ON MOLECULAR PHENOMENA IN MAGNETISED IRON. 145 
Report of the Committee on Molecular Phenomena associated 
with the Magnetisation of Iron. (Phenomena occurring at a red 
heat.) Professor G. F. FirzGEraLp (Chairman), H. F. NEwatt, 
F, Trovuton, and Professor W. F. Barrert (Secretary). 
in the interim report presented last year it was stated that this Com- 
mittee, which was appointed some time ago to enquire into the various 
molecular changes connected with the magnetisation of iron, proposed to 
confine itself, as we believe was the original intention on the appointment 
‘of the Committee, to those remarkable physical phenomena which are 
found to occur in iron and steel, abont the temperature of a red heat 
when iron ceases to be a magnetic metal. 
The suddenness with which iron loses its magnetic susceptibility at a 
red heat has often been noticed by different observers. Professor Row- 
land! was the first to point out that for small magnetising forces, the 
susceptibility of iron increases as the temperature rises, reaches a maxi- 
_ mum ata red heat, and then falls suddenly to zero, but that the suscepti- 
_ ‘bility diminishes as the temperature rises when large magnetising forces 
are used. Bauer” subsequently established the same fact. Later, one of 
us (H. F. Newall?) has experimented with small spheres of iron and 
steel enclosed between closely fitting hemispherical caps of brass, so that 
the iron sphere was held by and heated in the brass, and thus allowed to 
‘heat and cool slowly, the susceptibility being tested by means of a mirror 
magnetometer. It was found that during cooling from a white heat the 
reappearance of magnetic susceptibility was much more leisurely in steel 
than in soft iron, the rate of return to the magnetic state corresponding 
with the rate of recalescence ; where recalescence was absent the suscep- 
tibility suddenly returned; where the reglow was pronounced the return 
to the magnetic state was slow. This is to be expected, for the rise of 
temperature during recalescence is more than sufficient to carry the iron 
out of the magnetic condition it had just entered upon by cooling; hence 
there will be a sudden oseillation at this critical temperature. Ledeboer 4 
was, we believe, the first to assign the exact temperature of the loss of 
‘susceptibility. By means of a thermo-electric couple formed of wires of 
platinum and an alloy of platinum with10 per cent. of rhodium, he found 
the susceptibility of iron to disappear at a temperature ranging from 750° 
‘to 770°C. Hopkinson® more recently, in his well-known paper, has 
investigated the effect of different magnetising forces on the loss of per- 
“meability of iron and steel with increasing temperature, more especially 
near the critical temperature. Measuring the resistance of copper wire 
(exposed to the same temperature) from its known temperature coefficient, 
Hopkinson estimated the temperature, and found that with very low 
_ Magnetising forces, less than 1 C.G.S. unit, the permeability of iron gradu- 
_ ally rose up to a temperature of 785° C., when it almost suddenly dropped 
_ down to unity; in like manner mild steel at first rose and then suddenly 
fell at a temperature of 735° C.: hard steel behaved similarly, falling off 
‘ata temperature of 680° C. 
' Phil. Mag., Nov. 1874. ? Wied. Ann., xi. (1880). 
* Proc. Camb. Phil. Soc., vol. vi., Part 4 (1888). 
160 ee Electrique, t. xxvii., No. 2. . 5 Phil. Trans., May 1889. 
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