the upper limit of the mercury thermometer. The success of many metallurgical and other operations carried out in furnaces often depends upon correct regulation of temperature, and for this reason pyrometers are extensively used to control processes conducted at high temperatures. A number of different types of pyrometer are in use, each having its special advantages; and the choice of instrument depends upon the nature of the opera- tion. In order that pyrometers of every pattern may agree in their readings, each is calibrated by reference to a number of fixed points, determined by the gas thermometer. The National Physical Laboratory scale is used in Great Britain, and represents the melting-points or boiling-points of a number of pure materials, chosen so as to be separated by convenient intervals of tem- perature. Suitable points for graduating pyrometers are the melting-points of zinc (419-4 C.), antimony (630), common salt (801), silver (961), copper (1083), nickel (1452) and palladium (1549). Above 15 50 C. direct comparison with the gas scale is not possible at present, and instruments designed to read higher temperatures than this are calibrated by reference to the laws of radiation. The features of the different types may conveniently be considered in separate categories.
Thermo-electric Pyrometers. These depend upon the electromo- tive force developed when a junction of two dissimilar metals is heated. The couple used should show a steady increase in E.M.F. with rise in temperature, and should not be destroyed or show an alteration in E.M.F. on prolonged heating. These conditions are best fulfilled by couples made of the platinum series of metals, but owing to the high cost of these, " base metal couples are now largely used, which can be renewed when necessary at a trifling expense. The upper limit of temperature at which a thermocouple may be used must be some degrees below the point at which destruction or change in E.M.F. would commence. Thermocouples in common use are platinum and rhodio platinum (10 % Rh), which may be used to 1400 C. and generates an E.M.F. of i-l millivolts per 100 C.; iron and constantan, upper limit 900 C., E.M.F. 6-7 milli- volts per 100 C. ; and two different nickel-chromium alloys (Hoskin's alloys), upper limit liooC., E.M.F. 7-4 millivolts per iooC. Vari- ous other couples are also used. It will be noted that the base-metal couples develop a much higher E.M.F. than those made from the platinum group. Most couples require protection from furnace gases, which would cause corrosion, and are provided with shields of silica, porcelain, fireclay, or other refractory material, which should be non-porous.
The indicator used with a thermocouple is usually a millivolt- meter, the range of which is determined by the couple used, and the temperature to be measured. The deflections shown when the couple is subjected to the standard temperatures enable the scale to be marked so as to read temperatures directly. Due allowance must be made for the temperature of the other junctions in the cir- cuit, as the deflection, in general, depends upon the excess tem- perature of the heated junction over the " cold " junction or junc- tions. Errors in this direction may be avoided by (i) locating the cold junctions in oil in a thermos flask, so as to maintain a constant temperature; (2) by water-cooling the cold junctions; (3) by the use of compensating junctions (Peake and others) ; or (4) by compensated indicators (Bristol, Paul, Darling and others). For measuring special ranges such as 500 to 1000 an opposing E.M.F. from an ex- ternal source is appliel to the indicator, so that deflection does not commence until the junction has attained 500. Instead of a milli- voltmeter, the indicator may take the form of a potentiometer, in which the E.M.F. due to the junction is balanced against a known difference of potential (Northrup, Brown, Rosenham and others), with the advantages that a delicate galvanometer may be used, and that the indications are independent of the resistance of the leads.
Continuous records of temperature may be obtained by photo- graphic means (Roberts-Austen), a mirror galvanometer being used as indicator, and the spot of light directed on a sensitized paper mov- ing at a known rate. Records in ink are obtained by depressing the pointer of the indicator at regular intervals, and causing it to make a dot in ink on a chart mgving at a known rate, thus recording the deflection at any moment. In the " Thread " recorder (Cambridge and Paul Instrument Co.) an inked thread is pressed onto the chart ; in Siemens' and Paul's recorders an inked ribbon is made to touch the paper; in Foster's recorder a special pen, at the end of the pointer, makes the dot. Recorders may be actuated by clock- work or electric motors.
The present practice is to employ thermo-electric pyrometers for all ordinary work up to noo or 1200 C., when an accuracy of 5 or 10 suffices. This range embraces the temperatures involved in the heat treatment of ordinary steel and other metals and alloys.
