  Thermoelectric effects in Kondo insulators are attracting interests because
of the emerging possibility of developping better thermoelectric materials for
a portable refrigerator without liquid coolant. In this article, the theory of
thermoelectric effects are reinvestigated for insulators or semiconductors at
low temperatures. It is found that the famous relations established by Lord
Kelvin for metals in 1851 must be modified for insulators in order to be
consistent with the third law of the thermodynamics. Effects of strong
correlation are discussed.
