Thermoelements are usually made from crystalline solids. They are not commonly single crystals, but their crystalline nature can be observed under a microscope. Sometimes, the transport properties vary with crystalline orientation but, until we deal with specific examples, we shall suppose that the properties are uniform in all directions.The transport of electric charge is due to quasi-free electrons in the solid. The solids of interest to us are metals and semiconductors. In such materials, the electrons carry not only the charge but also the thermal energy. In other words, there is an electronic component of the thermal conductivity. As we shall see later, heat can also be carried by the thermal vibrations of the atoms in a crystal but, for the moment, we confine ourselves to the electronic effects.The idea of conduction by electrons was proposed by Drude and Lorentz using the principles of classical physics. The classical free electron theory predicted that the specific heat should be much larger for a metal than for an electrical insulator but, in reality, there is very little difference. This discrepancy disappeared when Sommerfeld [1] took account of the newly developed quantum theory but neither the classical nor the quantum mechanical-free electron theories were able to explain why some solids are metallic conductors and others are insulators. It was only when notice was taken of the interaction of the electrons with the periodic potential that exists in a crystal lattice that further progress could be made. It was shown that, through this interaction, the energy of the electrons must lie in discrete bands that are separated by forbidden regions or energy gaps. Interestingly, Sommerfeld's theory can still be applied to the current-carrying electrons, if they are assigned an effective mass rather than the mass of a free electron.According to quantum theory, the probability that an electron state of energy, E, will be occupied is given by the Fermi distribution function f 0 .E/ D