Imagine two systems \(A\) and \(B\), separated by an adiabatic wall, while each is in contact with a third system \(C\), vla a conducting wall [Fig. 12.2(a)]. The states of the systems (1.e., their macroscopic variables) will change until both \(A\) and \(B\) come to thermal equilibrium with \(C\). After this is achieved, suppose that the adiabatic wall between \(A\) and \(B\) is replaced by a conducting wall and \(C\) is insulated from \(A\) and \(B\) by an adiabatic wall [Fig. 12.2(b)]. It is found that the states of \(A\) and \(B\) change no further 1.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 1931 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.

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