LABORATORY OPTICAL PYROMETER 
81 
gether. The telescope must then be placed at such a distance from 
the lamp that the filament may be seen distinctly. If the back- 
ground is large it will be found very convenient to use a lamp hav- 
ing a filament larger than 0.033 mm. ; one of twice this diameter 
is very commonly used in practice. For each setting of the lens, 
lamp and telescope, a calibration will be required. 
Millimeter profile paper was used for plotting the calibration 
curves. For the ammeter curves the writer found that a scale of 
0.0002 ampere per mm. for the current and 0.2 division per mm. 
for the deflection, gave current-deflection curves from which cur- 
rents could be read with an error of less than 0.0001 ampere. The 
pyrometer-lamp current-temperature curves were drawn to a scale 
of 0.0002 ampere per mm. for the current and 2° per mm. for the 
temperature ; this was sufficiently accurate for the needs of the 
writer. Degrees-correction plotted as a function of the observed 
brightness-temperature gave a useful curve because the observa- 
tions were made through an absorbing mediurn. Since a large 
number of temperature determinations of the same substance were 
made, a curve connecting brightness-temperature and true-temper- 
ature saved much laborious calculation. The slope of the pyrome- 
ter-lamp curve changed from point to point. For the set-up of 
figure 7 it was such that, in the neighborhood of 1000° K, a change 
of one milliampere in the current through the lamp represented a 
change of 13° in the temperature of the background; at 1100° the 
variation was 9° per milliampere; at 1700° the rate of change was 
3° per milliampere. 
The writer in conclusion wishes to thank the Physics Depart- 
ment for placing at his command such good workshop facilities ; 
Professor L. P. Sieg, under whose supervision the work was done, 
for his keen interest; Mr. J. B. Dempster for much valuable in- 
formation regarding workshop methods ; and Dr. A. G. Worth- 
ing of the Nela Research Laboratory for many valuable sugges- 
tions concerning optical pyrometry and for furnishing the cali- 
brated standard-lamp and a pyrometer-lamp. 
REFERENCES 
n) Worthing & Forsythe, Phy. Rev., 4, 163-176, 1914. 
(2) Pyrometry (see note below), 293, 1920. 
(3) Worthing, Phy. Rev., 10, 377-394, 1917. 
(4) Hyde, Cady & Forsythe, Astrophy. Jour., 42, 294, 1915. 
(5) Pyrometry, 319. 
(6) Forsythe, Gen. Elect. Rev., 20, 749, 1917. 
(7) Pyrometry, 306. 