Resistance Pyrometers. These instruments were introduced by Siemens in 1871, and are still in use. The principal utilized is the increase in resistance to electricity shown by elementary metals when heated, which, in the case of platinum, has been proved by
Callendar to bear a definite relation to the rise in temperature. The working part consists of a coil of platinum wire, suitably shielded from furnace gases, and connected by platinum leads to one arm of a Wheatstone bridge, or to one branch of a differential galvanometer circuit. Compensation for the leads is effected by dividing the bridge at one end of the coil, so that the leads are in opposing arms (Sie- mens), or by dummy leads connected to the opposing arm of the bridge (Callendar). The reading consists in adjusting the bridge until a balance is obtained, when the resistance of the coil may be read, and the corresponding temperature deduced. For industrial use, indicators are provided in which the temperature is indicated on a dial when the bridge is adjusted to balance, so as to avoid calculation (Whipple, Siemens). Paul's indicator is a special form of ohmmeter, which requires no adjustment, and gives direct readings of temperature. Callendar's recorder is an automatic Wheatstone bridge, controlled by the galvanometer, a pen moving over the bridge wire giving an inked record. The Leeds-Northrup recorder achieves the same end by automatically balancing the pyrometer resistance against an opposing resistance in a differential galva- nometer circuit. The resistance pyrometer is not now greatly used for industrial purposes, and is not suited for continuous use above 1000 C., owing to an alteration fn its indications due to the va- pourizing of the platinum (Crookes). It is more costly and difficult to use than a thermo-electric pyrometer, but is capable of giving closer readings under steady conditions.
Total-Radiation Pyrometers. The energy radiated by a " black body " or full radiator is proportional to the fourth power of the absolute temperature, and if the energy be measured the temperature can be deduced from the above relation. An enclosure at a constant temperature, such as a furnace, gives black-body radiations, and enables the laws of radiation to be applied to measuring tempera- tures without serious error. In most existing forms of total-radiation pyrometers, the rays from the heated enclosure are directed on to a blackened thermal junction, the temperature of which is raised in the proportion of the energy received. A galvanometer in circuit with the junction serves to indicate, by its deflections, the relative amounts of energy absorbed by the junction, and its scale may there- fore be marked to read temperatures directly by applying the fourth- power law.
In the form due to Fery, the rays are focussed by means of a con- cave mirror on to a small metal disc to which the junction is attached, a different focus being required for different distances from the furnace. In Foster's fixed-focus instrument a concave mirror is placed at the closed end of a narrow tube, the radiations being ad- mitted through a diaphragm at the open end, and reflected on to a thermocouple. So long as the lines joining the extremities of the mirror with the edges of the diaphragm fall, if produced, within the heated source, the reading will be the same at any distance. In the form due to Thuring, and made by Paul, the rays are made to enter a polished cone, at the apex of which a thermal junction is placed. Various other modifications have been used by different makers. Records may be taken by attaching the junction to any form of thermo-electric recorder, and employing a chart divided in terms of the fourth-power law.
Radiation pyrometers are used for temperatures ranging from 800 to 2000 C., and are particularly valuable under circumstances which preclude the introduction of an instrument into the furnace, as in the case of rotary cement kilns.
Optical Pyrometers. In the most reliable of these instruments the brightness of the red rays from the heated source is matched against a standard, and calibration effected by applying Wien's laws for the distribution of energy in the spectrum. The red rays from standard and source are obtained either by spectroscopic means, or by viewing through monochromatic red glass. In Wanner's pyrometer, and the Cambridge optical pyrometer, a polarizing device is used for match- ing the colours, the position of the analyser being made to indicate the temperature. In the Holborn-Kurlbaum type, made by Siemens, the filament of an electric lamp is placed in the focal plane of a tele- scope, and the image of the heated object brought into the same plane. The adjustment consists in increasing or decreasing the bright- ness of the lamp by means of a rheostat in its circuit, until the fila- ment disappears into the background, the current taken by the lamp being then read and the temperature deduced from a law connecting this current with temperature. In Fery's optical pyrome- ter equality of tint of standard and source is obtained by means of absorbing wedges of glass which slide over each other. In all these cases experience is needed to secure an exact match. Optical pyrometers of the " extinction " type depend upon the complete absorption of the rays from the heated object, which may be ef- fected by lowering a wedge of dark glass in front of the image as received in a telescope, as in the " Wedge " pyrometer, or by using a layer of densely coloured liquid, the depth of which may be ad- justed, as in Heathcote's pyrometer. The temperature calibration in both these cases is obtained by taking readings at standard tem- peratures, and marking the instruments accordingly.
No satisfactory recording apparatus for optical pyrometers has yet been devised. Their chief advantages are the indefinitely high range from 800 C. upwards and the possibility of obtaining readings from a considerable distance, and under conditions which would make it difficult to use any other type of pyrometer.
