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The best precision is obtained when an optical pyrometer is used each time under similar conditions of light and the same observer.

The pyrometers used at present are the Wanner optical pyrometer and the Fery radiation pyrometer.

Optical System and Electrical Circuit of the Leeds & Northrup Optical Pyrometer.—For extremely high temperature, the optical pyrometer is largely used.

The temperatures of slag or metal in furnaces or running through tap-holes or troughs are often measured with optical pyrometers.

Another type of optical pyrometer is focused on the body, the temperature of which is to be measured.