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There are three (3) temperature scales in use today, Fahrenheit, Celsius and Kelvin.  The Fahrenheit temperature scale is a scale based on 32º for the freezing point of water and 212º for the boiling point of water.  The interval between these two temperature points is 180º.  The 18th-century German physicist Daniel Gabriel Fahrenheit originally made 0º equal to the temperature of an equal amount ice-salt mixture and selected the values of 30º for the freezing point of water and 90º for normal body temperature.  These values were later revised to 32º and 96º.  The final scale version required an adjustment to 98.6º for normal body temperature.

The Fahrenheit temperature scale is still in common use in most English-speaking countries even though many countries use the Celsius (centigrade) scale.  The conversion formula for a temperature that is expressed on the Celsius (C) scale to its Fahrenheit (F) representation is:

The Celsius temperature scale is based on 0º for the freezing point of water and 100º for the boiling point of water.  Invented in 1742 by the Swedish astronomer Anders Celsius, it is sometimes called the centigrade scale because of the 100-degree interval between the defined points.  The Celsius scale is in general use wherever metric units have become accepted and is used in scientific work.  The following formula can be used to convert a temperature from its representation on the Fahrenheit (F) scale to the Celsius (C) value:

The Kelvin scale does not measure temperature; rather, it measures molecular activity.  The Kelvin is defined as 1/273.16 of the triple point of pure water (equilibrium among the solid, liquid, and gaseous phases.)  The Kelvin scale was named for the British physicist Baron Kelvin William Thomson.  The Kelvin has as its 0 point, absolute zero - which is the theoretical point at which the molecules of a substance have their lowest energy.  Since many physical laws and formulas can be expressed more simply when an absolute temperature scale is used, the Kelvin scale has been adopted as the international standard for scientific measurement.  The Kelvin scale is related to the Celsius scale in that the difference between the freezing and boiling points of water is 100 degrees in each scale; hence, the Kelvin scale has the same magnitude as the Celsius scale.  The following formula can be used to convert a temperature from its representation on the Celsius (C) scale to the Kelvin (K) scale: