thermodynamic (Kelvin) scale of temperature: Symbol K. The efficiency of Carnot engine* is equal to,

e = (Q₁ - Q₂) / Q₁ = 1 - Q₂/Q₁

where Q₁ is the heat extracted from the reservoir and Q₂ is the heat rejected at the sink, by the engine. The ratio of Q₁ and Q₂ depends only on the temperature of the reservoir and the sink and nothing else. It was proposed by Lord Kelvin that if this fact is used in defining a temperature scale then it would be independent of the properties of any particular substance. The Kelvin temperature of the reservoir (T₁) and the sink (T₂) between which Carnot engine operates are defined by the relation,

Q₁ / Q₂ = T₁ / T₂

Further the scale is fixed by assigning arbitrarily the temperature of triple point* of water as 273.16 K, which is 0.01^oC in the Celsius scale. Therefore if T is the temperature in the Kelvin scale, the corresponding temperature in the Celsius scale is,

t^oC = T K - 273.15

The temperature 0 K or -273.15^oC is called `absolute zero', since this temperature is the lowest temperature theoretically possible.