Thermometer:

             A device that is used to measure the temperature of a body is called a thermometer.

Principle of thermometer:

          Mercury thermometer is based on the fact that material (in this case, the liquid mercury) expands when heated.

Basic properties of thermometric liquid:

 A thermometric liquid should have the following properties:

• It should be visible.
• It should have uniform thermal expansion.
• I should have a low freezing point.
• It should have a high boiling point.
• It should not wet glass.
• I should be a good conductor of heat.
• It should have a small specific heat capacity

Preference for mercury:

         Mercury has a uniform thermal expansion, is easily visible, has a low freezing point, has a high boiling point, and has less specific heat.

Note:  Due to these properties’ mercury is used in a mercury thermometer. Since it is opaque, it is easy to see the capillary.