Disappearing-filament pyrometer
![Technician measuring the temperature of molten silicon at 2650°F with a disappearing-filament pyrometer, in Czochralski crystal growing equipment at Raytheon transistor plant in 1956. The knob she is turning (bottom right) controls current through the filament.](/Images/godic/202501/09/Silicon_grown_by_Czochralski_process_19565221.jpg")
The disappearing-filament pyrometer is an optical pyrometer, in which the temperature of a glowing incandescent object is measured by comparing it to the light of a heated filament. Invented independently in 1901 by Ludwig Holborn and Ferdinand Kurlbaum in Germany and Harmon Northrup Morse in the United States, it was the first device which could measure temperatures above 1000°C. Disappearing filament pyrometers have been used to measure temperatures between about 600°C and 3000°C. Like other optical pyrometers they are used to measure the temperature of objects too hot for contact thermometers, such as molten metals. Widely used in the steel and ceramics industries as well as for research, they have been almost totally superseded by electronic spectral-band pyrometers.