PYROMETER (from Gk.7riip, pyr, fire + Airpov, metron, measure). An instrument used for measuring high temperatures where the or dinary mercurial thermometer is not available, as mercury boils at 358° Centigrade. The first pyrometer was that invented by the Dutch physi cist Mussehenbroek, about 1725, and consisted of a metallic bar which expanded under the influence of beat. Wedgwood's pyrometer, which was de vised about 1780. employed cylinders of fire-clay which were exposed to a high temperature and then allowed to cool. The degree of heat was as certained by measuring the amount of contraction in length experienced by the clay. This method, however, was found unreliable, and the apparatus invented in 1821 by Daniell formed the first really serviceable pyrometer. Here the temperature was measured by the relative expansion of a bar of metal such as iron or platinum contained in a tube of black-lead earthenware. The metal rod was in contact with a lever which moved an indi cator over a graduated scale. This instrument was subsequently improved by the substitution of graphite for the platinum rod, and could be used for the measurement of temperature as high as 1000° Centigrade. Numerous other pyrometers have been devised depending on the property pos sessed by metals expanding with an increase in temperature, but the most accurate instrument is the air thermometer (q.v.), which is the ultimate standard of reference in all high-temperature measurements. It is usually constructed with a bulb of porcelain or platinum containing either air or hydrogen, and the expansion of the gas under the influence of heat increases its pressure and raises the level of a column of mercury. As the pressure of a gas is proportional to its tem perature, it follows that by determining the height of the mercury column supported by the air at the different temperatures, a determination can be made with considerable accuracy, though there are numerous practical difficulties necessitating an experienced .observer. A form of pyrometer much used is that invented by Dr. C. W. Siemens, in which the temperature is measured by the in crease in the electrical resistance of platinum wire exposed to the heat. This wire is wound on a cylinder of refractory fire-clay inclosed in a shield of platinum wire, which can be determined easily and quickly with a galvanometer and re sistance coils by the application of the Wheat stone bridge method. The platinum thermom
eter, now made in many forms, can be standard ized by direct comparison with an air thermom eter, and a reference table constructed enabling an observer to ascertain readily the temperature of a furnace or substance with a high degree of accuracy. The pyrometer which is considered the most useful for extremely high temperatures is that used by Le Chatelier, and may he taken as typical of those making use of the thermo-eouple. This instrument consists essentially of two pieces of wire of but slightly different composition. which are inclosed in a long tube of porcelain or fire-elay. These wires are platinum and an alloy of 90 per cent, platinum and 10 per cent. of the rare metal rhodium, and the current produced at an increase in temperature is measured by a gal vanometer. The instrument measures and accurately temperatures in the neighborhood of 1200° Centigrade. To Le Chatelier is also due a photometric method of determining high temperatures by measuring the intensity of the light emitted by a glowing metal. The spectro scope can also be used to measure high tempera tures as well as the calorimeter, in which ease the temperature of a given piece of metal whose specific heat is known is determined by noting the increase in temperature of a mass of water in which it is immediately placed upon removal from the furnace. This principle is em ployed in a number of instruments, but it does not afford especially accurate results, as the spe cific heat of metals is different at different tem peratures. The thermocouple and the platinum are on the whole the most useful forms of pyrom eters, as they can be used over an extended rause of temperature, from that of liquid air in hydrogen almost to the inciting point of and readings can be made without undue difficul ties either of manipulation or of calculation. Con sult: Preston. Theory of Heat (New York. 1894) ; Barns. "Thermo-Electrie Measurement of High Temperatures," Balirtia raitcd Ntates ical Aurrey: id.. Beintrt on High Temper011re Measurements, In ternatiwzal Congress of Physi cists (Paris. 1901).