LABORATORY OPTICAL PYROMETER 
75 
and the pyrometer. Since such media absorb some of the incident 
light it is necessary to correct for their absorptions before the true 
brightness-temperature of the source can be stated. If the absorb- 
ing medium is on such an apparatus that it is possible to put a 
small incandescent lamp behind the medium, then the temperatures 
of the lamp for fixed currents can be determined through the ab- 
sorbing medium and without the medium present. From such data 
the temperature-corrections may be found for the desired range. 
If the apparatus is in the form of a closed glass tube it can be 
placed in the usual position for the sectored-disc, its transmission 
determined, and the corrections calculated.^^, 
Currents through the pyrometer-lamp filament must be accurate- 
ly known. A potentiometer gives the highest accuracy but, if a 
number of settings are to be made, the time required is rather long. 
Therefore a deflection potentiometer is used for the highest grade 
work. For ordinary purposes a high grade ammeter equipped 
with an external adjustable (by steps) shunt will be found very 
satisfactory. The steps should be so chosen that it will always be 
possible to keep the deflection of the pointer between 100 and 120 
on a 150-division scale. The variable resistance to be used in the 
pyrometer-lamp circuit should be capable of rather fine adjust- 
ment. This is readily accomplished by connecting two rheostats of 
the sliding-contact type in parallel ; one should have a resistance of 
about three times that of the other. The rough adjustment is made 
on the rheostat of low resistance and the final adjustment on 
the one of high resistance. Best results will be secured if the ad- 
justment is always made in the same manner, as for example, the 
brightness-match might be made by gradually increasing the cur- 
rent until the filament disappears against the background-image. 
If all the settings are made jn this manner consistent results can 
be obtained with a minimum of experience. 
A LABORATORY OPTICAL PYROMETER 
Figure 7 shows an optical pyrometer which meets all the condi- 
tions listed in the preceding section of this article. It also has a 
number of adjustments which help make it flexible enough to be 
of general use in a physical laboratory. 
The objective lens A is a Bausch & Bomb rapid rectilinear lens 
having a diameter of 28 mm., and a focal length of 21 cm. Be- 
tween the two lens-combinations is an iris diaphragm which, when 
set at the largest aperture, allows the lens to have a speed of 
U. S.4. 
