598 
Proceedings of the Royal Society 
iron-zinc, iron- copper, iron-cadmium, &c., circuits, this function has 
been proved to be simply the absolute temperature itself (at least, 
within the range of mercury thermometers), it is probable that such 
is the general law, at least for ranges of temperature short of those 
which materially alter the molecular structure of the metals em- 
ployed. 
The second method consisted in employing two pairs of circuits, 
all four hot junctions being in the same heated substance, and all 
four cold junctions kept at a common temperature. The members 
of each pair acted on a differential galvanometer (as explained in 
Proc. Dec. 19, 1870) in such a way as to eliminate the term containing 
the square of the absolute temperature. In this case the readings of 
the galvanometers should be simply proportional to one another, 
and likewise to the differences of absolute temperature of the junc- 
tions. The method is exact in theory, but by no means easy in 
practice, especially with the very limited number of metals capable 
of resisting a high temperature which I could manage to obtain. 
That a very exact and useful thermometric arrangement can be 
made on this principle admits of no doubt, when we examine the 
results of the experiments. 
The third method consisted in assuming the parabolic law, and 
the following consequence of it which follows directly by the use 
of Thomson’s general formulse. These may easily be reproduced 
as follows : — Suppose a sliding ring or clip to be passed round the 
wires, so as to press together points of the wires which are at the 
same temperature, t. Its effects are known by experiment to be nil, 
whatever be its material. Let it be slid along so that the tempera- 
ture of what is now effectively the hot junction becomes t + St, then 
the two laws of thermodynamics give, respectively, 
SE = J(SH + (crq — o~ 2 ) St^ , 
and 
o =s n + £i^&. 
Here E is the electromotive force, n the Peltier effect at a junction 
at temperature t , and <r v cr 2 , are the specific heats of electricity in 
the two metals. 
