AKUS.] 
PORCELAIN AIR THERMOMETRY. 
215 
Sven this correction is small, and for 1,500° will not much exceed 5°. 
t is permissible therefore to put T=2T' and to insert for V a mean 
'alue. Table 57 is of the kind here referred to, and exhibits the values 
>f T^W f ° r each Value of T and for *'= 15 ° > v '/ v is found b .Y measure- 
nent to be 0.00043. 
Table 57. — Errors of thermometer formula' . 
T 
S 
T 
s 
aM* 
T 
S 
°G 
°G 
°0 
100 
— 0.0 
GOO 
— 0.G 
1100 
— 1.8 
200 
— 0.1 
700 
— 0.8 
1200 
- 2.2 
300 
— 0.2 
800 
— 1.0 
1300 
- 2.6 
400 
— 0.3 
900 
— 1.2 
1400 
— 2.9 
500 
— 0.4 
1,000 
— 1.5 
1500 
- 3.4 
It will be seen by comparing* this table, 57, with the similar one above 
Table 45) for the constant- volume method that the errors here are 
rery much smaller in magnitude. A result of this kind was to be antici- 
)ated, and the occurrence of small stem corrections, added to the fact 
;hat measurements are made in such a way that the tension of the gas 
nside the bulb need not exceed atmospheric pressure, is the salient 
tdvantage of the present method of high-temperature measurement 
>ver the constant-volume method. 
Compensator. — In calculating the results of the last table no allow- 
mce is made for fissures or for the porosity of porcelain. Hence bet- 
er results may be anticipated by using the compensator, though it 
ilways remains questionable whether the volumes of two nominally 
dentical porcelain capillary stems are at all identical in fact. llow- 
ver, it is only the part of the compensator along which temperature 
raries from the high value T to that of the room that need be identical 
ivith the stem of the air thermometer. The remainder of the compen- 
sator may have any (capillary) volume, which need not be exactly the 
>ame as the volume of the capillary canals of the air thermometer, 
rhis appears fully from the formulae below. 
The theoretical introduction of the data of this apparatus is here 
juite simple; for it is seen that the quantity marked 8 in the equa- 
tion, page 212, is the one immediately given by the compensator. It 
vill be remembered that this apparatus is essentially a porcelain stem 
dentical with the stem of the air thermometer, provided, however, with 
i manometer tube of much smaller caliber than the tube B C of the 
nanometer, page 210. Supposing therefore that the observations are 
nade in the way already described for constant-pressure air thermom- 
>try, we have at once 
s=^'(/(T)-nt'))=-}.f(h) 
(869) ' 
