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. It is greatly to be regretted that all pyrometers are not made to indicate in Centigrade degrees, as confusion often arises through the use of the two scales. An agreement on this point between instrument makers would overcome the difficulty at once, as the Centigrade scale is now so widely used that few purchasers would insist on Fahrenheit markings.