Definition: If two bodies A and B are separately in thermal equilibrium with a third body C, then A and B are also in thermal equilibrium with each other.
– This implies that it is meaningful to define a quantity (called temperature) that can be used to describe the thermal state of the body. If two bodies have the same temperature, they are in thermal equilibrium.
– In short, temperature is the hotness or coldness of a body.
– The difference in temperature is the driving force for heat flow, just as the difference of electrical potential is the driving force for electric current flow.
– This law was not thought to be important until the early 1920s, long after the first three laws were already widely known and named as such, hence the zero numbering.