Temperature is a numerical representation of hot or cold compared against baselines, typically the point at which water freezes and boils.

Intuitively, people have known about temperature for a long time: fire is hot and snow is cold. Yet, a full appreciation of the complexities of temperature and its measurement has been relatively slow to develop. Today, Temperature is by far the most measured parameter. It impacts the physical, chemical and biological world in numerous ways ….

Measuring temperature …

We do not know much about if the ancient Greeks or Chinese had ways of measure temperature, as far as we know measuring temperature started during the Renaissance.

Several inventors were working on it during the latter part of the 16’s century but Galileo is the most well-known and he was reported to have built a device that showed changes in temperature sometime around 1592. This appears to have used the contraction of air in a vessel to draw up a column of water, the height of the column indicating the extent of cooling. However, this was strongly influenced by air pressure and was little more than a novelty.
The thermometer as we know it was invented in 1612 by Santorio Santorii (Italy). He sealed liquid inside a glass tube, observing how it moved up the tube as it expanded. A scale on the tube made it easier to see changes, but the system lacked precise units.

Scales and units

There are three major temperature scales used today. Fahrenheit is the standard used for measuring temperature in the United States, but most of the rest of the world uses Celsius. The Kelvin scale is commonly used in scientific applications.

  • The Fahrenheit scale (by German physicist Daniel Gabriel Fahrenheit around 1720), divided the freezing and boiling points of water into 180 degrees. It established 32 degrees was the freezing point of water and 212 degrees was the boiling point of water.
  • Swedish astronomer Anders Celsius came out with the Celsius scale (sometimes called the centigrade scale). It is divided into 100 degrees separating the boiling and freezing points of water.  The original scale set 0 as the boiling point of water and 100 as the freezing point, but he flipped it around soon after inventing the scale. The term Celsius was adopted in 1948 by an international conference on weights and measures and the scale is the preferred temperature gauge for scientific applications as well as most of the world outside of the United States.
  • Lord Kelvin of Scotland came up with his temperature gauge in 1848, known as the Kelvin scale. He based it on the idea of absolute temperature, a theoretical temperature at which all substances have no heat energy.  There are no negative numbers on the Kelvin scale, 0 K being the coldest temperature possible. (Absolute zero converts to – 273.15 °C and – 459.67 °F.) The units on the Kelvin scale are the same size as those of the Celsius scale, except that the Kelvin scale sets the lowest temperature at 0.

Today, temperature measurement scales are defined in a document titled International Temperature System 90, ITS-90.