The temperature is a property that indicates whether two objects would be in
‘thermal equilibrium’ if they were in contact through a diathermic boundary.
Thermal equilibrium is established if no change of state occurs when two objects A
to B are in contact through a diathermic boundary. Suppose an object A (which we
can think of as a block of iron) is in thermal equilibrium with an object B (a block
of copper), and that B is also in thermal equilibrium with another object C (a flask of
water). Then it has been found experimentally that A and C will also be in thermal
equilibrium when they are put in contact (Fig. 1.3). This observation is summarized
by the Zeroth Law of thermodynamics: