1907-8.] Sensitive State induced in Magnetic Materials. 
239 
XIV. — On a Sensitive State induced in Magnetic Materials by 
Thermal Treatment. By James G. Gray, B.Sc., Lecturer on 
Physics in the University of Glasgow, and Alexander D. Ross, 
M.A., B.Sc., Assistant to the Professor of Natural Philosophy in 
the University of Glasgow. Communicated by Professor A. Gray, 
F.R.S. 
(MS. received January 31, 1908. Read February 3, 1908.) 
Part I. 
Introductory . — Whilst engaged in carrying out a series of tests on the 
magnetic properties of annealed steel, the authors detected a peculiar 
variation in the susceptibility of the specimens. In the tests referred to, 
the magnetometric method was employed, the specimens being completely 
demagnetised prior to the annealing. It was noticed that the magnetisa- 
tion curve obtained with a specimen in the freshly annealed condition 
differed materially from any subsequent magnetisation curve obtained with 
that specimen. Thus, a cylindrical rod was demagnetised, annealed at a 
high temperature, and a test gone through. A certain I-H curve was 
obtained. The rod was now demagnetised by reversals, the test repeated, 
and a new I-H curve constructed. The values of I obtained in the second 
test were in all cases lower than those obtained in the first ; in other words, 
the magnetic quality of the material had been somewhat destroyed in the 
process of magnetisation. 
As the presence of this effect is important in magnetic testing generally, 
the authors commenced a detailed examination of the phenomenon. They, 
however, subsequently found that the effect had been previously detected 
and partially investigated by Ewing. * His experiments have apparently 
been carried out with the object of obtaining merely a general idea of the 
nature of the sensitive state. The specimens, in the form of long wires, 
were annealed by passage through a bunsen flame, and thus only a rough 
estimate can be formed of the temperature to which the specimen was 
raised, and of the rate at which it cooled. As will be seen from the 
experiments to be described, these are important factors in the case. The 
authors’ tests also show that the effect depends largely on the chemical 
constitution of the specimens and on their previous history. 
* Phil. Trans. Roy. Soc., 1885, p. 570, §§ 54-58. Similar tests have been carried out by 
Searle and Bedford (Phil. Trans. Roy. Soc., 1902, p. 70), but, with the exception of some 
quantitative determinations of the magnitude of the effect, little further information is given. 
