192 
MEASUREMENT OF HIGH TEMPERATURES. 
[BULL. 54„' 
Hon. By way of example, the following data for k'+k", taken froiuj 
tables of the kind in question, may here be inserted : 
Table 45.— Values of n' -\-n" for divers T and t". 
T= 
100° 
500° 
1,000° 
1,500° 
V =. 20° 
t" = 30° 
0. 0060 
0. 0058 
0. 0057 
0.0121 
0.0117 
0. 0114 
0. 0197 
0.0191 
0. 0185 
0. 0272 
0. 0263 
0. 0255 
This table shows at once that if T" and t" differ by only a few degrees,! 
the effect on the result will be less than t |q of one per cent, per degrea 
of difference of T", and t"< Now, the walls of the furnace were pur-i 
posely chosen thick. They are well jacketed, and it is therefore possible] 
to screen off the radiation from the " cold" 25 cm of stem entirely. The] 
use of capillary tubes of metal enables the operator to place the manom-j 
eter at some distance from the furnace and in an environment of con^ 
stant temperature. The high furnace table conduces to this purpose,] 
since the mercury apparatus may be expediently placed below and oi 
one side of the furnace plane, quite screened from it by the table. 
Hence the approximation I"=zt" involves no greater error than a fe^ 
hundredths of one per cent. 
From these considerations the practical convenience of equation (6), 
when many values of Tare to be reduced, appears at once. T computec 
simply as (H—h ) (ahn—fSH) is in the extreme case within 3 per cent 
of the correct result. By introducing this value into the corrective 
h'+h" the effect on the result (T) is in the extreme cases less than T fj 
of one per cent. Having therefore calculated a table of double entry foi 
H'+x"={l + aT) 
« 1+ 4 
v" l + fit 1 
1 + a 
T ' v 1 + at" 
2 
. . (7) 
of which the variables are T and t" in the manner suggested, the ap- 
proximate value Ti=(H—h„) (ah —/3H) may be corrected at once. II 
is in the interest of expeditious calculation moreover to compute i 
smaller table for fSR. Mention has been made that h is a mean value 
of measurements made before and after the calibration. 
Compensator. — This is an ingenious device of MM. Deville and Troost, 
by which the stem error may be corrected directly. A second stem 
closed at one end, but otherwise identical in diameter, length, caliber, 
etc., with the stem of the air thermometer, is exposed simultaneously 
with it. The two stems are placed contiguously and so as to be in tli 
same environment. MM, Peville and Troost, using volumetric methods, 
(84G) 
