Introducing temperature


What is temperature?

Temperature is the physical quantity which determines the degree of hotness or coldness of a body. More precisely, temperature is that property which governs net heat transfer between two systems. When two systems that are at different temperatures are brought into contact with each other there is a flow of heat from the hotter to the colder system until thermal equilibrium is established. This heat transfer may occur by conduction, convection or radiation.

On a microscopic scale, thermal energy is related to the kinetic energy of atoms and molecules that make up a system (a body or a fluid). The thermal agitation of its constituent molecules is manifested both in linear and vibrational motion. It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy.

The basic unit of temperature in the International System of Units is the kelvin (K). Two other units are currently used: the degree Celsius (°C) and the degree Fahrenheit (°F).

The current and official scale is the International Temperature Scale ITS-90.

It is defined from fixed points (freezing points, triple points or boiling points of single elements) and interpolating equations between fixed points.

A good and simple overview of temperature and heat can be found at the Wikipedia website, which can be read in several languages. There is brief discussion of temperature measurement, some elementary theory, ITS-90, temperature scales and a temperature conversion calculator. There are also links to other sites.

Why is temperature important?

  • Temperature is one of the most important and frequently measured quantities in industry
  • Temperature is an important design or control parameter in a wide range of applications, e.g. process monitoring, optimisation, manufacturing, quality control, storage and distribution
  • Temperature plays a vital role in health, environment and safety and there are many legal requirements and regulations related to temperature

What is special about temperature?

Among the base quantities of the International System of Units (Système international d'unités, abbreviated SI), temperature is special because it is an intensive thermodynamic quantity. This means that temperature is not "additive". While you can easily realise the mass 2m or the length 2l if you have two masses m or two gauge blocks of length l, respectively, having two systems with temperature T does not help you in realising the temperature 2T. Bringing the two systems together does not double the temperature, but leaves its value at T.
Therefore, it is an important issue of thermometry to set up a complete temperature scale starting from one or two temperature values.

What is a thermometer?

The temperature of a body or a medium is measured by a means of a thermometer, which has the property to change with temperature. The range of techniques is extensive: the expansion of a liquid, change of gas pressure, change of electrical quantity (resistance, voltage), thermal radiation, etc.
A thermometer consists of a sensor (a temperature sensing element) connected to a readout instrument. The characteristics of the instrumentation are just as important as the sensor.



Two masses each of 1 kg have a combined mass of 2 kg. For the units of electric current (ampere, A), amount of substance (mole, mol), and luminous intensity (candela, cd) the situation is the same.


However, combining two systems with temperature T produces a combined temperature of T, not 2T. Temperature is said to be an intensive quantity, not an extensive quantity like mass. Therefore, setting up a temperature scale requires some thought.

What is a temperature scale?

The history of temperature scale development is long and far from straightforward. A sketch of the main development from 1878 to 1990 can be found at the BIPM site.

The International Temperature Scale ITS-90 is the current and official scale. It is based on fixed points (phase transitions of pure materials) and interpolating equations. It is the scale which accredited labs refer to in calibration certificates.

There is another scale for low temperatures: The Provisional Low Temperature Scale of 2000 (see below) is mainly for temperatures below 0.65 K, the lower limit of the ITS-90. The PLTS-2000 was recommended by the CIPM in 2000 to extend the ITS-90 to temperatures in the temperature range 0.9 mK to 1 K. T2000 is defined in terms of the melting pressure of 3He.

For further details: