1. R.B. Scott, Cryogenic Engineering, Chap, 8, D. Yam Nostrand Company, Inc., Princeton. N.J. (1959).
2. G.K. Whie, Experimental Techniques in Low-Temperature Physics, Chap. 2, Orford University Press, London (1959).
3. R. C. Mathewson, Rev, Sci, Instr., Vol. 26, 616 (1955).