1910-11.] Young’s Modulus under an Electric Current. 187 
behaviour quite different from what was found when the load was small. 
It became necessary, therefore, to find out how each metal behaved as the 
load was increased step by step to the maximum. 
As the curves for iron and steel are somewhat similar in nature to 
those obtained when these bodies are subjected to tensile strain in a 
magnetic field, the further necessity was felt of examining nickel and 
cobalt. These two metals give results different from each other, and also 
from iron, when under tensile strain in a magnetic field. Now, if there 
is a similarity between the variation of the modulus with magnetic field 
and that of intensity with magnetic field, it might be concluded that there 
was some relation between the modulus and magnetic induction. 
In all my preceding experiments a definite weight was put into the pan 
before any current was passed through the wire, then the current was 
gradually increased up to the maximum, and finally diminished to zero. 
Throughout each cycle of heating and cooling, therefore, the only change 
in the stress to which the wire was subjected was putting on and taking 
off the weight employed to produce the change of length. To vary the 
conditions of the experiments, another method was tried, viz. to alter the 
load while the magnetising force was kept constant. This method of 
investigating the effects of stress on magnetisation is described by Ewing,* 
and the results are quite different from those obtained by keeping the 
load constant and varying the magnetic field. The idea was to find out 
whether, under these conditions, there would be any change in the modulus 
as compared with the previous conditions of experiment, and, if so, whether 
the variation in the modulus was at all similar to that of the magnetic 
intensity. I thought that this comparison would be more accurate and 
complete if the strength of the magnetic field were known, and I determined 
* Magnetic Induction in Iron , 3rd ed., p. 216. 
