It was indicated previously that an understanding of the behavior of the Fermi level is necessary to complete the models for ATP because it is the most important factor in Eqs 5-17, 5-18, 6-82, and 6-85. The discussion in Chapters 5 and 6 assumed that the thermoelements were in the ideal state; it was considered that the metals had perfect, isotropic crystal lattices and that they contained no impurities. Questions thus arise as to the effects of temperature, composition, stress, etc., upon the Fermi level of the metal being considered. These factors have important influences upon thermoelectric properties and upon the stability of thermoelements.