PROPERTIES OF IRON AT A HIGH TEMPERATURE. 
463 
is small, and because the accuracy of the results may be affected by the possible 
oxidation of the copper. The temperatures of change of coefficient, 855° C. and 
812° C., are higher than any critical temperature I had observed. It was necessary 
to determine the critical temperatures for magnetisation for the particular samples. 
A ring was formed of the respective wires, and was wound with a primary and 
secondary coil, and the critical temperature was determined as in the preceding 
magnetic experiments ; it was found to be for the soft iron 880° C., for the hard 
pianoforte wire 838° C. These temperatures agree with the temperatures of sudden 
change of resistance coefficient within the limits of errors of observation.'" 
Some interesting observations were made on the permanent change in the resistance 
at ordinary temperatures caused in the wires by heating to a high temperature. In 
the following table are given the actual resistances of wires at the temperature of the 
room :— 
Before 
heating. 
After first 
heating. 
Second 
heating. 
Third 
heating. 
Soft iron.'. 
0-629 
0-624 
0-72 
0-735 
Pianoforte wire .... 
0-851 
0-794 
0-79 
0-74 
Manganese steel . . . 
1-744 
1-656 
1-61 
1-61 
In a second experiment the resistances before heating were : soft iron 0’614, 
pianoforte wire 0'826 ; after heating, soft wire 0‘643, pianoforte wire 072. 
The effects are opposite in the cases of soft iron and pianoforte wire. 
Recalescence of Iron. 
Professor Barrett has observed that, if an iron wire be heated to a bright redness 
and then allowed to cool, this cooling does not go on continuously, but after the wire 
has sunk to a very dull red it suddenly becomes brighter and then continues to cool 
down. He surmised that the temperature at which this occurs is the temperature at 
which the iron ceases to be magnetisable. In repeating Professor Barrett’s experi¬ 
ments, I found no difficulty in obtaining the phenomenon with hard steel wire, but 
I failed to observe it in the case of soft iron wire, or in the case of manganese steel 
wire. Although other explanations of the phenomenon have been offered, there can 
never, I think, have been much doubt that it was due to the liberation of heat owino- 
* \_Note added July 2, 1889.—Sir Joseph Whitworth and Co. have kindly analysed these two wires 
for me, with the following results ;— 
C Mn S Si P 
Soft iron wire .... ‘006 '289 ’015 ’034 T41 per cent. 
Pianoforte wire . . . 724 '167 '010 T32 '030 ,, ] 
