Disappearing-filament pyrometer

[1][2][3] Invented independently in 1901 by Ludwig Holborn and Ferdinand Kurlbaum in Germany and Everett Fleet Morse in the United States,[1] it was the first device which could measure temperatures above 1000 °C.

[1] 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Widely used in the steel and ceramics industries as well as for research, they have been almost totally superseded by electronic spectral-band pyrometers.

A thin wire (filament), placed at the focal plane of the objective lens, is heated by electric current.

[2] In other designs the current through the filament is kept constant, and the radiation allowed through from the target object is varied with calibrated attenuating wedges in the optical path,[3] or a prism is used to place the images of the target object and a calibrated glowing surface next to each other, e.g. as a disk inside a ring.