the Magnetic Permeability of Nickel and Iron. 195 
during heating at a temperature of 867°, and an evolution of 
heat during cooling at about 855°. 
No such effect as has been described in the last section was 
observed in the case of nickel. 
It appears that as the quantity of impurity in a metal is re- 
duced, these two critical points approach one another. Nodoubt 
the fact that the sample of nickel used in these experiments is 
purer than the iron is sufficient to account for the absence of 
any temperature-hysteresis effect in the former metal. 
Above the chief critical point in iron, Morris observed 
another maximum in the curve representing the variation of 
maximum permeability with temperature. The author finds 
no such increase in permeability above the critical point, but, 
as is seen in fig. 8, the drop in permeability becomes less 
rapid above 830° (i. e., = becomes smaller). 
This bending outwards in the wT curve is also remarked on 
by Pitcher (loc. cet.). It is evidently not directly due to the 
alteration in temperature coefficient of the iron as Pitcher 
suggests, since in the present experiments temperature was 
not deduced from the resistance of the iron, but from that of 
the platinum tube. 
(GENERAL CONCLUSIONS. 
For the purpose of correlating these magnetic changes. 
with variations that occur in other physical properties of iron 
and nickel at high temperatures, it will be found advisable 
not to confine attention to the actual critical temperature, but 
to regard as a critical interval the range of temperature 
extending from the sudden drop in magnetic intensity to its 
vanishing point *. 
The following values were obtained for this critical interval, 
in three separate experiments for nickel, and one for iron. 
Temperature of | a8 | re ‘Temperature at 
Metal. | sudden drop in | T drial becuse which specimen | 
permeability- epee. | 7) oP easeecmenled. 
yas et Oo | ign y © ° | ° 
320°0 3740 5+:0 | 500 
NICKEL . 300°0 373°0 73:0 600 
S180) willl, BY40 640 600 
Tron ... 760°0 t855°0 95 0 | 1000 
* See the articles by Du Bois and Warburg, tome 2, Rapports présentés 
au Congres International de Physique, Paris, 1900. 
+ 855° is the mean of the two critical temperatures referred to in the 
previous section. 
O02 
