EXPERII\IENTAL RESEARCHES ON DRAWN STEEL. 
n 
is about 11 or 12 units is nearly three times greater than tlie suscei)tihility of the 
wire cold, and throughout, up to the maximum force employed, the liot curve always 
lies above the cold curve. On the application of a demagnetising force, after 
maximum induction, the hot curve droops faster than the cold curve, and crosses it 
when — H is about 9 or 10 units, and for this force the intensity is the same 
whether the wire be hot or cold. 
9. This suggests that if a suitable demagnetising force be applied the temperature 
coefficient would be zero, and previous experiments'^' have verified that a wire of this 
kind does yield a zero coefficient with an appropriate self-demagnetising force. But 
it is not 230ssible to calculate with precision, from curves of induction, the requisite 
demagnetising factor, and consequently the dimension ratio to which such a wire 
must be cut, in order that the coefficient may be zero, because of complexities in 
magnetic behaviour arising in j)art from the irreversible effects of changes of tem¬ 
perature. Nevertheless, an estimate can be made, for, in order that the point of 
intersection of the curves should he on the ordinate of no external force, the curves 
must be sheared by an amount equivalent to about 10 units of force, and as the 
corresponding intensity is about 800, this would mean that the force per unit of 
intensity, that is to say the demagnetising factor, must be 0-0125, and in the case of 
a cylinder the dimension ratio for this factor is nearly 59. Experiment shows, how¬ 
ever, that a piece of this kind of wire has an approximately zero coefficient when it is 
8 centims. long, and thus the dimension ratio for this condition is 43 instead of 59. 
10. Again, the irreversilde changes which occur on heating and cooling do not 
allow the temperature coefficient to l^e simply calculated fi'om tlie difference of the hot 
and cold residual intensities which are left after removal of the maffnetisinn force. 
O O 
Experiments were made on the wire in two ways. Beginning with the wire hot, the 
intensity fell from 1294 to 1139 during a series of heatings and coolings, and the 
coefficient was + 0-000358. Then, repeating the experiment, but beginning with tlie 
wire cold, the intensity fell from 1204 to 112G, and the coefficient was + 0-000327, a 
value not very different from the former, but less than half the coefficient calculated 
from the difierence of the hot and cold intensities left immediately after the with¬ 
drawal of the magnetising force. 
Nevertheless, the relation of these hot and cold curves throws llnht on the fact 
o 
that the temperature coefficient of a magnet of drawn steel is positive or negative, 
according as the demagnetising factor is below or above a certain value, a result which 
has been fully established {vide Table I.). 
11. In another series of experiments the temperature coefficient was determined 
at different stages of the curve, the wire l^eing kept during heating and cooling 
under a constant force. The wire, the same one as before, was, in the first place, 
demagnetised carefully, and then a field of ll-Q units was applied and maintained; 
the intensity at 16° was 120-7, but on passing steam through the tube the intensity 
* ‘Roy. Soc. Proc.,’ vol. 62, p. 210. 
