PYROMETER

PYROMETER, an instrument for measuring high temperatures (approximately above 300°, when ordinary mercury thermometers are not applicable), based mainly on the properties of solids, liquids, and gaseous bodies to expand from heat, as well as on the different fusibility of certain solids, or on changes in their optical or electrical properties. The gas pyrometer of Pouillet enjoys the greatest fame, based on the expansion of gases. In the most widespread modification of Regnault and Jolly it appears in the following form (see the figure): the reservoir A by means of a capillary tube a can be by means of the washer m connected with a mercury manometer cdef. The latter consists of two tubes cd and ef containing mercury, which can be poured through the opening f and released through the cock k. On the tube cd, near its upper neck, a horizontal mark is made for a constant level of mercury. The reservoir A and the tube a, filled with dry gas, are connected to the manometer by means of the washer m, and the mercury in cd is brought to the horizontal line. The reservoir is disconnected from the manometer and with the cock g open is placed in the medium whose temperature is to be measured. After sufficient time has elapsed for the gas in the reservoir to take on the temperature of the medium, the reservoir with the cock g closed is immersed in a vessel with melting ice, and after cooling it is again connected to the manometer, in which the mercury in the tube cd is brought to the original level. From the difference in the levels of mercury in the elbow ef before and after heating the reservoir, the degree of expansion of the gas is determined, from which, using the formula of Gay-Lussac and the coefficient of expansion of the gas, the desired temperature is found.

Besides gas pyrometers, which are usually used for calibrating other pyrometers, alloys of different fusibility are used in factories, as for example the Seeger scale, consisting of 20 numbers, where each subsequent number of alloy has a melting point 29° higher than the previous one, so that with the help of this scale temperature measurements are possible in the range from 1150° (No. 1) to 1700° (No. 20).

Of electrical pyrometers, the Siemens pyrometer enjoyed the greatest fame, based on the change in the electrical conductivity of platinum under the influence of temperature, and in the Apleyard pyrometer, constructed on the same principle, the temperature is read directly on a Wheatstone bridge.

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D. Romashov.