1. W.E. Ruder, U.S. Patent 1,110,010.
2. T.D. Yensen, Trans. AIEE, 43, pp. 145–175, 1924.
3. T.D. Yensen and N.A. Ziegler, Trans. ASM, 23, pp. 556–576, 1935.
4. T.D. Yensen and N.A. Ziegler, Trans. ASM, 24, pp. 337–358, 1936.
5. ASTM Standard A 596-69 (Reapproved 1979).