núllulögmálið

Núllulögmálið, also known as the Zeroth Law of Thermodynamics, is a fundamental principle that underpins the concept of temperature. It states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. Thermal equilibrium means that there is no net flow of heat between the systems when they are in contact.

This law is crucial because it establishes the existence of temperature as a measurable property. Before the Zeroth Law was formally recognized, scientists had already developed thermometers. However, the Zeroth Law provides the theoretical justification for why thermometers work. A thermometer, acting as the third system, can be used to measure the temperature of another system by establishing thermal equilibrium. If the thermometer shows the same reading when in contact with system A and system B, then systems A and B must have the same temperature.

While seemingly intuitive, the Zeroth Law was one of the last thermodynamic laws to be articulated. It was proposed by Ralph H. Fowler in 1931, after the first, second, and third laws had already been established. Its foundational nature made it logical to assign it the "zeroth" position, preceding the others chronologically in its conceptual importance. It is essential for defining and measuring temperature consistently across different objects and substances.