1906-7.] Variation of Young’s Modulus under Electric Current. 355 
differed from the others, inasmuch as the final readings with the diminishing 
current gave a higher value for the modulus than it had with the initial 
increasing current. 
It is interesting to examine in more detail the results for soft iron and 
steel. Here we see the effects on the same chemical substance, and therefore, 
on the assumption that a change in the modulus is caused by a regrouping 
of the molecules, are able to follow the changes in the groups of molecules, 
to see when they break up, and how the regrouping has affected the 
modulus. When the rate of change is small, we may assume that the 
groups are more stable than when the rate is great. 
There is a general similarity in the behaviour of soft iron and steel, 
but there are also marked differences. With an increasing current the 
results are, qualitatively speaking, the same. The effect of a weak current 
is to produce a fall in the modulus ; next, as the current increases the 
modulus also increases, and rises to a higher value than it had before any 
current was passed; then a maximum is reached, after which a steady 
decline sets in. There is a great difference, however, in the quantitative 
results ; for, while the increase from the lowest value to the maximum is 
143 per cent, in soft iron, it is only 23 per cent, in steel. 
With a diminishing current, there is a marked difference between them. 
The value of the modulus is smaller in the soft iron than at the same 
temperatures with the increasing current ; whereas, in the steel, it has 
at first a higher value than it had at the same temperatures with an 
increasing current. They are, however, alike in this respect, that the 
modulus in both attains a maximum and then diminishes, the 
final value being less than with the increasing current at the same 
temperatures. 
After cooling to the temperature of the room, the modulus in each case 
undergoes a distinct increase. In the soft iron this value is not so great 
as the greatest value reached with the current, but in the steel it is the 
highest of all the readings. 
It is to be observed that the variation of the modulus in steel is not so 
great as in iron, and from this we may infer that the groups of molecules 
are more stable than they are in iron. This result might be expected, for 
the molecules of steel will be affected to a smaller extent by the circular 
magnetic field produced by the current than those of soft iron. This 
expectation, however, is not borne out at all points, for, when one would 
have looked for the coercive force of the steel to manifest itself, we find 
a distinct change in the modulus after the current has been stopped. 
In all cases, after the wire had been allowed to cool to the temperature 
