Theory:

Thermometer:
It is a device that is used to measure the temperature or temperature gradient (the degree of hotness or coldness of the object).
 
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Thermometer
  
There are various kinds of thermometers. Some of them are like glass tubes that looks thin and filled with some kind of liquid. Often alcohol and mercury are used in thermometers.
 
Why mercury or alcohol is used in thermometers?
  • Mercury and alcohol both remain in a liquid state even when the temperature changes.
  • Both have a high coefficient of expansion, which helps measure the small changes in temperature by measuring the expansion in the volume of the liquid.
Coefficient of expansion -  The amount by which unit length/area/volume of material changes when the temperature is changed by one degree is called the Coefficient of expansion.
 
Properties of mercury:
  • It has a uniform expansion (For equal amounts of heat, mercury expands by equal lengths).
  • It is opaque (non-transparent) and shining.
  • It does not attach to the sides of the glass tube.
  • It is the only element in the liquid state at room temperature.
  • It is a good conductor of heat (but comparatively low compared to other metals).
  • The range of temperatures measured using a mercury thermometer is high because mercury has a high boiling point of \(357\)\(°C\) and a freezing point of \(−\)\(39\)\(°C\).

Properties of alcohol:

  • The freezing point of alcohol is less than \(−\)\(100\)\(°C\). So we can use it to measure very low temperatures.
  • Its expansion per degree Celsius increase in temperature is very large.
  • It can be coloured brightly and hence it is easily visible.
Types of thermometers:
There are different types of thermometers available to measure the temperature. Each type has its own merits to measure the temperatures in different things like body temperature, air, food and automobiles, etc.,
  • Clinical thermometer
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  • Laboratory thermometer
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  • Digital thermometer
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  • Infrared thermometer
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Reference:
https://commons.wikimedia.org/wiki/File:CelsiusKelvin.svg