BABUS.J 
PORCELAIN AIR THERMOMETRY. 173 
Table 4:3. — Capacity, etc., of porcelain gas-thermometer bulbs. 
No. 
2 
3 
Bulb. 
Stem. 
Equatorial < ,f l ean 
Capacity. 
Length. 
Thick- 
ness. 
Mean 
caliber. 
Volume 
per cm. 
cm. cm. 
9 0. 27 
9 0.26 
cc. 
300, 
307.4 
cm. 
40 
40 
cm. 
0.8 
0.8 
cm. 
0.122 
cc. 
0. 0116 
Probably owing to difficulties in burning tbe polar diameter is usually 
slightly less than the equatorial diameter. 
To use this bulb for calibration it is necessary to have a space of very 
constant temperature, for the indications of the thermoelement are in- 
stantaneous, and refer only to the little space immediately surrounding 
the thermo-electric junction, whereas the gas thermometer passes rela- 
tively slowly from one temperature to another, and the temperature 
datum refers to the internal mean temperature of the whole exposed 
surface. These differences of character in the respective temperature in- 
dications may, of course, be seriously large. They are entirely arbitrary. 
With the object of eliminating these errors I had a bulb made in the 
shape of Eig. 32, the bottom of which is reentrant, forming a cylindri- 
4Qcms. 
Fig. 32. Non-inglazecl re-entrant air thermometer bulb. Scale \. 
cal tube, n m, the closed end m of which projects inward as far as the 
center of the bulb. It is into this tube that the properly insulated 
thermoelement is introduced with its junction at m. The insulators 
are sufficiently large to practically close the tube n m as with a plug, by 
which loss of heat by radiation is made imperceptible. The tempera- 
ture of the thermo-element and of the gas thermometer may therefore 
be regarded identical. I add that the stem of the form (Fig. 32) is thicker 
than the stem in Fig. 31, in order that at high temperatures there may 
be less liability to bending, supposing the thermometer to be held in a 
horizontal position and the stem to be slightly viscous. The widening 
(827) 
