282 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
a form which also includes equation (12), since A and B are constants. 
In the case of any instrument the capillary bore of which is variable 
along the length of the tube, or which can not be determined, A and B 
may be found by exposing the viscosity pyrometer to two known tem- 
X>eratures. For greater accuracy such an instrument may be directly 
compared with the air thermometer in the way soon to be indicated. 
In this place it is pertinent to call to mind certain valuable properties 
of the explicit equations (12) and (13). In the first place it is clear,; 
inasmuch as the right-hand member of the equations varies as thej 
f power of absolute temperature, that the transpiration thermometer 
is unusually sensitive to variations of temperature. Again it appears 
that the one consideration in which the equations might seem to be of 
questionable applicancy, viz, the occurrence of the fourth power of 
(1+/J0"), becomes of less serious moment because this expression only 
effects l-\-a 6" in the 2.4 power of (1+fid"). Hence the coefficient of 
thermal expansion ot platinum, it known with an accuracy of only, 
10 per cent., would not affect the result more than 0.2 per cent, at 1,000°. 
Furthermore, in the case of known fixed thermal data, like those dis- 
cussed in Chapter II, j3 may be directly determined from transpiration 
measurements made with the instrument itself. For it is merely neces- 
sary to solve equation (14) with respect to fim order to determine this 
constant with the same degree of accuracy with which it is to be used. 
Finally the quantity 
7 J0 
which enters into equations (12) and (13) can also be directly determined 
from measurements made at the low temperature. For, disregarding 
the unessential correction members, equation (5), on page 253 shows at 
once that 
x . R<f_ « t" 
7) 16.76. L V ( p2 ~^ 2 ) 
in which if the measurements are made at 0, t] may be reduced to y] by 
Holman's coefficient. With this operation the fiducial zero of the vim 
cosity apparatus may be said to be determined. 2jJ 4 Ao is more ac-i 
curately determinable in this way than by gravimetric measurement 
of R. 
Results. — To give emphasis to the remarks on the probable excellence 
of the viscosity pyrometer, I will use the data in Tables 86 to 89 for 
the calculation of temperatures from data based on the viscosity ol 
the gases operated with. In Table 92, 6" denotes the temperature,' 
thermo-electrically measured, at the points of the helix shown in the 
diagram, Fig. 47a. [8"] is the corresponding datum of the transpiration 
pyrometer. 
(936) 
