1. S. C. Cowley, W. J. Timsoiij and J. A. Sawdye, in: Advances in Cryogenic Engineering, Vol. 7, Plenum Press, New York (1961), p. 385.
2. K. D. Timmerhaus, in: Applied Cryogenic Engineering, R. W. Vance and W. M. Duke (eds.), John Wiley and Sons, Inc., New York (1962), p. 131.