“If two bodies are each in thermal equilibrium with a third body, they are also in thermal equilibrium with each other”. If we take two bodies ‘A’ and ‘B’, one hotter than the other, and bring them in to contact with each other, the heat energy will be transferred from body at higher temperature to the body at lower temperature and after some time when there is no further heat transferred between them, then the bodies are said to be in thermal equilibrium with each other now if a third body ‘C’ is brought in contact with the two bodies (let us take thermometer as a third body).
Now suppose there is no change in mercury level of the thermometer (body ‘C’) then we can say that the bodies ‘A’ and ‘B’ are each in thermal equilibrium with the third body ‘C’ (thermometer).
Now suppose there is no change in mercury level of the thermometer (body ‘C’) then we can say that the bodies ‘A’ and ‘B’ are each in thermal equilibrium with the third body ‘C’ (thermometer).