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Summary The phenomenological theory of barium titanate given in an earlier paper has been extended, and expressions obtained for the piezoelectric constants, elastic coefficients for constant field and dielectric constants for constant strain in terms of other physical constants of the material. Curves are drawn showing these quantities as functions of temperature, and some comparison made with experiment. The relations of the constants of the ceramic to those of the single crystal are then briefly discussed.
A. F. Devonshire (Mon,) studied this question.