Calorimetric pyrometers, based on Regnault’s “method of mixtures,” were first made for industrial purposes by Byström, who patented an instrument of this type in 1862. This method has been widely applied, and a simplified form of “water” pyrometer, made by Siemens, is at present in daily use for industrial purposes. It is not capable, however, of giving results of the degree of accuracy demanded by many modern processes.
The resistance pyrometer was first described by Sir W. Siemens in 1871, and was made by him for everyday use in furnaces. Many difficulties were encountered before this method was placed on a satisfactory footing, but continuous investigation by the firm of Siemens & Co., and also the valuable researches of Callendar and Griffiths, have resulted in the production of reliable resistance pyrometers, which are extensively used at the present time.
In 1872 Sir William Barrett made a discovery which indirectly led to the present development of the science of pyrometry. Barrett observed that iron and steel, on cooling down from a white heat, suddenly became hotter at a definite point, owing to an internal molecular change; and gave the name of “recalescence” to the phenomenon. Workers in steel subsequently discovered that this property was intimately connected with the hardening of the metal; thus Hadfield noticed that a sample of steel containing 1·16 per cent. of carbon, when quenched just below the change-point was not hardened, but when treated similarly at 15° C. higher it became totally hard. The demand for accurate pyrometers in the steel industry followed immediately on these discoveries, for even the best-trained workman could not detect with the eye a difference in temperature so small, and yet productive of such profound modification of the properties of the finished steel. In this instance, as in many others, the instruments were forthcoming to meet the demand.
The researches of Le Chatelier, published in 1886, marked a great advance in the progress of pyrometry. He discovered that a thermo-electric pyrometer, satisfactory in all respects, could be made by using a junction of pure platinum with a rhodioplatinum alloy, containing 10 per cent. of rhodium; a d’Arsonval moving-coil galvanometer being used as indicator. This type of galvanometer, which permits of an evenly-divided scale, is now universally employed for this purpose, and has made thermo-electric pyrometers not only practicable, but more convenient for general purposes than any other type. Continuous progress has since been made in connection with this method, which is now more extensively used than any other.
Attempts to deduce temperature from the luminosity of the heated body were first made by Ed. Becquerel in 1863, but the method was not successfully developed until 1892, when Le Chatelier introduced his optical pyrometer. This instrument, being entirely external to the hot source, enabled readings to be taken at temperatures far beyond the melting point of platinum, which would obviously be the extreme limit of a pyrometer in which platinum was used. The quantitative distribution of energy in the spectrum has since been worked out by Wien and Planck, who have furnished formula based on thermodynamic reasoning, by the use of which optical pyrometers may now be calibrated in terms of the thermodynamic scale of temperature. Other optical pyrometers, referred to in the text, have been devised by Wanner, Holborn and Kurlbaum, Féry, and others; and the highest attainable temperatures can now be measured satisfactorily by optical means.
The invention of the total-radiation pyrometer by Féry in 1902 added another valuable instrument to those already available. Based on the fourth-power radiation law, discovered by Stefan and confirmed by the mathematical investigations of Boltzmann, this pyrometer is of great service in industrial operations at very high temperatures, being entirely external, and capable of giving permanent records. Modifications have been introduced by Foster and others, and the method is now widely applied.
Recorders, for obtaining permanent evidence of the temperature of a furnace at any time, were first made for thermo-electric pyrometers by Holden and Roberts-Austen, and for resistance pyrometers by Callendar. Numerous forms are now in use, and the value of the records obtained has been abundantly proved.
For scientific purposes, all pyrometers are made to indicate Centigrade degrees, 100 of which represent the temperature interval between the melting-point of ice and the boiling point of water at 760 mm. pressure, the ice-point being marked 0° and the steam-point 100°. In industrial life, however, the Fahrenheit scale is often used in English-speaking countries, the ice-point in this case being numbered 32° and the steam-point 212°; the interval being 180°. A single degree on the Centigrade scale is therefore 1·8 times as large as a Fahrenheit degree, but in finding the numbers on each scale which designate a given temperature, the difference in the zero position on the two scales must be taken into account. When it is desired to translate readings on one scale into the corresponding numbers on the other, the following formula may be used:—
| (C. reading) | (F. reading - 32) | |
| ————— | = | —————— |
| 5 | 9 |
Thus by substituting in the above expression, 660° C. will be found to correspond to 1220° F. and 1530° F. to 832° C.