The Kelvin scale
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.