A and B change no further i.e. they are found to be in thermal equilibrium with each other . This observation forms the basis of the Zeroth Law of Thermodynamics , which states that ‘two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other’ . R.H.
Fowler formulated this law in long after the first and second Laws of thermodynamics were stated and so numbered. The Zeroth Law clearly suggests that when two systems A and B, are in thermal equilibrium, there must be a physical quantity that has the same value for both. This thermodynamic variable whose value is equal for two systems in thermal equilibrium is called temperature ( T ). Thus, if A and B are separately in equilibrium with C , T A = T C and T B = T C .
This implies that T A = T B i.e. the systems A and B are also in thermal equilibrium. We have arrived at the concept of temperature formally via the Zeroth Law. The next question is : how to assign numerical values to temperatures of different bodies ?
In other words, how do we construct a scale of temperature ? Thermometry deals with this basic question to which we turn in the next section. Fig. .
(a) Systems A and B are separated by an adiabatic wall, while each is in contact with a third system C via a conducting wall. (b) The adiabatic wall between A and B is replaced by a conducting wall, while C is insulated from A and B by an adiabatic wall.