1. R.H. March and O.G. Symko, IIR Bull. Annexe
1965(2), 57.
2. E.B. Osgood and J.M. Goodkind, Cryogenics
6, (1966).
3. Naval Research Laboratory Report No. 7133;PM Berglund,1970
4. R.G. Gylling, Acta Polytech. Scand., 81, 1 (1971);
5. P.M. Berglund, G.J. Ehnholm, R.G. Gylling, O.V. Lounasmaa, and R.P. Søvik, Cryogenics
12, 297 (1972).