Fahrenheit-Celsius Converter

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What is temperature?

An object's temperature is a measure of the average energy of its molecules. This energy causes all the molecules in the object to move at random, constantly colliding with and rebounding from their neighbours. The hotter an object, the faster its molecules are movinging, and so the harder their collisions with each other. With every 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 glass capillary tube. When the liquid is warmed, its molecules vibrate faster, bouncing off each other harder, and conseqently taking up more space. This causes the liquid to expand and rise up inside the capillary tube - by measuring how far it rises we can work out how much the liquid has expanded, and so discover its temperature.

Why are there different temperature scales?

A form of the Celsius scale was suggested by the Swedish astronomer Anders Celsius in 1742 - strangely, he defined the boiling point of water to be 0° and the freezing point 100°, the numbers decreasing as the temperature got hotter. After the death of the astronomer, the botanist Carolus Linnaeus reversed this scale for use on the thermometers that monitored the temperature in his greenhouses. For a long time 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 renamed the Celsius scale.

The Fahrenheit temperature scale was first proposed in 1724 by the physicist Daniel Gabriel Fahrenheit. He partly based his scale on an earlier temperature scale, and initially his scale defined the freezing point of water as 30°, normal 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. Using this adjusted scale, freezing point was 32° and body temperature 96°. Using this scale, the boiling point of water was measured at around 212°. In later years, it was decided to precisely 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 around 98.6°F as is accepted today.

The Fahrenheit scale was used widely throughout the English-speaking world until the late 1960's when it was largely replaced by the Celsius scale. Some countries, notably the US, still cling to the old-fashioned Fahrenheit scale but the use of the Celsius 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 with falling temperature. Logically, there should be a temperature at which all motion ceases, and it will be impossible for anything to be colder than this temperature. That point is known as Absolute Zero, and although it is impossible in practice to cool anything to exactly absolute zero, modern laboratories can reach temperatures astoundingly close to it, within a billionth of a degree. Since there is this physical limit to temperature, it makes scientific sense 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.