A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. The word thermometer is derived from two smaller word fragments: thermo from the Greek for  heat and meter from Greek, meaning to measure.

A thermometer has two important elements, the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer).

Industrial thermometers commonly use electronic means to provide a digital display or input to a computer.

The Liquid in Glass Thermometer:


Parts of Thermometer:


working Liquid

Reference point


Expansion chamber

Contraction chamber

Working Principle:

The Liquid in Glass thermometer utilizes the variation in volume of a liquid in temperature. They use the fact that most fluids expand on heating. The fluid is contained in a sealed glass bulb and its expansion is measured using a scale etched in the stem of the thermometer If we consider that the thermometer does not expand then as physical property it utilizes the variation of length of liquid with temperature.

Liquids commonly used include Mercury and Alcohol.

Basic Two Parts:

  1. a) The bulb: Acting as a reservoir holding the liquid whose volume changes with temperature. The Bulb   also acts as a sensor or gauge which is inserted in the   body whose temperature is to be measured.
  2. b) The Stem: containing the scale that is measuring the temperature and a capillary through which the liquid         can accordingly expand and contract.


  1. They are cheap to manufacture
  2. Easy to carry and handle.


They tend to have high heat capacities. They are not sensitive enough, that is they cannot measure rapid temperature changes.


Liquid in Glass thermometers have been used in science, medicine, metrology and industry for almost 300 years.


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