ds & Northrup optical pyrometer.]

The manipulation is simple and rapid, consisting merely in the turning
of a knurled knob. The setting is made with great precision, due to
the rapid change in light intensity with change in temperature and
to the sensitiveness of the eye to differences of light intensity.
In the region of temperatures used for hardening steel, for example,
different observers using the instrument will agree within 3 deg.C.

[Illustration: FIG. 124.--Too low.

FIG. 125.--Too high.

FIG. 126.--Correct.]

Only brightness, not color, of light is matched, as light of only
one color reaches the eye. Color blindness, therefore, is no hindrance
to the use of this method. The use of the instrument is shown in
Fig. 127.

OPTICAL SYSTEM AND ELECTRICAL CIRCUIT OF THE LEEDS & NORTHRUP OPTICAL
PYROMETER.--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 can 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.

[Illustration: FIG. 127.--Using the optical pyrometer.]

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.

CORRECTION FOR COLD-JUNCTION ERRORS