Zeroth Law of Thermodynamics
If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other.
Formula / equation
T_A = T_C and T_B = T_C ⟹ T_A = T_B
Unit
K
In depth
This seemingly obvious statement is what makes temperature a meaningful, measurable property and makes thermometry possible: a thermometer reaches equilibrium with a body and then reports a value comparable with any other body. It was named the zeroth law because it was formalised after the first and second.
Examples in the real world
A thermometer reading 37 °C in two different patients establishes that they are at the same temperature.