Modern Gasworks Practice

THE MEASUREMENT OF HIGH TEMPERATURES 241 (<z ) Titan couples for temperatures up to 1,000° C. (b) Iron-Constantan rod thermo-couples for temperatures up to 800° C. (c) Iron-Constantan wire thermo-couples for temperatures up to 600° C. With. a thermo-electric pyrometer the electro-motive force indicated by the galvanometer is proportional to the difference in temperature between the hot and cold junctions of the thermo-electric circuit. Hence, in order to obtain correct indicationJ it is important for the temperature of the cold junction to be known. To get over the difficulty of keeping the pyrometer head, cooled by water or otlier devices, compensating leads are supplied which have the cffect of transferring the cold. junction to a position of known temperature, i.e., to the indicator or recorder (or junction box if the connecting leads are of great length), and thus the temperature of the pyrometer head itself does not affect the readings. Where a still doser accuracy is required, or in places where the instrument or junction box cannot be erected in a steady temperature, the cold junction may be placed in a thermos flask filled with oil, or in a steam vessel or hypsometer, through which. steam at atmospheric pressure is passing. The success of a pyrometric installation depends largely on the care taken in installing the pyrometers. Care should. be taken in the method of protectmg the leads used for connecting the pyrometer to the junction box or indicator. It will usually be found advisable to run the leads through flexible metallic sheathing either to the galvanometer ot to the junction box. Where the leads are subjected to a high temperature they should be protected by asbestos before being placed in the Steel sheathing. If the temperature is not high, then cotton braided leads may be used. EXPANSION PYROMETERS The Féry spiral pyrometer is a more compact and cheaper instrument than the Féry thermo-electric type, but it is less accurate. The methods of receiving tlie radiant heat and of focusing are exactly the same, but instead of using a thermo-couple which must be connected to a galvanometer outside the instrument, a very small bi-metallic spiral is used, and this Controls the movement of a pointer which swings across a scale calibrated in degrees of temperature. The instrument. is, therefore, entirely self-contained. Fig. 142 is a diagram showing the metallic spiral (much enlarged). The action of the spiral when heated will be understood from the following consideration. If two strips of metal having different coefficients of expansion with temperature be soldered together to form one strip of double thickness, then, if the temperature be changed, the two sides of the strip will expand or contract by different amounts, thus causing the strip to bend. In the Féry spiral pyrometer the strip built up of two dissimilar metals is rolied flat and very thin and. coiled into a spiral shape, so that as its temperature is raised the spiral uncoils. This spiral is very small, actually measuring less than -’-inch (3 mm.) in diameter, and /y-inch. (2 mm.) wide. It is fixed at the centre, and its free end carri.es a light aluminium R