What is Thermometer?

A thermometer is a simple device of measuring normal temperature. It may be Liquid-in-glass thermometer for measuring the temperature of our environmental condition or a digital type, used mostly by doctors.

By Anonimski – Own work, CC BY-SA 3.0,

Glass thermometer is the most common temperature measuring device. It consists glass tube filled with working fluid that is usually mercury

In the LIG thermometer (Liquid-in-glass ) the thermally sensitive element is a liquid(mercury) contained in a graduated glass envelope of thermometer scale.

Working Principle

Working Fluid(mercury) expand with temperature increases so the temperature can be determined by measuring expanding volume of fluid.

LIG thermometer work on the principle of thermal expansion of the liquid, which simply means material (Like mercury) properties expand over in the variation of temperature.

The least count of digital Thermometer is 0.1-degree Celsius or degree Fahrenheit.

Types of Thermometer

Usually there are two types of thermometer

  1. Mercury In Glass Thermometer
  2. Bimetallic Strip Thermometer

Mercury In Glass Thermometer

In Mercury In Glass Thermometer consist a glass scale which allows reading temperature. A blank space is maybe vacuum filled with nitrogen.

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Why mercury in glass thermometer?

Due to following reasons mercury used as working fluid

  • Mercury freeze at -38.83 degree Celsius
  • Not expand upon solidification

Many countries banned mercury in medical department because of it toxicity.

Mercury can be replace by mercury-thallium for performance upon solidification.

Bimetallic Strip Thermometer

Bimetallic Strip Thermometer is normally used to control temperature. It consists of a metal strip of different thermal expansion coefficients which are joined together.

By Patrick87 – Translation of Bimetallstreifen.svg, CC BY-SA 3.0,

Working Principle

Different metal have different expanding properties upon rising the temperature.

Let say,if we bong two metal strip together that can bear high temperature and can be use as a temperature controller.

A backyard dial thermometer consists a very long strip so make it more sensitive to small temperature changes.


A furnace thermostat or mercury switch follow above principle to ON or OFF the device.

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