Note:  Do not rely on this information. It is very old.


Pyrometer is an instrument for measuring very high temperatures, and is specially required to measure the heat of furnaces. An air or gas thermometer is sometimes used, the temperature being calculated from the expansion undergone by the gas, the pressure' being constant, or from its increased pressure at constant volume. A simple form of it consists of a bulb (of glass or some other suitable material) which is drawn out into a fine neck. It is filled with dry air, exposed to the heat of the furnace, and when it has attained this temperature the neck is sealed up with a blowpipe. It is removed, allowed to cool, and the point of the neck broken off under mercury. This liquid rises into the bulb, the levels of the mercury inside and outside the bulb are equalised, the opening is temporarily closed, and the bulb with the mercury it now contains is weighed. This weight, together with the weight of the bulb when full of mercury, are data from which the expansion of the air and hence the temperature, can be deduced. Hydrogen, nitrogen, mercury vapour, and iodine vapour have in their turn all been used instead of air. Another method of measuring high temperatures is to expose pieces of various alloys to the hent of the furnace, and note which of them melt, the melting-points of these alloys having been previously determined. Prinsep arranged a series of such alloys (of silver, gold, and platinum), whose melting points ranged from 945° to 1775°. Again, a piece of platinum may be exposed in the furnace for some time and then plunged into water in a calorimeter. From knowing the specific heat of platinum for a long range of temperatures, and noting the temperature to which it raises the water, the temperature of the furnace can be calculated. Measurements of the expansion of earthenware or metal rods have also been used, the most successful being Bregnet's device, in which strips of gold, silver, and platinum are fastened together in a spiral, whose end moves as the temperature alters. In electric pyrometers a spiral of platinum wire is heated in the furnace, the temperature of which is estimated from the alteration in its resistance.