1910-11.] Influence of Temperature on Carbon Steels. 515 
magnetism. Hopkinson’s curves give no indication of the presence of such 
a transformation point. For hard steel the permeability -temperature curve 
obtained by this investigator is, for a field strength of 1*5 c.g.s. units, and 
for temperatures lying between 15° C. and 550° C., practically a straight 
line. Morris demonstrated the existence of several minor transformation 
points for soft iron lying between room temperature and the critical 
temperature. And in this connection it should be stated that Morris 
submitted the specimen under test to a process of reversals immediately 
before carrying out a test : the magnetisation curves yielded were there- 
fore the true magnetisation curves corresponding to the various tempera- 
tures. It is not pointed out by Morris, however, in his paper, that it is 
essential that the process of reversals should be carried out at the particular 
temperature at which it is desired to test the specimen. As a matter of 
fact, had the process of reversals not been so applied the small waves in 
the curves would have been completely masked by the want of neutrality 
brought about by the thermal changes. 
Summary. 
1. Experiments of Hopkinson, Curie, and D. K. Morris. 
2. Precautions necessary in magnetic testing. If the temperature of a 
test-specimen is changed, the specimen must be rendered neutral at the 
new temperature previous to carrying out the test. 
3. The effect of increasing the temperature of a specimen of cast iron 
from 15° C. to 190° C. is to improve its magnetic quality for moderate 
values of the magnetising force; from 190° C. to 260° C. the magnetic 
quality falls off, after which there is a further improvement. Finally the 
specimen becomes non-magnetic at the critical temperature. These results 
indicate the presence of a transformation point for carbon alloys of iron 
in the neighbourhood of 200° C. 
4. Steel containing P64 per cent, of carbon resembles cast iron in its 
behaviour. The set-back in magnetic quality takes place at much the 
same temperature as that which occurs in cast iron, the magnitude of the 
set-back being much the same for both. 
5. Steel containing 0*8 per cent, of carbon exhibits the transformation 
point, but in a less marked degree. The first maximum point in the 
susceptibility curve occurs at 180° C., and the first minimum point 
at 220° C. 
6. Low carbon steel containing 0 - 3 per cent, of carbon shows the 
transformation point ; it is, however, very much less marked than in the 
case of the steels containing the higher percentages of carbon. 
