506 
Proceedings of the Royal Society of Edinburgh. [Sess. 
and cobalt at different temperatures was published by M. Curie,* who 
employed fields up to 1350 c.g.s. units and temperatures extending from 
room temperature to 1400° C. He found the critical temperature of iron to 
be 7 60° C., which is somewhat lower than Hopkinson’s value ; he showed 
also that as the temperature is further increased the permeability remains 
practically zero up to 1300° C., when there is an abrupt increase in magnetic 
quality, indicating the existence of a further transformation point. 
In 1897 H. K. Morris f repeated Hopkinson’s work for soft iron, special 
attention being paid to the influence of temperature upon the magnetic 
hysteresis of the materials tested. It was found that for small and 
moderate magnetising forces heating the specimen resulted in a general 
improvement of the magnetic quality, as had already been shown by 
Hopkinson. The experiments showed, however, that for small field 
strengths the permeability-temperature curve passes through several 
maximum and minimum points before the critical temperature is reached. 
Morris verified the existence of a transformation point for iron at a 
temperature considerably above the critical temperature. The fact that 
turning points occur in the permeability-temperature curves indicates that 
as the temperature is increased the crystalline structure undergoes modifica- 
tions ; and Morris’ work showed that for iron, in addition to the main critical 
point, there are at least three further such points lying between room 
temperature and the critical temperature. 
It is interesting to notice that the changes which the magnetic qualities 
of iron and steel undergo on being heated are accompanied by changes in 
the other physical properties of the materials. Gore, in 1869, found that 
an iron wire cooling from a bright red heat undergoes a momentary elonga- 
tion at a dull red heat, and then goes on contracting as before. Barrett, | 
in 1874, showed that at the temperature at which Gore’s phenomenon takes 
place the material passes from the non-magnetic to the magnetic condition. 
Tait § found that at certain temperatures the thermo-electric powers of iron 
and nickel undergo very remarkable alterations. 
In Hopkinson’s work, and in that of many of his successors, the 
procedure adopted was as follows. The ring specimen was first heated up 
above the critical temperature and allowed to cool to the temperature at 
which it was desired to carry out a test ; the cooling was then arrested and 
the test carried out by the method of reversals. The specimen was next 
submitted to the action of an alternating magnetic field of gradually 
diminishing strength ; the initial value of this field being great and the 
t Phil. Mag., vol. xliv. p. 213, 1897. 
§ Trans. Roy. Soc. Edin., 1873. 
* Journ. de Phys., vol. v. p. 289, 1895. 
1 Phil. Mag., Jan. 1874. 
