The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. On the measurement scale of everyday life, a thermoelectric device creates a voltage when there is a different temperature on each side. Conversely (and, thermodynamically speaking, reversibly) when a voltage is applied to it, it creates a temperature difference. On the scale of atoms (specifically, charge carriers), an applied temperature difference causes charged carriers in the material, whether they are electrons or holes, to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence, the thermally-induced current.