A pyrometer is a device for measuring high temperatures, specifically those above 673°F (356°C), the boiling point of mercury. John Frederic Daniell (1790-1845), the first professor of chemistry at King's College, London, invented an instrument known as a register pyrometer in 1830. This used the expansion of platinum to indicate, for example, the temperature of liquid silver. The platinum bar was placed in a hollow cylinder of plumbago, and the expansion was registered using a lever system and scale. Daniell also went on to invent an electrical battery that became known as the Daniell cell, and a dew-point hygrometer.
As the nineteenth century progressed, the accurate measurement of temperature became ever more Important in manufacturing processes involving such things as pottery kilns and steel furnaces. Devices were also required that would measure temperatures up to around 5,400°F (3,000°C). Since these devices had to be distant from the high-temperature object, the mirror pyrometer invented by Frenchman Charles Fery used a reflecting telescope system to focus emitted radiation onto a detector.
Optical pyrometers were first patented in 1899, and these compared the color of the incandescent body to that of a hot current-carrying wire. This instrument relied on the fact that the color of the radiation emitted by a heated body changes from red to yellow to white as the temperature increases.
