208 
MEASUREMENT OP HIGH TEMPERATURES. 
Table 55. — T and e 20 from Table 54. 
[Non-re-entrant inglazed bulb No. 2 ; thermo-couple No. 37.1 
[BULL. 
T 
620 
T 
Cm 
T 
C-20 
o(7. 
microvolt. 
0(7. 
microvolt. 
OO. 
microvolt. 
333 
3240 
907 
10650 
767 
8270 
345 
3360 
919 
10800 
707 
7550 
366 
3640 
931 
11000 
602 
6320 , 
379 
3780 
994 
11860 
528 
5320 
396 
4000 
1019 
12220 
452 
4430 
409 
4100 
1036 
12400 i 
396 
3710 
411 
4110 
1041 
12500 
369 
3410 
655 
7300 
325 
2940 
716 
8040 
903 
10150 j 
307 
2720 
722 
8100 
828 
9100 
267 
2300 
In the up-going series, in which observations were made only durir 
periods of very constant temperature, the values of electro-motive for< 
are normal in comparison with values of temperature. The boilii: 
point of* zinc, for instance, is fixed at 930°. In the down-going seri< 
thermo-electromotive force is too small, as usual. The effect, howeve 
is possibly exaggerated by the great difficulty of making these bull 
absolutely tight. Nor is it possible to estimate the leak effect as 
function of time, for the capillary canals change with temperature au 
even by accidental disturbances. 
Difficulties such as are here described led me to the constructic 
of the re-entrant form of bulb, in which, by a simple device, they a] 
wholly obviated. This will be shown at length in the final sectioi 
which follows. I have purposely given a full series of data in Tabh 
50 to 55, in order to bring before the mind the extreme difficulty e: 
countered in making comparisons between the thermo-couple and tl 
air thermometer, when the conditions to be met are accuracy and exp 
dition. 
CONSTANT PRESSURE AIR THERMOMETRY — APPARATUS. 
The above data were investigated in the rational endeavor to ada] 
Jolly's very convenient form of air thermometer to high-temperatu; 
measurements. 1 To do this I found it desirable to use low-pressure (< 
atmosphere) manometers, so that at high temperatures, when porcela: 
shows a tendency to become viscous, the pressures on the interior and c 
the exterior of the hot bulb may not differ by an amount sufficient to d 
form the bulb seriously. But while on the one hand this can never 1 
perfectly accomplished, the difficulty of maintaining the air in the bu 
and manometer at an invariable low pressure makes this instrument u: 
usually liable to errors or accidents on the other. In theconstant-pressui 
method of air thermometry, all hurtful excesses of pressure on the bu] 
1 Weiuhold, Erhardt, and Schertel tried it before (cf., p. 33, 34). 
(8G2) 
