Four Laws That - Drive The Universe By Peter Atkins -.pdf-

The Zeroth Law, introduced by Ralph Fowler in 1931, states that if two systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This law allows us to define a temperature scale, such as the Celsius or Kelvin scale, which measures the thermal energy of a system. The Zeroth Law provides a fundamental concept in thermodynamics, enabling us to compare the temperatures of different systems and understand the concept of thermal equilibrium.