1. American Society for Testing of Materials Publication, Vol. 3 (1949) p. 467.
2. J. F. CLEMENTS and J. VYSE,Trans. Brit. Ceram. Soc. 53 (1934) 134.
3. F. R. HIMSWORTH, ibid.56 (1957) 345.
4. F. KELEMEN,Angew. Z. Phys. 17 (1964) 562.
5. J. GATEEL and G. WEILL,J. Phys. Radium 23 (1962) 95.