61. V o l u m e C h a n g e s i n H a r d S u p e r c o n d u c t o r s . H. ROHRER. InstituJc fiir Kalorische Apparate und K~ltetechnik, Swiss Federal Institute of Technology, Ziirich, Switzerland. The change in length on destruction of superconductivity by magnetic fields in the temperature range 1.5 to 4.2°K was investigated. An optical lever arrangement allowed changes in length of 1 part in l0 s to be observed with an accuracy of 2-10% on specimens 10 cm long. We have been able to improve the accuracy of our previously published measurements 1) 2), on Va and Ta, and have extended the measurements to La and Nb. The apparatus is described and the experimental results compared with recent theoretical work. 1) Olsen, J. L. and Rohrer, H., Helv. Phys. Acta 30 (1957) 49. 2) Mfiller, L., Olsen, J. L. and Rohrer, H. Proc. 5th int. Conf. Low Temp. Phys., Madison, 1957.
62. T h e effect of i m p u r i t i e s on t h e c r i t i c a l t e m p e r a t u r e of a l u m i n u m a n d i n d i u m . E. A. LYNTON and B. SERIN. Rutgers Univ., New Brunswick, N.J., U.S.A. Measurements of the critical temperature of both aluminum and indium alloys indicate a qualitative behavior strikingly similar to t h a t recently reported 1) for impure tin samples. For sufficiently small amounts of all solutes Tc again decreases, and we believe t h a t this is a repetition of the mean free path effect noticed in tin. W i t h increasing impurity Tc varies more complicatedly in a manner also resembling the previous results : the curves of Tc versus impurity concentration tend to fall into groups according to the valence difference between solvent and solute. F o r higher valence solutes Tc has a sharp upward trend; for those with lower valence the curves tend to flatten out. The similarity of these impurity effects for different solvents suggest float they are a fundamental property independent of the detailed nature of the superconductor. 1) Lynton, Serin and Zucker, J. Phys. Chem. Solids 3 (1957) 165.
63. S u p e r c o n d u c t i v i t y of V a n a d i u m A l l o y s . G. B u s c h and Federal Institute of Technology, Zurich, Switzerland.
Very pure samples of v a n a d i u m alloys containing small amounts of chromium, titanium and cobalt have been obtained by simultaneous thermal dissociation of the iodides. The superconducting transition temperatures and critical magnetic fields have been measured as a function of the concentration. In the chromium alloy series, the transition temperature shows a rapid decrease and vanishes at approximately eight per cent Cr. W i t h the addition of cobalt, the decrease of the transition temperature occurs at considerably lower solute concentrations. These results can be explained by considering the exchange interaction of the conduction electrons with the 3d electrons 1). Similarly, as already reported for pure V 2), in those alloys for which an accurate determination of the critical field was possible, this field snows deviations up to four or five per cent from the parabolic form. These deviations are in agreement with the theoretical prediction of B a r d e e n et al. l) Baltensperger, W., This conference, Short commun. No. 64, 2) Busch, G. and Mfiller, J., Helv. Phys. Acta 30 (1957) 230,