PROPERTIES OF IRON AT A HIGH TEMPERATURE. 
449 
ten times. The last currents gave magnetising forces 1'2, 0'6, 0’3, 0'15, 0‘075, O’Oo. 
The inequality of successive observations is due to the residual effect of the current 
last applied ; it is remarkable to observe how greatly this small force affects the 
result. In Curve XI. the first deflection was caused by a reversal of a current 
opposite to the last demagnetising current. 
Comparing Curves X. and I. we see that the effect of working with the sample is 
to diminish its magnetisability for small forces, a fact which will be better brought 
out later. 
Referring now to the temperature effects, we see that as the temperature rises the 
steepness of the initial part of the curve increases, but the maximum magnetisation 
diminishes. The coercive force, that is, the force required to completely demagnetise 
the material after it has been exposed to a great magnetising force, also, judging from 
the form of the ascending curves, diminishes greatly. 
In Curves XII., XIII., and XIV. the abscissae are temperatures, and the ordinates 
are induction-magnetising force, called by Sir William Thomson the permeability, 
and usually denoted by p,. These curves correspond to constant magnetising forces of 
0’3, 4‘0, 4.5'0. They best illustrate the facts which follow from these experiments. 
Looking at the curve for 0‘3, we see that the permeability at the ordinary temperature 
is 367 ; that as the temperature rises the permeability rises slowly, but with an 
accelerated rate of increase ; above 681° C. it increases with very great rapidity, until 
it attains a maximum of 11,000 at a temperature of 775° C. Above this point it 
diminishes with extreme rapidity, and is practically unity at a temperature of 786° C. 
Regarding the iron as made up of permanently magnetic molecules, the axes of 
which are more or less directed to parallelism by magnetising force, we may state the 
facts shown by the curve by saying that rise of temperature diminishes the magnetic 
moment of the molecules gradually at first, but more and more rapidly as the critical 
temperature at which the magnetism disappears is approached, but that the facility 
with which the molecules have their axes directed increases with rise of temperature 
at first slowly, but very rapidly indeed as the critical temperature is approached. 
Whit'ivo7'th’s Mild Steel. — This sample was supplied to me by Sir Joseph Whit¬ 
worth and Co., who also supplied me with the following analysis of its composition :— 
C Mn S Si P 
Per cent. . . ’126 ”244 '014 '038 ‘047 
The dimensions of the ring were as shown in the accompanying sketch. 
MDCCCLXXXIX.-A. 
3 M 
