140 PROFESSOR TAIT-ON A THERMO-ELECTRIC DIAGRAM. 
and it is obvious that it is a matter of great difficulty to estimate with precision 
where they lie. The wire called B Thin shows so remarkable a resemblance to 
steel in its thermo-electric properties—though it is certainly not steel—that, 
as a verification, I tried the electro-motive force of a circuit formed of it and of 
B Thick which so nearly coincides with pure iron. The result is given by the 
lower curve in Plate VIII., which is easily seen to be in entire agreement with . 
the curves in the upper part of the plate, the ordinates of one being the differ- 
ences of those of the other two. 
In Plate IX. I have endeavoured, by drawing tangents to the curves of 
Plate VIII., to construct (to the same distorted temperature-scale) the thermo- 
electric diagram for N, and the various specimens of iron and steel. It will be 
seen that all of these specimens have at least two changes of sign of the specific 
heat of electricity. It is to be remarked, however, that as the heating of the 
junctions was effected by means of a white-hot iron cylinder (as described in one 
of the extracts above), the diagram belongs to specimens of iron and steel 
which have been raised to a white heat and are cooling. This process generally 
produces a marked change in the thermo-electric properties of steel, though a 
very slight one in those of iron. 
In the same Plate, [X., I have attempted (by means of the parabolic law, 
assumed for M,N) to approximate to the diagram for pure iron in terms of 
absolute temperature. The result is indicated by a double line, which may 
be compared with the line for nickel in Plate VII., to which it has more than a 
mere general resemblance. But this figure also shows one way of forming a 
thermo-electric circuit which shall give a current without any Peltier effect at 
either junction, and without electric convection of heat in one of the two metals 
concerned. 
Note.—Since this paper was read to the Society I have seen in the Phil. 
Mag. for December 1873 a paper by Prof. BARRETT, in which he recalls attention 
to Mr Gore’s singular observation regarding the sudden changes of length 
which take place in an iron wire at a low red heat, and adds his own very 
curious discovery of the sudden glow which occurs simultaneously with them. 
I have for some time been seeking for other physical changes, besides the 
well-known magnetic ones, and the above described thermo-electric ones, which 
may be expected to take place in iron about this temperature. A brief note on 
the change of electric resistance of iron appears in Proc. R.S.EL. (Dec. 16, 1872) 
as a first instalment which I hope soon to be able considerably to extend. 

