216 MEASUREMENT OF HIGH TEMPERATURES. 
[bull. 5ii 
where v- } and t x have the same signification for the compensator that 7J 
and T\ have for the air thermometer. Inserting the expression 
into the equation (5) this becomes 
in which the correction is evaluated experimentally. 
It will be shown below that the stem error is not of such serious im- 
portance in the constant-pressure method as it was in the constant- 
volume method; that the stem error rather falls below the other possi- 
ble errors of measurement. Hence the use of the compensator is not? 
to be as strenuously advised as it was above, and the correction derived 
merely from calibration and computation may be regarded satisfactory. 
This is, of course, a convenience, since it obviates the manipulation foi 
the measurement of the two additional magnitudes Vi and t x * Such! 
statements, however, must be made with caution, for it will appear that 
in the constant-pressure method, inasmuch as the volume of the bulb 
enters fundamentally into the computation, the real object of the com 
pensator is to define the volume of the bulb. The true volume of the 
bulb, when the compensator is used, is its own volume plus the volum< 
of the part of the capillary tube, the length of which is the difference 
of length of stem and of compensator. 
CONSTANT-PRESSURE THERMOMETER — EXPERIMENTAL RESULTS. 
Manipulation. — Before proceeding to the tabulation of final results it! 
is expedient to refer to a few details of manipulation. On inserting the! 
thermo-couple into the central tube ot the air-thermometer bulb it isj 
necessary to have the wires quite enveloped by the fire-clay insulators*! 
This is done to avoid silicificatiou. It is well to have the insulator within 
the air thermometer of smaller diameter than the straight tube which] 
passes from the bulb to the outside of the furnace, for the porcelain glaze] 
becomes viscous at high temperatures, and is liable to-be absorbed by the! 
insulator. Hence it is desirable to avoid such close contact as would! 
be given by a tube filling the hole snugly. With due precautionary! 
preventives of this kind silicification is nil. Of course, the thermo-j 
couple must be kept in place while the furnace is cooling. If different! 
thermo-couples are to be inserted this must be done while the furnace] 
is heated or during the stages of rising temperature. With a well-j 
dried bulb, symmetrically adjusted in the revolving muffle, the heatingj 
and cooling may be repeated almost indefinitely without breakage.! 
Breakage is liable to occur when thermometers or muffles are ex-j 
changed. Wherever it is possible, it is well to avoid glass cocks.j 
(870) 
