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Temperature

thermal equilibrium

We all have an intuitive notion of temperature via contact with objects. When we want to heat up our food, then we bring it in contact with a hot object, for example a flame. We put food in a refrigerator when we want to cool it down. Some of the properties of substances, called thermometric properties, change when their temperature changes. When objects are heated, they expand, when they are cooled, they shrink. When a gas is heated, its pressure will increase and it will decrease when the gas is cooled. Electrical conductivity can change with temperature.
When two objects are in thermal contact the hotter object transfers heat to the colder object. This can be observed by the changes of their thermometric properties. When no more changes are observed, the objects are in thermal equilibrium.
When a hot metal bar is in thermal contact with a cold metal bar, the hot bar will shrink, whereas the cold bar expands. Their temperatures are equal when no more shrinking or expanding is observed. That process can be used for thermometers.

thermometers