W. A. Wooster, N. Wooster, J.L. Rycroft and L.A. Thomas: J. Instn Elect. Engrs 94(1947) 927.
K. Aizu and T. Hirai: Reports in the Autumn 1969 Meeting of The Physical Society of Japan whose preliminary notes are given on p. 37 of The Lecture Notes 4 for the same meeting (part I in English and part II in Japanese).