Temperature is how we measure how hot stuff is. According to a book I was reading recently,

The Farenheit scale was introduced in 1720 A.D.
The Centigrade or Celcius scale was introduced in 1742 A.D.
and the Kelvin scale (with Absolute Zero at the bottom) was introduced in 1854 A.D.

Though apparently there is some 'evidence' of an understanding of temperature comparison as early as 2000 B.C.

There are three temperature scales in common use today. The United States uses the Fahrenheit scale, nations that have adopted the metric scale use Celsius (sometimes called Centigrade), and the sciences use the Kelvin (or Absolute) scale.

All of these scales are related, and you can convert from one scale to another using temperature equations.

```
5
Celsius = --- * ( Fahrenheit - 32 )
9

9
Fahrenheit =  ( --- * Celsius ) + 32
5

Kelvin = Celsius + 273

```
Water boils at:

• 100° Celsius

• 212° Fahrenheit

• 373° Kelvin

Water freezes at:

• 0° Celsius

• 32° Fahrenheit

• 273° Kelvin

• There are also two less known temperature scales: Réaumur and Rankine:

French scientist René Antoine Ferchault de Réaumur (1683-1757) created his scale at about the same time Fahrenheit did, unaware of Fahrenheit's work. Réaumur used the freezing point of water as the zero mark for the scale and the set boiling point to 80 °R. The scale was used in some European countries for a while but was later replaced by the Celsius scale.

William Rankine (1820-1872) invented the Rankine scale which is an absolute scale like Kelvin but with steps the size of Fahrenheit degrees. (Rk = °F + 459.69)

Sources: Eric Weisstein's Treasure Trove of Scientific Biography (http://www.treasure-troves.com/bios/), Dictionary of Units (http://www.ex.ac.uk/cimt/dictunit/dictunit.htm)

Temperature is actually a rather subtle concept.

Currently there are two well established, consistent but independent definitions of temperature:

1. From classical thermodynamics: temperature is equal to the derivative of internal energy with respect to entropy.

2. From statistical mechanics: temperature is defined by the shape of the Maxwell-Boltzmann distribution for the system if it is a system with mass, and by the Planck distribution if it is a system without mass (i.e. a system of photons).

These two definitions are consistent, as it can be shown that for systems in which both are well defined, they are equal -- but both are not always well defined. For example, in plasmas, it is possible for the temperature of the system to be different for different directions (x,y,z)!

One rather interesting side effect of the coexistence of these two systems: if the system has an energy maximum (and a few other interesting properties), it may have a thermodynamically negative temperature.

One more note about the preceding discussion on this topic. According to the zeroth law of thermodynamics, there is an absolute temperature scale. That scale is the kelvin scale. Because the scale is not set by measuring reference points and then dividing the area between, kelvins are not measured by degrees. Temperatures given in units of kelvin are simply "kelvin", not "degrees kelvin", and there is no degree mark beside units of kelvins.

Tem"per*a*ture (?), n. [F. température, L. temperatura due measure, proportion, temper, temperament.]

1.

Constitution; state; degree of any quality.

The best composition and temperature is, to have openness in fame and opinion, secrecy in habit, dissimulation in seasonable use, and a power to feign, if there be no remedy.
Bacon.

Memory depends upon the consistence and the temperature of the brain.
I. Watts.

2.

Freedom from passion; moderation. [Obs.]

In that proud port, which her so goodly graceth,
Most goodly temperature you may descry.
Spenser.

3. (Physics)

Condition with respect to heat or cold, especially as indicated by the sensation produced, or by the thermometer or pyrometer; degree of heat or cold; as, the temperature of the air; high temperature; low temperature; temperature of freezing or of boiling.

4.

Mixture; compound. [Obs.]

Made a temperature of brass and iron together.
Holland.

Absolute temperature. (Physics) See under Absolute. --
Animal temperature (Physiol.), the nearly constant temperature maintained in the bodies of warm-blooded (homoiothermal) animals during life. The ultimate source of the heat is to be found in the potential energy of the food and the oxygen which is absorbed from the air during respiration. See Homoiothermal. --
Temperature sense (Physiol.), the faculty of perceiving cold and warmth, and so of perceiving differences of temperature in external objects. H. N. Martin.

Tem"per*a*ture, n. (Physiol. & Med.)

The degree of heat of the body of a living being, esp. of the human body; also (Colloq.), loosely, the excess of this over the normal (of the human body 98°-99.5° F., in the mouth of an adult about 98.4°).