623 
1907-8.] Sensitive State in Magnetic Materials. 
folded into cylindrical form was placed within the porcelain tube, and 
assisted in keeping the temperature uniform within the furnace. As in the 
earlier experiments, the temperatures were measured with a platinum, 
platinum-iridium pyrometer. 
A steel specimen whose critical temperature was about 845° C. was 
heated from 15° to 800° C. and slowly cooled. When tested at 15° C. it 
gave for a field of 12 c.G.s. units a “ sensitive state ” of 29 per cent. The 
specimen was then demagnetised and heated up to 800° C., and tested when 
the temperature had become uniform. During the process of heating it was 
found to have acquired a “ sensitive state ” of 8 per cent, for the same mag- 
netising field. After demagnetisation it was slowly cooled to 15° C. and 
tested thereat. A “sensitive state” of 12 per cent, had been brought on by 
the cooling. Similar tests were made, taking the specimen through the 
■smaller range from 15° to 430° C. The heating and cooling then gave 
10 per cent., the heating alone 13*5 per cent., and the cooling alone 25 per 
cent. These results are grouped in Table VII. for the sake of comparison. 
Table VII. — Sensitive States by Heating and Cooling. 
Thermal Treatment. 
Percentage Sensitive State. 
From 15° to 800° C., and then to 15° C. . 
29* 
From 15° to 800° C. . . . 
8* 
From 800° to 15° C. 
12- 
From 15° to 430° C., and then to 15° C. . 
10- 
From 15° to 430° C 
13-5 
From 430° to 15° C. 
25- 
The “ sensitive states ” induced by equal augmentations or reductions of 
temperature are of widely different amounts, depending on the position of 
the temperature ranges on the temperature scale and on the composition of 
the steel. Even a small increment, say of only 25° C., in the neighbourhood 
of 180° C., produces in most steels a relatively large “ sensitive state,” in some 
cases as much as 10 per cent. The authors are engaged on an extensive 
investigation of these changes in magnetic quality in various steels for 
different ranges of temperature between 0° C. and the critical points. The 
results of the research they hope to lay before this Society in a subsequent 
paper. 
Rods of steel of varying composition were tested at 100° C., 200° C., 
400° C., and just below the critical point, to ascertain whether any “ sensitive 
state ” was induced by prolonged heating at constant temperature. As was 
to be expected from the results of the liquid air tests, no such improvement 
