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1907-8.] Sensitive State induced in Magnetic Materials. 
Soft Iron . — An iron specimen cooled from 900° showed no indication of 
the “ sensitive state.” 
Cobalt . — In a specimen of cobalt, cooled from 900° C., for H = 8 an 
increase in susceptibility amounting to about 15 per cent, of the final value 
was observed. 
In figs. 1 to 5 the values of I and H are given in c.G.S. units. In 
calculating the effective from the applied magnetising field, the factors 
investigated by Du Bois for cylindrical rods have in all cases been 
employed. 
Tests are being made on specimens of nickel, and further investigation 
of the effects detailed above is in active progress. The authors hope to 
publish the results of these tests in a second part of this paper, to be 
brought before the Society at an early date. 
The experiments described in the present paper were carried out in the 
Physical Laboratory of the University of Glasgow, and the authors desire 
to express their thanks to Professor Gray for the very helpful interest he 
has taken in the research. 
Summary. 
1. Specimens of steel cooled from high temperatures are, as regards their 
magnetic properties, in a “ sensitive state.” 
2. After magnetisation, reversal of the field reduces the susceptibility. 
3. Cyclic reversals of the field between definite limits bring the 
magnetic quality to a definite lower value for fields lying between these 
limits. 
4. So long as the field is not changed in sign, the “ sensitive state ” 
persists. 
5. Cyclic reversals of the field between narrow limits reduce the suscep- 
tibility for fields beyond these limits, but not to its lowest possible value. 
6. The rate of cooling from the high temperature— if not very rapid — 
has little influence on the magnitude of the effect. 
7. The “ sensitive state ” does not wear off with time. 
8. Mechanical vibration of the specimens considerably reduces the 
phenomenon. 
9. The “ sensitive state ” is most pronounced in hard steels. 
10. The effect is induced to a slight degree in some varieties of steel by 
temperatures as low as 100° C. It increases with increasing temperatures 
to about 700° C. Further increase in temperature has little influence. 
