M MEASUREMENT OF HIGH TEMPERATURES. 
8. In view of the facts summarized in 2, insulation of the wires is not 
difficult even when the couple is to be used under pressure. This does 
not apply in case of the resistance thermometer. 
9. When destroyed by silicificatiou or metallic corrosion the thermo- 
couple may easily be purified by fusing it over again on a lime hearth 
and drawing to wire. With good metal the variation of constants thus 
produced is almost negligible. 
10. Finally, the thermo-couple has this important property, that 
between temperatures lying not too far apart (100° to 200°) any inter- 
mediate temperature may be interpolated with great accuracy by the 
quadratic equation devised by Avenarius and Tait, and such interpo- 
lation, curiously enough, seems to be more trustworthy in proportion as 
temperature increases above the regions of incipient red heat. It is this 
property which specially recommends the thermo-couple for the meas- 
urement of relatively very small increments of temperature added to 
relatively large temperatures, possibly under conditions of high pres- 
sure, as is the case, for instance, in investigation relating to melting 
point and pressure of solids. 
To use the thermo-element it is necessary to find the thermal equiva- 
lent of the electromotive force for. all temperatures of the junctions. 
This is at present possible only by making detailed comparison with the 
air thermometer. An experimental problem of some difficulty is thus 
encountered at the outset. Inasmuch as the measuring part of a thermo- 
couple is not much more than a. sensitive point, and the corresponding- 
part of the air thermometer is a sphere of relatively enormous dimen- 
sions, it is not easy to devise an environment which at temperatures 
high and. low shall be thermally identical for both. When temperature 
varies, the indications of the air thermometer necessarily lag behind 
those of the thermo-element. It is therefore one of the chief purposes 
of the present volume to devise a method such that the observed indi- 
cations of air thermometer and thermo-element may be rigorously equiv- 
alent; in other words, to carry forward the methods of calibration to a 
degree of perfection subject only to the improvement of the air ther- 
mometer. I hope in some future publication to show the feasibility of a 
fire-clay air thermometer which will be available for temperature meas- 
urement much above those at which porcelain becomes too viscous for 
further use; but in much of the present volume the object is less to de- 
vise new methods, than to bring the old ones more within the scope of 
easy application than has hitherto been done. However, in Chapter 
V I submit a new method of pyrometry. 
In calibrating thermo-couples I make use of two methods. The first 
of these is preliminary. The measurements made are based on known 
values of high boiling points. The second method, however, is a direct 
calibration with the air thermometer. In this way I arrive at an ulti- 
rior result; for a comparison of the two sets of data thus obtained is 
to some extent a criterion of the degree of accuracy with which thermal 
(70S) 
