This means that when two bodies have equality of temperature separately with the third body, they in turn will have equality of temperature with each other.
Application
This law is used in comparing the temperature of any two or more systems by the common use of thermometer. For example, in order to compare temperature of two bodies X and Y, the thermometer (analogous to body C) is allowed to come into thermal equilibrium first with body X and then with body Y by placing it in contact with X and Y turn by turn. The temperature readings on thermometer give the comparative ideas of degree of hotness of two bodies X and Y. As the thermometer is a small body compared to X and Y the exchange of energy between X and C or between Y and C is insignificant and individual energies of body X and Y remain unchanged during temperature measurement.
