The concept of temperature springs from the observation that a change in physical
state (for example, a change of volume) can occur when two objects are in contact
with one another, as when a red-hot metal is plunged into water. Later (Section 2.1)
we shall see that the change in state can be interpreted as arising from a flow of energy
as heat from one object to another. The temperature, T, is the property that indicates
the direction of the flow of energy through a thermally conducting, rigid wall. If
energy flows from A to B when they are in contact, then we say that A has a higher
temperature than B (Fig. 1.2).