Leeds And Northrup Optical Pyrometer
: PYROMETRY AND PYROMETERS
: The Working Of Steel
The principles of this very popular method of measuring temperature
are sketched in Fig. 123.
The instrument is light and portable, and can be sighted as easily
as an opera glass. The telescope, which is held in the hand, weighs
only 25 oz.; and the case containing the battery, rheostat and
milliammeter, which is slung from the shoulder, only 10 lb.
A large surface to sight at is not required. So long as the image
formed by the objective is broader than the lamp filament, the
temperature can be measured accurately.
Distance does not matter, as the brightness of the image formed
by the lens is practically constant, regardless of the distance
of the instrument from the hot object.
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.
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