How does the zeroth law of thermodynamics make thermometers possible?

The zeroth law of thermodynamics makes thermometers possible by establishing the principle of thermal equilibrium. The law states that if two bodies are each in thermal equilibrium with a third body, they must also be in equilibrium with each other.1

In this context, a thermometer acts as the "third body." When you place a thermometer in contact with an object, the thermometer and the object reach thermal equilibrium, meaning they are at the same temperature. Because the zeroth law guarantees that the thermometer will also be in equilibrium with any other object at that same temperature, it allows the thermometer to provide a consistent, objective measurement of temperature across different systems.123

By observing changes in the thermometer's physical properties—such as its volume or electrical resistance—scientists can assign numerical values to these states to define temperature scales, enabling accurate and reliable measurement.23