1. W. E. Gifford and H. O. McMahon, “A Low Temperature Heat Pump,” in: Proceedings of the Tenth International Congress of Refrigeration, Vol. 1, Copenhagen, Denmark (August 1959).
2. W. E. Gifford and T. E. Hoffman, in: Advances in Cryogenic Engineering, Vol. 6, Plenum Press, New York (1961), p. 82.
3. W. E. Gifford, in: Progress in Cryogenics, Vol. III, Heywood and Co., London (1961), p. 49.
4. W. E. Gifford, U.S. Patent 2,966,035.
5. W. E. Gifford, U.S. Patent 3,119,237.