156 PROFESSOR TAIT ON A FIRST APPROXIMATION 
were led to a sort of switch, by means of which either circuit could be instantly 
made to include the galvanometer. Readings were taken of each circuit as 
fast after one another as possible (with the galvanometer I employed about 6°5 

seconds was the necessary interval), and the mean of two successive readings of 
one circuit was taken as being at the same temperature as that of the imter- 
mediate reading of the other. 
“The indications of these curves are very curious as regards the effect of 
even small impurities on the thermo-electric relations of some metals. It 
is probable, from analogy, that the curve for iron and pure platinum, in terms 
of temperature, would be (approximately, at least; even if it should be the 
iron, and not the platinum metal, which is represented by a broken or curved 
line) a parabola with a very distant vertex. And it appears probable that 
when the wire of curve III. is analysed it will be found to contain even a 
larger percentage of iridium (?) than that of curve IT. 
“T find by tracing these curves on ground glass, allowing for the difference 
between temperatures and the indications of an Fe-Pd circuit, and superposing 
them on a nest of parabolas with a common vertex and axis, that they can be 
closely represented by successive portions of different parabolas (with parallel 
axes) whose tangents coincide at the points of junction, though the curvature 
is necessarily not continuous from one to the other. Hence, as at least a fair 
approximation to the electro-motive force in terms of difference of temperature 
in the junctions, we may assume a parabolic function, which up to a certain 
temperature belongs to one parabola, then changes to another without dis- 
continuity of direction, and so on. 
“Hence either the iron, or the hard platinum and the platinum-iridium 
alloys, will be (approximately, at least) represented on my form of THomson’s 
thermo-electric diagram (ante, p. 132) by broken lines, of which the successive 
parts are straight. This, contrasted with the (at least nearly) straight Imes 
for pure metals, seems to show that some bodies take successively different 

