1. G. Busch, R. Jaggi undP. Braunschweig, Helv. Phys. Acta26, 392 (1953).
2. W.van Roberts, Phys. Rev.24, 532 (1924).
3. E. van Everdingen, Comm. Leiden53 (1899);58 (1900).
4. H. Kamerlingh Onnes undB. Beckman, Comm. Leiden129 (1912);132 (1912).
5. C. W. Heaps, Phys. Rev.29, 332 (1924);30, 61 (1927).