Absolute Temperature: Symbol: T , A temperature defined by the relationship:

T = θ + 273.15

where θ is the Celsius temperature. The absolute scale of temperature was a fundamental scale based on Charles’ law applied to an ideal gas:

V = V₀(1 + αθ)

where V is the volume at temperature θ, V₀ the volume at 0, and α the thermal expansivity of the gas. At low pressures (when real gases show ideal behavior) α has the value 1/273.15.

Therefore, at θ = –273.15 the volume of the gas theoretically becomes zero. In practice, of course, substances become solids at these temperatures.

However, the extrapolation can be used for a scale of temperature on which –273.15°C corresponds to 0° (absolute zero). The scale is also known as the ideal gas scale; on it temperature intervals were called degrees absolute (°A) or degrees Kelvin (°K), and were equal to the Celsius degree. It can be shown that the absolute temperature scale is identical to the thermodynamic temperature scale (on which the unit is the kelvin).