National Physical Laboratory

What is a thermometer? (FAQ - Thermal)

A thermometer is a device which has a measurable property which changes with temperature.

  • For a liquid-in-glass thermometer, the property measured is the length of the liquid column inside a glass tube.

  • For a platinum resistance thermometer or for a thermistor, the property measured is the electrical resistance of a piece of 'sensing' material.

  • For a thermocouple, the property measured is the voltage generated along the wires making up the thermocouple.

  • For a pyrometer (radiation thermometer), the property measured is the current generated by a photodiode situated at the focus of a lens system.

Of course, nearly everything changes in some way with temperature, but not everything is a thermometer! So to qualify as a useful thermometer, a device must have some other properties too.

  • It must be reproducible. This means that whatever the measured property of the device, that property should have the same value (or very nearly so) whenever the temperature is the same.
  • It must be insensitive to things other than temperature. This means that whatever the measured property of the device, that property should not depend on factors such as the humidity or pressure, or on the materials of which it is made, or on special properties of the thing being measured such as its colour or size.
  • It must be calibrated. This means that we must know how to convert the measured property (length, resistance, etc) to temperature. To do this, the device must be exposed to some environments where the temperature is known, and the value of its measured property must be recorded in those environments. In some cases, for example in a mercury thermometer, the scale reads directly in temperature, and in this case calibration serves to show how accurate the thermometer scale is.
  • It should be convenient to use. Factors such as size, cost, speed of response, ruggedness, immunity to electrical interference, etc, will be important to varying degrees in different applications.

Links You can find some more background information about thermometers at:

Last Updated: 25 Mar 2010
Created: 8 Oct 2007


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