S279] 
XX. On the Variation with Temperature of the Magnetic 
Permeability of Nickel and Iron. By Ki. Putip Harrison, 
PhD. (Ziirich), 1851 Kehibition Research Scholar of 
University College, London; King’s College, Cambridge *. 
Introduction. 
T is a remarkable fact that many of the physical properties 
of iron, nickel, and cobalt undergo sudden changes at a 
temperature which is roughly particular to the metal in 
question, but which varies slightly with individual specimens. 
For instance, Tait + found that the thermoelectric power of 
iron altered at a red heat and Kohlrausch and Hopkinson f{ 
proved the same thing for its electric resistance, while as 
early as 1869 Gore had noticed a change in its coefficient of 
expansion. 
Barrett’s § more recent discovery of “ recalescence,” an 
evolution of heat which occurs in cooling steel wire at 
approximately the same temperature as Gore’s phenomenon, 
is only another instance of some molecular rearrangement, 
which doubtless takes place in iron, at a temperature of about 
800° C. But of all the phenomena connected with this 
structural alteration in iron at high temperatures, that which 
has been longest known and studied is the loss of magnetic 
quality at what Hopkinson called ‘the critical temperature ” 
for the metal. It is natural to suppose that this magnetic 
critical temperature is identical with that at which changes 
in other physical properties take place ; a supposition to 
some extent confirmed by experiment. 
In order, however, definitely to decide this point, it is 
important to use the same specimens in each experiment, 
since small and otherwise undetectable differences in chemical 
or physical state of a metal may lead to very considerable 
alterations in thermoelectric power, resistance, or magnetic 
induction. 
The present experiments form the third part of a research 
into the temperature variations of various physical properties 
of iron and nickel. The first two parts || dealt with thermo- 
electromotive force and electric resistance, and showed that 
* Inaugural Dissertation for the degree of Ph.D. in the University of 
Ziirich. Communicated by Prof. H. L. Callendar, F.R.S. 
+ Tait, Trans. Roy. Soc. Edin. 1873. 
{t Kohlrausch, Wied. Ann. vol. xxxili. (1888), 
§ Barrett, Phil. Mag. Jan. 1874. 
|| Harrison, Phil. Mag. Jan. 1902. 
N 2 
