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What is temperature?

The temperature of an object is a measure of the average energy of its molecules. This energy causes every molecule in the object to jiggle about, constantly colliding with and rebounding from its neighbours. The hotter an object, the faster its molecules are jiggling, and so the harder they collide with each other. With each collision, energy is shared between molecules - this is why temperature tends to even itself out over time in an object that is not being actively heated or cooled.

A glass thermometer contains a liquid, usually alcohol but occasionally mercury, in a narrow glass tube. When the liquid is warmed, its molecules vibrate faster, bouncing off each other harder, and so effectively taking up more space. This causes the liquid to expand and rise up the tube - by measuring how far it rises we can work out how much it has expanded, and so discover its temperature.

Why are there different temperature scales?

A form of the Celsius scale was proposed in 1742 by the Swedish astronomer Anders Celsius - curiously, he defined the boiling point of water to be 0° and the freezing point 100°, the numbers increasing with coldness rather than with heat. After the death of the astronomer, botanist Carolus Linnaeus reversed this scale for use on thermometers used to monitor the temperature in his greenhouses. Originally the scale was referred to as the centigrade scale, but that term (meaning simply 'one hundredth of a step') caused confusion in some European countries where it already had a different meaning, and so in 1948 the scale was officially named the Celsius scale.

The Fahrenheit temperature scale was first used in 1724 by its inventor, the physicist Daniel Gabriel Fahrenheit. He based his scale in part on an earlier scale, and initially his scale defined the freezing point of water as 30° and body temperature as 90°, and a freezing mixture of ice, water and ammonium chloride as 0°. However for practical reasons he redefined his scale so that there were 64 degrees between freezing and body temperature, since he could then graduate his thermometers by repeatedly halving an interval. This adjusted scale put freezing point at 32°, and body temperature at 96°. Using this scale, he discovered that water boiled at around 212°. In later years, it was decided to define the scale by setting the freezing and boiling points of water to be exactly 32°F and 212°F, with body temperature then being approximately 98.6°F as is accepted today.

This scale was used widely throughout the English-speaking world until the late 1960's when the Celsius scale largely replaced it. Some countries, notably the US, still cling to the Fahrenheit scale but the use of the Celsius scale in scientific and academic circles may eventually see it becoming a worldwide standard.

As mentioned above, temperature is a measure of molecular motion, and this motion decreases as the temperature falls. Logically, there should be a temperature at which all motion ceases, and it should be impossible for anything to be colder than this temperature. That point is known as Absolute Zero, and whilst it is in practice not possible to cool anything to exactly absolute zero, modern laboratories have reached temperatures astonishingly close to it, within a billionth of a degree. Since there is this physical limit, it makes sense in scientific circles to use it as the zero point of a new temperature scale, known as the Absolute or Kelvin scale. Temperature differences in Kelvin are the same as differences in Celsius, but the whole scale is shifted by 273.15° so that Absolute Zero is 0 Kelvin, the freezing point of water is 273.15 Kelvin, and the boiling point 373.15 Kelvin.

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