of Edinburgh, Session 1871-72. 
597 
5. Laboratory Notes : On Thermo-Electricity. By 
Professor Tait. 
For some time back I have been endeavouring to prove, by ex- 
periment, through great ranges of temperature, the result announced 
by me in December last, viz., that the electro-motive force of a 
thermo-electric circuit is in general, unless the temperature be very 
high, a parabolic function of the absolute temperature of either 
junction, that of the other being maintained constant. 
For moderate ranges of temperature the experiment presents 
little difficulty; but, when mercurial thermometers cannot be em- 
ployed, a modification of the experimental method must be made. 
I have employed in succession several such modifications, of which 
the following are the chief : — 
The simplest of all is to dispense altogether with thermometers, 
and to employ two thermo-electric circuits, whose hot and whose 
cold junctions are immersed in the same vessels ; and to plot the 
curve whose abscissae and ordinates are simultaneous readings of 
the electro-motive forces in the two circuits. In every case I have 
tried the curve thus obtained is almost accurately a parabola, most of 
the few deviations yet observed being in the case of silver and other 
metals at temperatures not very much below their melting points — 
under circumstances, in fact, in which we should naturally expect 
that the law would no longer hold. There are, also, cases in which 
the whole electro-motive force is so small, even for very large differ- 
ences of temperature, that very much more delicate apparatus would 
be required for their proper investigation. And there are cases in 
which the neutral point is so far off that for moderate ranges of 
temperature the curves obtained are sensibly straight lines. I 
intend to examine these cases with care — the former by using more 
delicate galvanometers ; the latter, by employing metals which are 
practically infusible. The difficulty of obtaining wires of such 
metals has been the chief one I have had to face. 
If we assume the experimental curve to be a parabola, then it is 
easily seen ( Proc . May 29, 1871) that in each circuit the electro- 
motive force must be a parabolic function of some function of the ab- 
solute temperatures of the junctions. And, as in the iron-silver, 
4 L 
VOL. VII. 
