Temperature is a Measure of Heat

Temperature is a measure of the heat content of a substance or system. It is an imperfect measure of the total heat content of a system because it doesn't directly measure all of the rotational or vibrational energy of molecules, but those can be calculated fairly accurately from the temperature.

We exploit the property of thermal expansion of materials to make devices to measure heat as temperature.

This figure illustrates what happens to a gas of atoms as the temperature is increased. In the bottom panel, atoms are closely spaced. The spacing is proportional to the average speed of the atoms, which reflects the heat of the gas. Slower-moving atoms have lower-energy collisions that don't result in large displacements after collision.

As more heat is introduced (top panel), the average speed increases, which makes collisions more energetic and rebounding atoms move farther away. The net effect is to increase the average distance between atoms. It works just the same for molecules.

Most materials expand over most temperatures as their internal heat energy is increased. Some materials expand more rapidly than others, a property that is tabulated as the coefficient of thermal expansion.

Mercury (Hg) has a large thermal expansion coefficient and was once widely used in thermometers until it was understood that toxic mercury from broken thermometers could be a significant health hazard, especially to children. Now liquid-based thermometers use colored alcohols.

thermal expansion cartoon

thermometer illustration


Bulb thermometers (glass thermometers) exploit the property of thermal expansion of materials to put a number on the amount of internal heat of a substance.

The illustration shows a thermometer at a relatively low temperature (left). When the bulb is exposed to a material with more internal energy (heat), the atoms/molecules of the material collide with the glass of the bulb, which transfers the collision energy to the molecules of the thermometer liquid, which causes it to expand.

All that is left is to calibrate the stem of the thermometer with degree markings. That is done thermometer-by-thermometer for higher precision instruments in order to compensate for differences in forming the narrow tube and bulb.

Another kind of thermometer, the bimetallic device, exploits the difference in thermal expansion properties of two fused metals.

There are other kinds of thermometers, as well. See for example, thermocouple, thermistor, liquid-crystal thermometer and others.

Temperature Scales


The Kelvin scale is the most modern of the temperature scales, and the one recommended for scientific reporting. Zero Kelvin (we don't say "degrees Kelvin" with this scale — a Kelvin is a unit) is the temperature at which all atomic and molecular motion stops. It is not possible to reach this temperature, so far as we know, as it would violate an important principle of quantum mechanics. It makes sense that zero should be absolute zero. There is no lower temperature and a substance at 0K contains no heat energy whatsoever. Temperatures of below 1 nanoKelvin (10-9 K) have been achieved in the lab, however.


Also (but less frequently) called "centigrade", the Celsius (C) scale is a base-ten scale calibrated to the melting and boiling temperatures of pure water at sea level. Zero Celsius is the temperature of melting ice; a convenient calibration of a Celsius thermometer is to place it in water containing ice. By definition, that system is at 0˚C (ice alone can



be much colder than 0˚C). The boiling temperature of water is 100˚C.

The size of the Celsius degree is the same as the Kelvin degree, and 0˚C is 273K, so conversions between degrees-Celsius and Kelvin are very simple, just add 273 to the Celsius temperature or subtract 273 from the Kelvin temperature.


The Fahrenheit (F) temperature scale is a relic that many nations have abandoned, but there's a lot of inertia for keeping it if it's what you grew up with. The melting temperature of ice is 32˚F and the boiling point of water is 212˚F, both inconvenient when compared with the Celsius scale. Additionally, the size of the Fahrenheit degree is 5/9 the size of the Celsius degree or Kelvin. Because of this difference, the Celsius and Fahrenheit scales are the same at one temperature: -40˚F = -40˚C

Here are some benchmark temperatures so you can compare the temperature scales.

Temperature point
All atomic motion stops
Liquid helium (evaporation)
Liquid nitrogen (evaporation)
Dry ice (sublimation)
Water freezes
Water boils
Surface of the sun


fahrenheit celsius conversions


arctic fox factImage: National Geographic

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