In unsteady or transient conduction the temperature of a system changes with time due to a thermal disturbance created within the system and/or the environment. For example, when a power transistor is suddenly energized, the heat generated within the transistor causes its temperature to increase with time. When a hot billet is withdrawn from a furnace and immersed in a coolant pool, it is the change in the environmental condition that causes the temperature of the billet to change with time.