1910-11.] Influence of Temperature on Carbon Steels. 
505 
XXXIII. — On the Influence of Temperature upon the Magnetic 
Properties of a Graded Series of Carbon Steels. By 
Margaret B. Moir, M.A., B.Sc., Carnegie Research Scholar 
in the University of Glasgow. Communicated by Professor A. 
Gray, F.R.S. 
(MS. received March 20, 1911. Read same date.) 
The magnetic properties of iron, steel, nickel, and cobalt at moderate and 
high temperatures have been investigated by numerous experimenters, 
perhaps the most notable work being that carried out by Hopkinson,* M. 
Curie, f and D. K. Morris.J Hopkinson employed the ring method of 
Rowland, insulating the windings from each other by means of asbestos, and 
deducing the temperature from the electrical resistance of a platinum wire 
wound upon the specimen. He obtained magnetisation curves, at temperatures 
lying between ordinary room temperature and the critical temperature of 
the material, for soft iron, mild steel, hard steel, nickel, and cobalt. In the 
case of soft iron he found that for low values of the magnetising force the 
effect of increasing the temperature was to bring about an increase in the 
permeability; as the temperature approached the critical temperature of 
the material an enormous increase took place in the magnetic quality. For 
a value of the magnetising force of 03 c.g.s. units the permeability was 
about 400 at room temperature ; as the temperature increased the permea- 
bility steadily increased, and at 600° C. had attained a value of 900. From 
this point on, the increase in permeability with temperature became more 
rapid; at 700° C. the permeability was about 1900, at 750° C. it had the 
value 4000, and at 775° C. it reached the maximum value of 11,000. 
Further heating brought about a very rapid loss of magnetic quality, and 
at the temperature of 790° C. the iron had become practically non-magnetic. 
At this temperature its permeability was about unity. For large fields the 
permeability remained practically constant until a temperature of 600° C. 
was reached ; there was then a steady falling off in magnetic quality up to 
790° C., at which temperature the iron became practically non-magnetic. 
Specimens of mild and hard steels exhibited much the same general 
behaviour; the critical temperature was found to be 740° C. for the former 
and 690° C. for the latter material. 
In 1895 an important paper on the magnetic properties of iron, nickel, 
* Phil. Trans. Roy. Soc., 1889, A, p. 443. t Journ. de Phys ., vol. v. p. 289, 1895. 
X Phil. Mag ., vol. xliv. p. 213, 1897. 
