If a hot^* body is in thermal contact with a cold body, energy is transferred from the hot body to the cold body. This is shown by the fact that the hot body "cools down" and the cold body "warms up".

Hot body: the cup of coffee

Cold "body": the air in the room, the table etc

If there is no other source of energy, the energy transfer eventually stops. At this stage we say that the two bodies have reached a state of thermal equilibrium. Note that energy only flows, unaided, from hot to cold.

Now consider a third body, which is first brought into contact with the cup of coffee (and allowed to come into thermal equilibrium with it) and is later allowed to reach a state of thermal equilibrium with the air in the room. If the third body does not change when moved from the coffee to the air it is reasonable to predict that the coffee and the air are in thermal equilibrium with each other.

This situation is considered in the zeroth^** law of thermodynamics, which is stated as follows.

As an alternative to the phrase "in thermal equilibrium", we can say that two bodies have the same temperature. Also, if energy flows unaided from body A to body B, then we say that body A is at a higher temperature than body B.

We can measure temperatures by using anything which experiences a measurable change in some physical property when energy flows into or out of it. Such a device is called a thermometer (and is the "third body" mentioned above).

There are many different types of thermometer, each having its own temperature scale. The physical properties used to define temperature scales include