1. H. Kamerlingh-Onnes, Communs Phys. Lab., Univ. Leiden, 120b, 122b (1911).
2. W. Meissner and R. Ochsenfeld, Naturwissenschaften, 21: 787 (1933).
3. B. Roberts, in: New Materials and Methods of Studying Metals and Alloys, Izd. Metallurgiya, Moscow (1966), p. 9.
4. B. T. Matthias, T. H. Geballe, and L. D. Longinott, Science, 156: 645 (1967).
5. H. Kamerlingh-Onnes, Communs Phys. Lab. Univ. Leiden, 133b, 134b (1913).