Quick The zeroth law of thermodynamics states that when two bodies have equality of temperature with a third body, they in turn have equality of temperature with each other. This statement seems quite obvious, but it is the zeroth law.
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Consider two blocks of copper and a thermometer. The first block of copper is brought into contact with the thermometer until equality of temperature is established, then it is removed. The second block of copper is then brought into contact with the thermometer, except that no change in the mercury level of the thermometer occurs during this operation. It can then be said that both blocks are in thermal equilibrium with the given thermometer.
Stated alternatively, if two bodies A and C are each in thermal equilibrium with another body B, A is also in thermal equilibrium with C. This law is really the basis of temperature measurement. Every time a body has equality of temperature with the thermometer, it can be said that the body is at the temperature that is read on the thermometer. The problem remains of how to relate temperatures that might be read on different mercury thermometers or obtain from different temperature-measuring devices, such as thermocouples and resistance thermometers. This observation implies the significance for the standard scales for temperature measurements.