Leeds And Northrup Optical Pyrometer

: 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.

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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

Fig. 127.