Since Galileo Galilei's invention of the thermometer in 1592, some 32 different scales have been used to represent temperatures. This entry is a brief introduction to some of today's most commonly used scales and how they came into being.

Fahrenheit (°F)

Daniel Gabriel Fahrenheit 1686 - 1736

Fahrenheit, although born in Germany, spent most of his adult life in The Netherlands. By profession he was a physicist - working in the field of meteorological instrumentation. He is credited with the invention of both the alcohol and mercury thermometers (in 1709 and 1714 respectively). Although it wasn't until over 150 years later that the mercury thermometer was first used for what is now its most common usage - an aid to medical diagnosis. This could have had something to do with the fact that the first medical thermometers were still over 10cm in length and took over five minutes to register a temperature.

His original scale was based upon three fixed temperatures:

• 0° - freezing point of a solution of 1 part (by mass) salt in 1 part water

• 30° - freezing point of water (later revised to 32°)

• 90° - temperature of the human body (later adjusted to 96° and finally to today's figure of 98.2°)

This resulted in a scale where the difference between the freezing and boiling points of water was divided into 180 equal parts.

The scale was widely used throughout the English speaking world until the 1970s but since then has gradually been replaced in official usage by the Celsius scale, with one exception: Fahrenheit continues to be used in both the USA (officially) and the UK (unofficially) when discussing atmospheric temperatures.

Centigrade/Celsius (°C)

Anders Celsius 1701 - 1744

Celsius, a Swede, was Professor of Astronomy in Uppsala University, Sweden from 1730 until his death in 1744.

During this time Anders was actively involved in a great deal of practical astronomical research: Taking part in an expedition to Lapland in 1736 to verify Sir Isaac Newton's theory that the world was not actually a perfect sphere. He was also the first person to connect changes in the Earth's magnetic field to the phenomenon known as the Aurora Borealis.

Sadly, he died of tuberculosis at the age of just 42.

Two years before his death he published details for a new temperature scale; to be known as the centigrade scale.

It was based upon two pre-set temperatures:

• 100° being the freezing point of water.

• 0° being the boiling point of water at standard atmospheric pressure.

These two figures were reversed after his death - leading to the current system.

The fact that the difference between the two points was broken up into 100 degrees gave the scale its original name: the Centigrade Scale - this was finally changed to honour the memory of the astronomer in 1948.

The Celsius scale has now replaced Fahrenheit throughout most of the world.

Other Current Temperature Scales

Kelvin (K)

In 1848, the Scottish physicist, William Thomson (later known as Lord Kelvin) introduced a new temperature scale. Based upon the Celsius scale, but with one important difference: The zero point for his scale was set at Absolute Zero (-273.15°C), meaning that 0°C/32°F is equivalent to 273.15K. This scale is widely used for astronomical and scientific work.

Note: Kelvin is the only temperature scale to be expressed without using 'degrees':

• 70°F - Seventy degrees Fahrenheit
• 5°C - Five degrees Celsius
• 300K - Three hundred kelvin

Kelvin is also the only temperature unit that can be used with standard numerical prefixes (micro, kilo, mega, giga, etc) and is the agreed SI unit of measurement for temperature.

The Rankine Scale (°R)

The 'American' equivalent of the Kelvin Scale. The zero point again being Absolute Zero, however, this time the scale is based upon the Fahrenheit system, meaning that O°C/32°F is now equivalent to 491.67°R.

Conversion Methods

Fahrenheit to Celsius

Celsius to Fahrenheit

An Aide-memoire to this Formula

One Researcher's method of remembering this formula is simply drawing a pair of thermometers side-by-side on a piece of paper, Make 0 on the Celsius scale align with 32 on the Fahrenheit scale. And 100 with the 212. You have 0-100°C corresponding to 32-212 (or 180) °F. That's a ratio of 100/180 or 5/9. Then you just have to figure out whether to subtract the 32 first or add it on after doing the division.

A Few Useful Temperature Equivalents