Zerotha

Meet Zerotha, three thermometers all reading the same temperature.

The zeroth law of thermodynamics says that if body A is in thermal equilibrium with body C, and body B is also in thermal equilibrium with C, then A and B are in equilibrium with each other. Thermal equilibrium means that when they touch, no net heat flows. It was named the zeroth law later because it was recognised as more basic than the first and second laws, which already had their numbers.

It is what makes thermometers work. Put a thermometer C into a cup of tea A and read 60 °C; put it in a second cup B and read 60 °C. Without touching the cups together, you know no heat will flow if they do.

The law justifies the very idea of temperature as a single number that all bodies in equilibrium share, and it underpins every temperature measurement, from medical thermometers to industrial sensors.

Equilibrium takes time. A thermometer must reach the temperature of whatever it measures before its reading means anything, and the law only applies to bodies that have settled.