The Kelvin scale of temperature is an absolute scale; that is the lowest possible temperature has the value 0 K. It is also an equal-interval scale; that is, the temperature difference between 100 K and 150 K, for example, is the same as between 150 K and 200 K.

One cannot measure Kelvin temperature directly. The best one can do is to measure another quantity, such as the electrical resistance of a wire or the pressure of a gas at constant volume, that is proportional to Kelvin temperature. If such a quantity is called A and Kelvin temperature is called T, the relationship between A and T may be stated

A α T

It is therefore determined the temperature of a given body of matter by the following:

Ab / Atp = Bb / Btp

where Tb is the temperature of the body, Ttp is the triple point of water, Ab is the value of A at the triple point of water.

To find Tb, simple algebra must be applied:

Tb = Ttp * Ab / Atp

Ttp is 273.16 K. Therefore, Tb = (273.16 K) * Ab / Atp

The equation above may be used to measure the Kelvin temperature of any body, provided A can be measured both at the body's temperature and at the triple point of water.

The constant volume thermometer is often used to measure Kelvin temperature. The reading at a given temperature is given determined by the pressure of a gas at constant volume. If the pressure is kept very low, the thermometer reading is independent of the gas used, because all gases at low pressure behave much the same. They approximate what is termed an ideal gas; an ideal gas is one that behaves as if its molecules occupy no space and exert no mutual attraction. Since the gas in a constant volume thermometer is nearly ideal, this thermometer is said to use the ideal gas temperature scale.

Kelvin is an absolute temperature scale. It is named after British physicist William Thomson, Lord Kelvin (1824-1907).

If you are looking for in-depth information, consult the other nodes on Kelvin.

And now... why you don't say "degrees Kelvin." (other then "it pisses engineers off")

The units of the Kelvin temperature scale are kelvins.

That is it. Just kelvin. Not "degrees Kelvin."

Don't say "35 degrees Kelvin" just "35 kelvins."

The 13th CGPM (Gelfand Correspondence Program in Mathematics) in 1967 adopted the name kelvin (symbol K), thus defining the temperature unit and shunning the "degree Kelvin" forever. Explanations for the decision are hard to find, but I like the Dr. Math view on the matter:

"I have never found an actual explanation of the reason for this change. My guess is that they want it to look like any other SI unit. The various kinds of degrees are labeled as "degrees" (the unit itself) followed by "Celsius" or whatever (identifying the system whose degree and 0 point are being used). Since the kelvin is the standard in SI, I suppose they feel there should be no need for such a two-level name, which suggests that there might actually be rivals to the SI!" {}

Why all the mix ups? We have gotten used to saying "degrees" because the units of both the Celsius and Fahrenheit temperature scales are called the "degree". We say "degrees Celsius" and "degrees Fahrenheit" to distinguish between the two. Otherwise "35 degrees" would make little sense.

Furthar adding to the confusion, the two degrees", are not the same "size". In other words, one degree change in Fahrenheit is different from one degree change in Celsius. This leads to annoying temperature conversions

Incidentally, you DO use the degree when referring to the Rankine temperature scale (whose degree is the same size as a Fahrenheit degree... but no one seems to care.

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