TO A THERMO-ELECTRIC DIAGRAM. 137 
states (i.¢., become diferent substances) at certain ‘ critical’ temperatures, retain- 
ing their thermo-electric properties nearly unchanged from one of those critical 
points to another. 
“The curve marked IV. in the woodcut was obtained by plotting against each 
other the simultaneous indications of the alloy of curve III. and iron, and of 
the alloy of curve II. and iron, so as to avoid any disturbance from possible 
peculiarities of palladium. Then, to obtain an idea of the share taken by iron 
in the results, it was found that the electro-motive force in a circuit formed 
by the two alloys, or by either with hard Pt, is (for a very great range of 
temperature) sensibly proportional to the temperature difference of the junctions. 
“The same result is easily seen from the plate, if we notice that the 
difference of corresponding ordinates in any two of curves L., II., III, is nearly 
proportional to the corresponding abscissa. Now, it seems a less harsh sup- 
position that the lines representing platinum and its alloys are nearly straight 
and parallel, while that of iron is a broken line, than that the latter should be 
straight and the former all broken at the same temperatures. On the other 
hand, this latter hypothesis would make & alternately negative and positive 
in iron, while the former would only require the platmum metals to have 
values of / alternately less and more negative than that of iron. 
“T may add that none of the above-mentioned effects can be due to altered 
electric resistance of the heated junctions, because the galvanometer resist- 
ance was about 23 B. A. units, while that of the iron and platinum wires 
together was in each case not more than one such unit. The palladium-iron 
circuit was so much more powerful than the others that a resistance coil of 
about 146 B.A. units had to be inserted in its course....... 
To this paper was added during printing the following postscript :—“ I 
have since made out that the lines of the diagram are approximately straight, 
and parallel to the lead line, for the platinum metals, that of hard platinum 
being below the lead line, while those of most of the other alloys are above 
it, and that the multiple neutral points depend upon the peculiar sinuosity of | 
the line for iron. I have also obtained curious results of a somewhat similar 
kind with steel wire. The method I employed was to explore the part of the 
thermo-electric diagram included between the lines of gold and palladium, by 
making a multiple arc of these two metals, and varying the ratio of their 
separate resistances. But I reserve details until I have carefully examined the 
behaviour of nearly pure iron.” 
The peculiarity thus exhibited by iron I afterwards found to be also pos- 
sessed by nickel, and with the farther advantage that the changes of sign 
of specific heat of electricity occur in that metal at temperatures within the 
range of mercury thermometers. (Proc. R.S.L., May 1873). These results I 
developed in the Rede Lecture of 1873, a full abstract of which was printed in 
VOL, XXVIL PART I. 2N 
