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Temperature Measurement: Thermometers

by Ron Kurtus

A thermometer is a device used to measure temperature.

Temperature is the average kinetic energy of a material's molecules or its average thermal energy. Heat is transferred to the thermometer, causing changes in its physical properties.

Thermometers can be classified according to the physical principles they employ to measure temperature, such as expansion, electrical resistance and color changes. Thermometers must be calibrated to be able to indicate accurate temperatures.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Measuring temperature

Measuring the temperature is not straightforward. Using a thermometer requires a transfer of energy or heat from an object or substance into the material in the thermometer. The thermal energy causes the thermometer to make some physical change, such as expanding. By calibrating with respect to some other physical process, such as the boiling and freezing points of water, you can have a device that measures temperature.

The unit of temperature is called a degree and is typically 1/100 of a high and low reading.

Types of thermometers


Most thermometers work on the principle that materials expand and contract with temperature.


By putting a liquid in a thin tube, you can readily see its expansion with increasing temperature.

You can put some colored water in a glass or plastic tube with one end closed. The water will move up or down the tube as the temperature changes. The thinner the tube, the greater the movement.

The problem with a water thermometer is measuring temperatures below the freezing point of water.

At one time silver-colored Mercury was used. It is the most accurate, but since it is dangerous to the environment it is only used today by physicians.

The red liquid in many thermometers is colored alcohol, which does not freeze until around −70 °C (−94 °F)

Bimetal thermometer

Another popular device is the bimetal thermometer that is used in many home thermostats. It consists of a copper and a steel strip of metal attached to each other. Since copper expands faster than steel, it causes the strip to bend. When it bends, it can make an electrical contact to start up your furnace.

Usually, these bimetal strips are in a coil. If you have an opportunity, take off the cover of your thermostat and observe the curved metal strip.

Other types of thermometers

The newest type of thermometers are electronic. They often use electrical resistance to measure temperature and show the degrees in digital numbers.

There are also thermometers that have a chemical that changes colors with temperature.

Calibration of a thermometer

Picking your two end points, such as the freezing and boiling points of water, you can calibrate the thermometer and set the degree units.

Expansion type

The assumption is that the expansion of the liquid or metal is relatively linear. In other words, it doesn't expand real much for one degree change at a high temperature as compared to a low temperature. If it is linear, you can have all the units on your scale the same size.

Note that the glass or plastic also expands or contracts with the change in temperature. But the expansion is relatively small, so it does not affect the readings.

Other types

Calibrating electrical or electronic thermometers is relatively easy since changes are linear.

Thermometers that use color changes do not measure temperatures accurately, but they must be calibrated for temperature ranges.


The temperature of an object is the average energy of its molecules. The fact that materials expand when heated resulted in the invention of the thermometer. There are three commonly used systems for measuring temperature: Fahrenheit, Celsius, and Kelvin.

Measure your capabilities

Resources and references

Ron Kurtus' Credentials


Alcohol Thermometers - Wikipedia

Physics Resources


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