This law was formulated after the enunciation of the first law of thermodynamics. But, since this law was considered to be of primary importance, it was called zeroth law.
The law states as follows:
If body 'A' is in equilibrium with body 'C' and body 'B' is also in equilibrium with body
'C', then bodies 'A' and 'B' are in equilibrium with each other.
The common use of thermometer in comparing temperatures of any two or more systems is based on this principle. The thermometer may be likened to the body 'C' in the above statement. While comparing temperatures of two bodies say 'A' and 'B', it is allowed to come into equilibrium first with the body 'A' and then with the body 'B', by placing it in contact with each other, turn by turn. The temperatures as read from the thermometer give comparative ideas of degrees of hotness of the two bodies 'A' and 'B' with which it had been allowed to come to equilibrium, one by one. Since thermometer is only a very small body, there is only an insignificant exchange of energy with large bodies 'B' and 'C'. Hence, the energies of bodies 'B' and 'C' remain unchanged during measurement of the temperature.