How are the temperatures below zero degrees celsius measured?

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Mercury thermometer is used to measure temperatures above zero degree Celsius. Mercury expands upon heating and is a good thermal conductor. It is also a bright liquid and thus convenient for temperature measurement. However, mercury cannot be used because it freezes and stops flowing at minus 38.87 degrees Celsius. Measurement of subzero temperatures are necessary in many areas. These include weather (daily temperatures of towns in Himalayan and such snow-covered regions), processes such as freeze drying to make milk powder, immunology where vaccines have to be preserved and so on.

Temperatures below zero degree Celsius (that is, the minus scale) can be measured by resistance thermometers. They work on the premise that the resistance of materials changes with temperature. Resistance decreases as the temperature is reduced in the case of metals (positive temperature coefficient), while the resistance increases with decrease of temperature in the case of semiconductors (negative temperature coefficient). Platinum resistance thermometers are used down to minus 170 degree Celsius. For measurement of even lower temperatures, Germanium and Silicon diode (semiconductor) thermometers are used. Thermocouples can also be used for measuring temperatures below zero degrees

Source: thehindu.com