## Abstract

Magnetic and electrical measurements have been made of the effect of impurity on the transitions to superconductivity in tin. Reproducible results were obtained only with well-annealed monocrystalline specimens. Solution of up to 6% indium in pure tin decreases the electronic mean free path l from about 3 $\times $ 10$^{-3}$ to 3 $\times $ 10$^{-6}$ cm, and over this range magnetic measurements show that there is only a small depression of the transition temperature T$_{c}$ and a small alteration in the critical field curve of H$_{c}$ and T. Electrical measurements show that if l > l$_{c}$, where l$_{c}$ = 8 $\times $ 10$^{-6}$ cm, the resistance transitions are sharp and almost concurrent with the magnetic transitions. However, if l < l$_{c}$ superconducting nucleation apparently occurs, since a state of partial superconductivity exists with zero resistance, but no exclusion of magnetic induction, in fields greater than H$_{c}$ but less than H$_{c}^{\prime}$, where it has been found that at any one temperature H$_{c}$/H$_{c}^{\prime}$ = l/l$_{c}$. This relation describes in broad outline the dependence of H$_{c}^{\prime}$ on l and temperature, although the interpretation of the results is complicated by considerable broadening of the resistance transitions and the appearance of a sensitive non-linear dependence on the measuring current of the temperature of nucleation. These complicating effects may wholly or partly be due to inhomogeneities in indium concentration. The concept of a range of coherence $\xi $ of the superconducting phase is used in formulating the thermodynamic conditions for the formation in a magnetic field of superconducting nuclei with cylindrical and spherical symmetry. It is shown that the main features of superconducting nucleation in homogeneous tin-indium alloys can be accounted for if $\xi $ = $\frac{2\lambda _{0}l}{l_{c}(1-t^{2})^{\frac{1}{2}}}$, where t = T/T$_{c}$ and $\lambda _{0}$ is the penetration depth at 0 degrees K. The implication that $\xi $ greatly exceeds l just below T$_{c}$ is supported by a consideration of the sharpness of resistance transition and the shape of the critical field curve near T$_{c}$. The formula for $\xi $ resembles that given in Pippard's phenomenological theory of superconductivity (1953).