barus.J DEGREE OF CONSTANT TEMPERATURE. 71 
set of data. Early values, i. e., such as were derived with couples of 
palladium or of platinum-silver alloy are discarded, because during 
the course of the measurements such elements were usually corroded 
through, and little confidence could, therefore, be felt in the use of the 
constants of the uncorroded element. Assuming equation (1) above, the 
temperature, T, of the hot junction of the couple is 
2&W + V s 
T '=— 
where a and b are the constants of the element, and where, if t be the 
temperature of the cold junction and e the observed electro-motive force 
for temperatures t and T of the junctions 
This value, £, is therefore the electro-motive force when the cold junc- 
tion is at zero, other conditions remaining the same. The passage from 
e to e usually involves only a small correction which may be interpolated 
from tables calculated for the purpose. Nevertheless the computation 
of T, where many results are in hand, is exceedingly tedious ; and it is 
therefore preferable to avoid it by the use of graphic methods, as ex- 
plained in Chapter II. In computing the values of 2\ the constants 
obtained in later and more refined apparatus are of course used, all older 
calibrations being allowed no more than corroborative importance. 
In Table 3 we give some of our earlier results, the first of which were 
obtained by submerging a protected thermo-couple in boiling zinc con- 
tained in a large fire-clay retort. The charge was from 5 to 10 pounds, 
but special data are not at hand. After this the iron apparatus de- 
scribed in Fig. 4 was used. Owing to difficulties of manipulation, we 
thereupon returned to the retort pattern, providing it with a suitable 
condenser; exchanging this, eventually, for a graphite crucible on the 
general plan of Fig. 5, but of much smaller dimensions. It is in this 
order that the results in the tables are given. The third column of the 
table contains the number of kilogrammes •distilled and the total num- 
ber of kilogrammes of zinc charged ; e 20 is the observed electro- motive 
force in microvolts, nearly when the hot junction is at T and the cold 
junction at 20° ; e 2() is preferred to e, which applies for t=0° because t 
in the average case is usually in the neighborhood of 20°, and therefore 
the correction to be added is small. T s and T %n are thermo-electric 
data for the boiling point, when the calibration interval within which 
the constants apply is respectively 0° to 450° and 0° to 1000°. T B is 
therefore the result of exterpolation. Further remarks regarding these 
quantities must be reserved for Chapter II. 
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