CHAPTER IV. 
THEC ALIBRATION OF ELECTRICAL PYROMETERS BY DIRECT 
COMPARISON WITH THE AIR THERMOMETER. 
DISPLACEMENT METHODS OF AIR THERMOMETRY. 
Some time after the methods for measuring high temperatures and of 
measuring vapor densities at high temperatures had been fully devel- 
oped in the admirable manner due to Deville and Troost, 1 a new method 
for high temperature vapor densities was published by V. Meyer. A 
modification was at once introduced by Crafts and Meier, by which V. 
Meyer's method became available for the measurement of high temper- 
atures. In these thermometers the gas used, instead of being kept at 
constant pressure or at constant volume, as in most air thermometers, or 
instead of being pumped out by a mercury air-pump, as in Deville and 
Troost's apparatus, is simply chased out by a second gas. If therefore 
the two gases be collected over a liquid in which the displacing gas only 
is soluble, the volume of the gas which fills the thermometer at any tem- 
perature is easily measurable. Air, for instance, may be used for meas- 
urement, expelled by HC1 or C0 2 , and collected over water. A few 
minutes suffice for the displacement. Since all operations are conducted 
under atmospheric pressure, it is obvious that the Crafts and Meier 
devices can be used at temperature at which porcelain is seriously vis- 
cous and permeable to gas. 
Special forms of the Crafts and Meier apparatus are made by MM. 
Morlent freres, among which the tubular form designed by Meyer 2 to 
fit the Fletcher organic combustion furnace would appear to be specially 
convenient for calibration work. In some earlier experiments Dr. Hal- 
lock and I endeavored to make use of it. The accompanying diagram, 
Fig. 28, drawn to the scale -fa indicates the method of adjusting the 
thermometer for calibration. 
The tubular thermometer of porcelain is shown at A B C. The ends 
A and C of both of the capillary tubes A F and E Care provided with 
three-way cocks of special construction, made of brass. Only one of 
these, D, is shown in the diagram, Fig. 28 being a longitudinal section 
with an end view cross section through the canals. The thermoelement 
to be compared with the air thermometer is stretched along its axis with 
the hot junction at a, the center of figure. The two wires a b and a c pass 
through the capillary tubes A F and E C and through a corresponding 
1 Cf. Introduction, p. 27 et seq., where the full references are given. 
2 V. Meyer: Chein. Ber., vol. 15, 1882, p. 1161. 
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