132 PROFESSOR TAIT ON A FIRST APPROXIMATION 
k does not vanish, would be a dislocation of the diagram, by a simple shear. 
This follows at once from the equation of one of the lines— 
y=hk, (a—T,). 
“The diagram gives the Peltier effect at the junction of a and 6 for any tem- 
perature ¢,, by drawing the ordinate at ¢,, and completing a rectangle cc’g/” on 
the part intercepted, its opposite end being at absolute zero. The area of this 

rectangle is to be taken positively or negatively according as the corner corre- 
sponding to @ is nearer to, or further from, the horizontal axis than that corre- 
sponding to 6, the current being supposed to pass from a to b. 
“The electro-motive force in a circuit of the two metals a and 8, with its 
Junctions at ¢, and ¢, respectively, is found by drawing ordinates at these tem- 
peratures, so as to cut off triangular spaces Acc’, Add’, whose vertices are at 
the neutral point. The difference of the areas of these spaces, cdd’c’, is propor- 
tional to the electro-motive force. When the higher temperature ¢, is above 
the neutral point, the electro-motive force is the difference of the areas Acc’, 
Ace’. The case above mentioned, in which, by a differential galvanometer, we 
get rid of the terms in ¢,, is obviously a process for making the curves of two 
separate complex arrangements into parallel straight lines. 
“In conclusion, I may give a few instances of the comparison of results of 
calculation of the neutral point of two metals from their observed neutral points, 
and differences of £, as regards iron, with calculation of the same neutral point 
from the portion of the curve (assumed to be a parabola) which expresses their 
electro-motive force within ranges of temperature where mercurial thermometers 
can be applied. 
“Thus with Fe, Cd, Pb, we have from the iron circuits 0:00112 — 000209 = 
— 000097, while the direct experiment with Cd, Pb gave —0-00096. 
“The neutral point, as calculated from the data for the iron circuits is — 69° 
C., while the calculation from direct experiment gives — 74° C, 

