166 
MEASUREMENT OF HIGH TEMPERATURES. 
[BULL. 54 
* 
straight capillary canal in the stop-cocks. To prevent escape of air a 
these holes the wires are, during the measurements, sealec i 
into them with resinous or other cement. 
The capillary stem of the air thermometer communi 
cates, however, with a second capillary canal in the stop 
cock Z>, at right angles with the other canal. It is througl 
this second canal and the three-way cock that either th( 
soluble or the insoluble gas may be introduced into the 
thermometer, the two lateral tubes d g and / 1 being it 
connection with the corresponding gasometers. There is 
also a hole at n, through which either gasometer may com- 
municate with the atmosphere. 
We did not carry this method of calibration into greal 
detail, chiefly because the temperature at any given point 
of the long tubular space of the Fletcher furnace, as well 
as the mean temperature of the whole length of tube, 
proved to be insufficiently constant. Nor can it be as- 
sumed that the temperature at the center a of Fig. 28 
(thermoelement) is identical with the mean temperature 
of the tubular column of gas. Moreover, since the capacity 
of the thermometers is not much over 100 cc , measure- 
ments of gas volume must be made with very great care 
to be in keeping with the accuracy of calibration required.! 
It is also inconvenient to insert a special thermo-elementt 
permanently for each series of measurements, the problem 
of calibration being usually of such a kind as to make it 
desirable to compare a series of thermoelements either 
at once or else in rapid succession. Finally, the thermo- 
element to be compared must necessarily be filiform and 
very long, whereas the constants of short thick thermo- 
elements are frequently in demand. Add to these even 
more serious sources of error, inasmuch as measurements 
are made in a tube not glazed internally and with a gas, 
the rigorous purity of which is not assured. Nevertheless 
this method of calibration may sometimes be convenient. 
(Of. p. 36.) 
Taking the elaborate gasometric apparatus into account 
the method is not as simple as it appears. It occurred 
to me, however, that this simplicity might possibly be 
reached by displacing dry air with slightly superheated 
steam. But I made no experiments. 
The application of Crafts's method, which can easily be made by in- 
serting a platinum capillary tube into the stem of the air thermometer, 
deserves special notice, and by the aid of the metallic tubing, to be de- 
scribed below (Chap. V), can be put to a rigorous test. 
(820) 
