of Edinburgh, Session 1875-76. 
629 
[ Deferred from jo. 491.] 
On the Electric Eesistance of Iron at a High Temperature. 
By Messrs C. M. Smith, C. Gr. Knott, and A. Macfarlane. 
(Plate.) 
The following paper is a continuation of a former brief one, 
communicated to the Society, and printed in the Proceedings , on 
the change of electric resistance of iron due to change of tempera- 
ture. In a note appended to Prof. Tait’s paper on a “ First 
Approximation to a Thermo-electric Diagram ” (Trans. R. S.E., 
1872-73), attention was drawn to the curious phenomenon observed 
by Grore, that at a temperature about dull red heat, iron wire 
undergoes sudden changes in length, and also to the further dis- 
covery by Prof. Barrett, that if the wire be cooling, a sudden 
reglow occurs simultaneously with these changes. These pheno- 
mena seemed to be connected with other known physical changes 
which take place in iron at this critical temperature, such as the 
loss of its magnetic properties, the remarkable bend of the iron 
line in the thermo-electric diagram, and the interesting alteration in 
the rate of change of electric resistance with respect to change of 
temperature, observable in iron at the same dull red heat. The 
following experiments were made mainly with the view of more 
thoroughly investigating this last peculiarity. 
The method employed in the first series of experiments consisted 
in comparing the change of resistance with time, the wire through- 
out the whole of the experiment being surrounded for the greater 
part of its length by an iron cylinder which had been previously 
heated to a white heat in a stove, and was then allowed to cool by 
radiation. By this means a sufficiently slow and uniform altera- 
tion of temperature was secured; and the curve (see diagram, 
Fig. I.) as plotted in terms of the resistance and the time as 
ordinate and abscissa, shows the remarkable and sudden change of 
^5. at a temperature about the dull red heat — a change observable 
in all the experiments in connection with the iron wire. Upon the 
substitution of an equal length of platinum wire for the Iron, ceteris 
‘ paribus , it was found that no similar change was observable— the 
