281 
1908-9.] Magnetic Properties of certain Copper Alloys. 
that the growth of hysteresis is specially rapid in the case of specimens 
exposed to temperatures between 200° and 250° C., and the maximum 
limit attained is greater than in specimens which have been subjected to 
prolonged heating at about 320° C. Indeed, a specimen which has been 
baked for some time at say 250° C., and thus gives a hysteresis cycle of 
large area, can have its hysteresis loss much reduced by a subsequent short 
heating at, say, 330° C. The variation in the liquid air effect shown in 
fig. 4 is also interesting. With the material in the condition as cast, cooling 
to — 190° C. improves the quality for all fields. In the case of' specimens 
which have been baked at temperatures in the neighbourhood of 250° C. 
the effect is reversed in sign for fields below 30 C.G.S. units, and it is much 
reduced in magnitude for high fields. The quality of specimens baked at 
330° C. is much the same as in the unbaked condition, both as regards tests 
at room temperature and at that of liquid air. In specimens exposed to 
still higher temperatures the susceptibility once more falls off, and this 
is again accompanied by a lessening and final reversal of the liquid air 
effect. 
Critical Temperatures . — In a paper * read before this Society by Mr 
J. G. Gray, of Glasgow University, results are given for the change in 
susceptibility produced by heating and cooling an alloy of similar composi- 
tion to B while a constant applied field is maintained. An interesting- 
contrast is also made with the results obtained by Hopkinson j* for nickel - 
steels. Similar tests have been carried out with alloys D and E, and the 
results are set forth in fig. 5. The ordinates are the intensities of magnetisa- 
tion, while the abscissa are the temperatures of the electric furnace within 
the magnetising solenoid as registered by a platinum, platinum-iridium 
pyrometer. The continuous lines indicate the values of I corresponding to 
an applied field of 125 units, and the dotted lines those for 11 = 30, while 
the arrow-heads distinguish the heating from the cooling curves. It will 
be observed that in the case of alloy D (shown in the upper diagram) the 
susceptibility falls off continuously with increasing temperature, as in the 
results given by Gray for alloy B. The critical temperature is, however, 
much lower, being approximately 280° C. instead of 500° C., and the deteriora- 
tion of the specimen due to the temperature cycle is more marked. In the 
case of alloy E there is a pronounced increase in susceptibility with increas- 
ing temperature to nearly 200° C., and then a rapid falling off as the critical 
temperature (345° C.) is approached. A similar maximum of susceptibility 
* “Experiments with Heusler’s Magnetic Alloy,” Proc. Roy. Soc. Edin., vol. xxviii., 
part 5, p. 403. 
t Proc. Roy. Soc., December 12, 1889 ; January 23, 1890 ; May 1, 1890. 
