330 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
When a bar whose diameter of cross section is comparable with its length is 
magnetised in a uniform field, there seems good reason to believe that the magnetisa¬ 
tion it possesses is very decidedly less in the interior near the axis than it is near the 
surface, and when the bar is possessed merely of residual magnetism, the difference 
between the interior and surface distributions seems completely proved. When, as 
usual, the magnetising field is not uniform, there is in general still less reason to expect 
a uniform intensity of magnetisation. Thus, in general, a bar may be regarded as 
composed of a large number of elements, throughout each of which the magnetisation 
may be treated as sensibly uniform, but which may differ widely amongst themselves. 
The term intensitij applied to the magnetisation of such a bar merely means some sort 
of average of the intensities throughout the several elements. 
Now the application of a given stress to such a bar may affect very differently the 
magnetism of different elements. It may, for instance, increase the magnetisation of 
some of them and diminish that of others. Experiments as a rule give merely a sort 
of integral of the effects on the different elements, and this ought to be clearly 
recognised. 
In particular I would point out that the magnetic moment of a bar, as determined 
by some definite method, may have the same value when the bar is exposed to the 
action of a certain current in a spiral, as it has after the break of a larger current in 
the spiral, and that notwithstanding there may be an important difference between 
the distributions of magnetism in. the two cases. There is thus no d ])riori reason to 
expect the same phenomena to follow the application of a given longitudinal stress in 
the two cases. 
Ejfects of Magnetism on the Length of Bars. 
§ 3. Such effects have been observed by Joule,* Mayer,! Professor Barrett,^ and 
Mr. Shelford Bidwell.§ 
Joule’s experiments dealt with iron and steel. His method of experiment con¬ 
sisted in running a current through a magnetising spiral and observing the instan¬ 
taneous changes in tlie length of the bar accompanying the make and break of the 
current. Observations were taken with several currents gradually rising in strength. 
Apparently Joule did not demagnetise his rods in the course of his experiments, nor 
does he seem to liave examined the eflect of making and breaking the same current 
more than once. He concluded that in iron or soft steel bars free from stress the 
institution of a current in a surrounding spiral is followed by an increase in the 
length of the bar proportional to the square of the intensity of the magnetisation 
* ‘ Phil. Mag.,’ vol. 30 (3rd series), 1847, pp. 76-87 and 225-241. 
t Ibid., vol. 46 (4th series), 1873, pp. 177-201. 
I Ibid., vol. 47 (4th series), 1874, p. 51; also ‘ Nature,’ vol. 26, 1882, p. 585. 
§ ‘ Phil. Trans.,’ A, vol. 179, 1888, pp. 205-230. Also ‘Roy. Soc. Proc.,’ vol. 40, 1886, pp. 109-133 
and 257-266. 
