On optical pyrometry

Abstract

The most convenient and accurate instrument for the measurement of high temperatures is undoubtedly the disappearing filament pyrometer. During some work on the vaporisation of palladium and nickel, Mr. C. W. Gibby and one of the present authors had occasion, some years ago, to design a modification of this pyrometer, so as to magnify small filaments of these metals, which were heated to incandescence electrically. Recently, having undertaken investigation of the carbon are at high pressures, the methods of pyrometry then used have been extended, with the object of diminishing some of the difficulties connected with optical phrometry. The first difficulty is that the readings of the oridinary opticl pyrometer are not independent of the distance, when sighted on a small source, such as a filament, for the reason that the cone of rays from the source does not fill the exit aperture completely. It is therefore essential to magnify the source of light. It is unnecessary to describe here the magnifying pyrometer which was constructed, because the design of the optical part of the instrument is similar to that detailed by Fair child and Hoover, and the position of the diaphragms and lenses for good definition agreed with those specified by those authors, although arrived at independently of their recommendations. Provision was made for two different magnifications by removal of the front component of the objective lens, the latter being at a fixed distance (35 cm.) from the lamp. When investigation the temperature distribution over a small source, it is very convenient to be able to suit the magnification to the size of the object or the bounds of the working distance. The working distance for the higher magnification (X 20) was 20 cm., and with the front combination removed about 45 cm. As a precision instrument, the magnifying pyrometer has advantage over an ordinary optical pyrometer, because it can be sighted on a source which, being smaller, is more easily maintained at a uniform and constant temperature.