Which one of the following Thermometers is known as a pyrometer?

The answer is B) Radiation thermometers. A pyrometer is a thermometer that measures temperature from the thermal radiation an object emits, without touching it. This makes it the standard tool for very high temperatures, such as furnaces, kilns, molten metal and stars, where contact sensors fail. The optical pyrometer and the total radiation pyrometer are the classic examples.

• A) Thermo-electric thermometers: These are thermocouples, which work on the Seebeck effect. They need physical contact with the body, so they are contact thermometers, not pyrometers in the standard sense.
• C) Gas thermometers: These use the pressure or volume change of a gas at constant conditions. The constant-volume gas thermometer is the reference for the ideal gas temperature scale. It is a primary standard, not a pyrometer.
• D) Liquid thermometers: These rely on the thermal expansion of a liquid such as mercury in glass. They are limited to a narrow range and are contact instruments.

The concept is the link between temperature and emitted radiation, which is governed by Stefan-Boltzmann and Wien's laws. In Mains, this comes up in GS-III under science and technology applications and in industrial measurement.

Trap: Many aspirants pick thermocouples because they are used at high temperatures. High-temperature use alone does not make an instrument a pyrometer. The defining feature is non-contact measurement through radiation.