iron than in steel. 
III. Increase in elasticity varies with the length of the rod. 
IY. Distinct agreement with results of the experiments on the flexure 
of rods. 
It is of importance to note that in the experiments on rigidity the rods 
were subjected to different couples, and that the increase was greater when 
the stress was smaller. This result is altogether in accordance with those 
I obtained, for in all cases I got a decrease in the modulus when the load 
was increased. Now, Young’s modulus depends on the rigidity, and we 
may assume that the one will vary with the other, and that those 
conditions which produce a change in the rigidity will also cause a change 
in the modulus. There is, therefore, distinct agreement between these two 
sets of experiments and my own. 
I. An increase in magnetisation produces an increase in elasticity. 
II. When the metal is subjected to various stresses, the modulus is 
lowest when the stress is greatest. 
This agreement, however, does not hold in every respect, for in one 
point their results differ from mine, viz. that within the range of their ex- 
periments there is no appearance of a maximum. This difference, however, 
can, I think, be easily accounted for if the different conditions be taken into 
consideration. In these the temperature of the rod was kept constant, 
whereas in mine it rose. But an increase in temperature produces a 
diminution in the modulus, and so a point must be reached at which the 
increase caused by the current is not sufficient to wholly counterbalance 
the decrease produced by the rise of temperature, and so the modulus falls. 
Gray, Blyth, and Dunlop in their paper examine the effect of change of 
temperature on Torsional Elasticity, and find in all cases that there is a 
VOL. XXVIII. 
* Phys. Rev., vol. x., p. 161. 
43 
