This is an instrument for measuring the temperature inside a furnace by comparing the light coming from it with that from a filament lamp viewed through crossed polarisers to adjust its intensity. Looking through the eyepiece an observer sees a patch of red light split into two halves, one from the furnace and one from the lamp. By rotating the analysing polariser the observer adjusts the two to appear the same. The temperature can then be read from the scale showing the position of the analyser.
The filament lamp is powered by an accumulator, rheostat and ammeter in the smaller wooden box. The system has to be calibrated by viewing the flame of an amyl acetate burner with the analyser set to a ‘Cal” position. The current in the lamp that gives a match is then used for subsequent observations.
There are two temperature ranges - 700-1400 and 900-2000 °C, the higher range being selected by inserting a filter in front of the light from the furnace.

Inscriptions / Translations:  CAMBRIDGE
OPTICAL PYROMETER
No 45637
THE CAMBRIDGE AND PAUL INSTRUMENT COY. LTD. ENGLAND