Isotope Effect in Superconducting Lead

The present work is a continuation of earlier measurements of the difference in critical fields of superconducting Pb specimens with various isotopic masses. The isotope samples have been repurified and the low-temperature behavior is now more fully understood. The results near $T_c$ are consistent with the relation $T_c=\mathrm{const}\times M^p$, where $M$ is the average isotopic mass, and yield a value $p=-0.478\mathrm{}\pm 0.014$. The measurements at lower temperatures confirm the similarity principle to within approximately 0.1% and indicate that $\gamma$, the coefficient in the normal electronic specific heat, is independent of isotopic mass to a similar accuracy.