## Abstract

The changes with temperature of penetration of a magnetic field into superconducting tin and mercury were studied by a method due to Casimir in which a mutual inductance with a superconducting core is measured using low-frequency currents. The results were found to be very sensitive to surface conditions, but single crystals with smooth surfaces gave reproducible measurements of $\lambda $(T) - $\lambda $ (2$\cdot $17 degrees K) as a function of temperature T. These were consistent with the formula $\lambda $(T) = $\lambda _{0}$(1 - (T/T$_{c}$)$^{4}$)$^{-\frac{1}{2}}$, where T$_{c}$ is the transition temperature, and $\lambda _{0}$ was found to be 5$\cdot $2 $\times $ 10$^{-6}$ cm. for tin and 4$\cdot $3 $\times $ 10$^{-6}$ cm. for mercury. For tin there was no significant difference between the values of $\lambda _{0}$ for current flow in different crystal directions, though a difference of up to 20% is not excluded. For mercury there is a suggestion that $\lambda _{0}$ is about 20% higher for current flow perpendicular to the principal axis than it is for current flow parallel to the principal axis, but this difference is little more than might be due to experimental errors. There was no evidence for any dependence of $\lambda $ on a steady magnetic field H, though an increase of 10% up to 80% of the critical field is not excluded.