Temperature Definition in Science

We think of temperature as a measure of hot or cold, but what we're really talking about is thermal energy.
We think of temperature as a measure of hot or cold, but what we're really talking about is thermal energy. Petra Schramböhmer, Getty Images

Temperature is an objective measurement of how hot or cold an object is. It can be measured with a thermometer or a calorimeter. It is a means of determining the internal energy contained within a given system.

Because humans easily perceive the amount of heat and cold within an area, it is understandable that temperature is a feature of reality that we have a fairly intuitive grasp on. Consider that many of us have our first interaction with a thermometer in the context of medicine, when a doctor (or our parent) uses one to discern our temperature, as part of diagnosing an illness. Indeed, temperature is a critical concept in a wide variety of scientific disciplines, not just medicine.

Heat Versus Temperature

Temperature is different from heat, although the two concepts are linked. Temperature is a measure of the internal energy of a system, while heat is a measure of how energy is transferred from one system (or body) to another, or, how temperatures in one system are raised or lowered by interaction with another. This is roughly described by the kinetic theory, at least for gases and fluids. The kinetic theory explains that the greater the amount of heat is absorbed into a material, the more rapidly the atoms within that material begin to move, and, the faster atoms move, the more the temperature increases. As atoms begin to slow down their movement, the material becomes cooler. Things get a little more complicated for solids, of course, but that's the basic idea.

Temperature Scales

Several temperature scales exist. In the United States, the Fahrenheit temperature is most commonly used, though the International System of Units (SI unit) Centigrade (or Celsius) is used in most of the rest of the world. The Kelvin scale is used often in physics and is adjusted so that 0 degrees Kelvin is equal to absolute zero, which is, in theory, the coldest possible temperature and at which point all kinetic motion ceases.

Measuring Temperature

A traditional thermometer measures temperature by containing a fluid that expands at a known rate as it gets hotter and contracts as it gets cooler. As the temperature changes, the liquid within a contained tube moves along a scale on the device. As with much of modern science, we can look back to the ancients for the origins of the ideas about how to measure temperature back to the ancients.

In the first century CE, the Greek philosopher and mathematician Hero (or Heron) of Alexandria (10–70 CE) wrote in his work "Pneumatics" about the relationship between temperature and the expansion of air. After the Gutenberg Press was invented, Hero's book was published in Europe in 1575, its wider availability inspiring the creation of the earliest thermometers throughout the following century.

Inventing the Thermometer

The Italian astronomer Galileo (1564–1642) was one of the first scientists recorded to have actually used a device that measured temperature, though it is unclear whether he actually built it himself or acquired the idea from someone else. He used a device called a thermoscope to measure the amount of heat and cold, at least as early as 1603.

Throughout the 1600s, various scientists tried to create thermometers that measured temperature by a change of pressure within a contained measurement device. English physician Robert Fludd (1574–1637) built a thermoscope in 1638 that had a temperature scale built into the physical structure of the device, resulting in the first thermometer.

Without any centralized system of measurement, each of these scientists developed their own measurement scales, and none of them really caught on until Dutch-German-Polish physicist and inventor Daniel Gabriel Fahrenheit (1686–1736) built his in the early 1700's. He built a thermometer with alcohol in 1709, but it was really his mercury-based thermometer of 1714 that became the gold standard of temperature measurement.