1910-11.] Influence of Temperature on Carbon Steels. 509 
to exclude atmospheric air from contact with the specimen. For the 
magnetic tests a Gray-Ross magnetometer,* provided with the newest type 
of electric furnace,]* was employed. The furnace is contained within the 
magnetising coil of the magnetometer, and by its means the specimen can 
be brought to any desired temperature while under the influence of the 
magnetising helix. The furnace is air-tight, and consequently there is no 
possibility of the carbon being burnt out of the specimen. The procedure 
followed in carrying out the magnetic tests was as follows. The test-bar, 
having been placed within the furnace, was submitted to a process of 
reversals and tested, all the operations being carried out at the temperature 
of the room. The temperature was next raised by a suitable amount, 
the heating arrested, the specimen submitted to a process of reversals, and 
a test carried out ; and so on. In this way a sufficient number of magneti- 
sation curves at different temperatures was obtained to admit of the 
variation in magnetic quality with temperature being deduced. 
In deducing the effective field strengths from the applied field strengths 
the demagnetising factors investigated by Du Bois were employed. 
Discussion of Results obtained. 
Cast Iron . — The magnetisation curves obtained on testing the specimen 
of cast iron are shown in fig. 1, and a series of susceptibility-temperature 
curves deduced from them is exhibited in fig. 2. As the tempera- 
ture increases from 15° C. to 180° C. the susceptibility of the material, for 
field strengths extending from 2 to 12 c.g.s. units, steadily increases. At 
the latter temperature the slope of the curve diminishes rapidly ; the curve 
exhibits a maximum point at about 190° C., and the effect of further heating 
is to bring about an abrupt diminution in magnetic equality. The suscep- 
tibility arrives at a minimum value at about 260° C., after which it again 
slopes steadily upwards until the temperature reaches 600° C. ; from 600° C. 
to 790° C. the magnetic quality falls off abruptly, becoming practically 
zero at 790° C. It will thus be seen that in cast iron there is a transfor- 
mation point in the neighbourhood of 200° C. Further, an inspection of 
fig. 2 shows that while the maximum point on the susceptibility curve in 
the neighbourhood of the critical temperature is more marked for low fields 
than for high ones, the maximum point in the neighbourhood of 200° C. is 
more marked for high fields than for low ones. 
High Carbon Steel. — The results yielded by the specimen of high 
carbon steel are shown in figs. 3 and 4. An examination of fig. 4 shows 
* Proc. Roy. Soc. Edin., vol. xxix. p. 182, 1909. 
t Proc. Roy. Phil. Soc. Glasg., vol. xii., 1910. 
