Celsius or Fahrenheit?

We all know that there are two serepate temperature scales that are commonly used around the world, the Fahrenheit and Celsius scale, but far fewer of us know the exact conversion between the two. To make things easy, we've put them below:

°C = (°F - 32) x ⁵/₉

°F = (°C x ⁹_/5) + 32

Fahrenheit Scale

Proposed by the German physicist Daniel Gabriel Fahrenheit in 1724, this scale takes the boiling point of water to be 212 °F and the freezing point as 32  °F, placing the freezing and boiling points of water exactly 180 degrees apart. The scale originally used the temperature of a mixture of ice, water and ammonium chloride as 0 °F, with the body temperature (found by placing the thermometer in the mouth or under his armpit) at 96 °F. Building on the work of Ole Rømer, Fahrenheit adjusted the scale so that the melting point of ice would be 32 degrees, so that 64 degrees would seperate that and body temperature (at 96 degrees). This allowed Fahrenheit to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).

Celsius Scale

The Celsius scale is traditionally defined as the freezing point of water as being 0 °C and the boiling point as being 100 °C at a pressure of one standard atmosphere. It was originally conceived by the Swedish astronomer Anders Celsius in 1742, albeit in a slightly different configuration. Celsius' original scale was the reverse of what we use today, with 100 °C being the freezing point of water and 0 °C being the freezing point. His reasoning was that water's boiling point varied with pressure, and as such the scale should be calibrated with 0 °C being the freezing point at sea level, at one standard atmosphere of pressure. Two years later, the same year as Celsius' death, Carolus Linnaeus (a Swedish botanist) effectively reversed the scale, creating the common scale we use today. Initially known as the Centigrade scale, this name was changed to the Celsius scale due to the fact that one 'centigrade' is also a unit of angular measurement in French and Spanish. The scale was formally renamed in 1948, although it has taken a while to become the sole name for this scale, with many communities, even some school textbooks, still referring to it as 'the Centigrade scale'.

 Kelvin Scale

The Kelvin scale is named after the British physicist and engineer, William Thompson, 1st Baron Kelvin, who wrote of the need for an 'absolute thermometric scale' (interestingly, Thompson's title was named after the River Kelvin, which flowed past his university in Glasgow). Zero degrees Kelvin (0K) relates to absolute zero, the point at which molecules within a body do not move, and therefore is the temperature at which nothing could be colder. It increases at the same increments as the Celsius scale, with the conversion rate being as follows:

°C = K - 273.15

K = °C + 273.15