Optical System And Electrical Circuit Of The Leeds & Northrup Optical Pyrometer

: The Working Of Steel

For extremely high temperature, the optical pyrometer is

largely used. This is a comparative method. By means of the rheostat

the current through the lamp is adjusted until the brightness of

the filament is just equal to the brightness of the image produced

by the lens L, Fig. 123, whereupon the filament blends with or

becomes indistinguishable in the background formed by the image

of the hot object. This adjustment ca
be made with great accuracy

and certainty, as the effect of radiation upon the eye varies some

twenty times faster than does the temperature at 1,600 deg.F., and some

fourteen times faster at 3,400 deg.F. When a balance has been obtained,

the observer notes the reading of the milliammeter. The temperature

corresponding to the current is then read from a calibration curve

supplied with the instrument.

As the intensity of the light emitted at the higher temperatures

becomes dazzling, it is found desirable to introduce a piece of red

glass in the eye piece at R. This also eliminates any question

of matching colors, or of the observer's ability to distinguish

colors. It is further of value in dealing with bodies which do

not radiate light of the same composition as that emitted by a

black body, since nevertheless the intensity of radiation of any

one color from such bodies increases progressively in a definite

manner as the temperature rises. The intensity of this one color

can therefore be used as a measure of temperature for the body

in question. Figures 124 to 126 show the way it is read.