The Zeroth Law of Thermodynamics states: If two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system, C, then A is also in thermal equilibrium with C.
This law was postulated in the 1930s, after the first and second laws of thermodynamics had been developed and named. It is called the "zeroth" law because it comes logically before the first and second laws (discussed in Atoms on the 1st and 2nd laws).
Two systems are in thermal equilibrium if they could transfer heat between each other, but don't. Indeed, experiments have shown that if two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system C, then A is also in thermal equilibrium with C. This conclusion may seem obvious, because all three have the same temperature, but zeroth law is basic to thermodynamics. Zeroth law justifies the use of thermodynamic temperature : the common "label" that the three systems in the definition above share is defined as the temperature of the systems.
Thermometer
A thermometer calibrated in degrees Celsius
Temperature
Thermometers actually take their own temperature, not the temperature of the object they are measuring. This raises the question of how we can be certain that a thermometer measures the temperature of the object with which it is in contact. The answer lies in the fact that any two systems placed in thermal contact (meaning heat transfer can occur between them) will reach the same temperature. That is, heat will flow from the hotter object to the cooler one until they reach exactly the same temperature. The objects are then in thermal equilibrium, and no further changes will occur. The systems interact and change because their temperatures differ, and the changes stop once their temperatures are the same. Thus, if enough time is allowed for this transfer of heat to run its course, the temperature a thermometer registers does represent the system with which it achieves thermal equilibrium.