An optical pyrometer (see illustration) allows very high temperatures to be measured at a distance from the temperature source [A]. It exploits the fact that two solids at the same high temperature radiate light with the same spectrum and show the same color. To measure the heat of a furnace [B], an electrically heated filament  fixed in the tube of a telescope with a special lens  is heated until it glows with the same color as that emitted by the furnace . Comparison with the background image shows if the furnace is hotter [C], as hot [D] or colder [E] than the wire. A meter  indicates the current passing through the wire and this can be calibrated directly in temperature degrees.
Related categories• HEAT AND THERMODYNAMICS
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