Which thermometers are used to measure extremely high temperatures like volcanoes?

Hint: The high temperature of the volcano is more than that of the temperature of the blast furnace. The furnace is maintained at . The temperature of the volcano is more than . A special instrument is set up in the blast furnace that can also be used to measure the high temperature of the volcano.

Complete answer:
The device which can be used to measure the high temperature of the volcano is called the “Pyrometer”. It can measure the temperature between the range of . Pyrometers work on the principle of heat radiation. They measure the radiation produced by the body and then identify the temperature. The device does not touch the material that is being measured.
In an Optical pyrometer that is used to measure the hot lava of a volcano, the standardized incandescent filament is adjusted so that it can measure the temperature visually. However, in a radiation pyrometer, the radiation is focused on a group of thermocouples i.e. the thermopile. The thermopile produces an electrical voltage that is dependent on the radiation. The electrical voltage is then calibrated to rectify the temperature of the object.
 Pyrometers are effective to measure very high temperatures because they do not have to be placed in the hot substance to measure the temperature. Because placing a device with such a high temperature is not feasible.

Note:
We commonly use a thermometer to measure the temperature that uses a glass tube filled with mercury to denote the temperature. The key difference between the thermometer and pyrometer is that the latter is a remote-sensing thermometer used to measure extremely high temperatures.