## Temperature and temperature units

What is temperature?

Temperature is an intensive quantity and describes the energy state of the matter. All materials have atoms and molecules that are constantly moving, vibrating or rotating. The temperature of an object can be defined by the average kinetic energy of its atoms and molecules.

Kelvin (K)

Kelvin is the base unit of temperature in the SI system (International System of Units). Kelvin unit’s abbreviation is K (no degree or degree sign). Kelvin unit was first presented by William Thomson (Lord Kelvin) in 1848. Kelvin is currently defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, absolute zero point being 0 K.

Celsius (°C)

Celsius is currently a derived unit for temperature in the SI system, Kelvin being the base unit. The unit and the actual Celsius scale was first presented by a Swede Andreas Celsius in 1742. The two main reference points of the Celsius scale were the freezing point of water (or melting point of ice) being defined as 0 °C and the boiling point of water being 100 °C.

Fahrenheit (°F)

Fahrenheit scale was first introduced by a Dutch Gabriel Fahrenheit in 1724.  The two main reference points of the scale are the freezing point of water being specified as 32°F and the temperature of human body being 96°F. Nowadays the Fahrenheit scale is redefined in a way that the melting point of ice is exactly 32 °F and the boiling point of water exactly 212 °F. The temperature of the human body is about 98 °F on the revised scale.

Rankine (°R, °Ra)

Rankine scale was presented by a Scottish William Rankine in 1859. The reference point of Rankine scale is absolute zero point being 0 °Ra, like in Kelvin scale. One Rankine degree is the same size as one Fahrenheit degree. So the freezing point of water equals 491.67 °Ra.

Réaumur (°Ré, °Re)

Réaumur scales was introduced by Réne de Réaumur in 1730. It has the reference points being the freezing points of water 0 °Re and boiling point of water being 80 °Re.

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