  A simple relation is established between the zero-$T$ penetration depth
$\lambda (0)$ and the slope of $\lambda^{-2}(T)$ near $T_c$, similar to
Helfand-Werthamer's relation for $H_{c2}(0)$ and the slope of $H_{c2}(T)$ at
$T_c$ for the isotropic s-wave case with non-magnetic scattering.\cite{HW} When
the scattering parameter $\rho=\hbar v/2\pi T_c\ell$ ($v$ is the Fermi velocity
and $\ell$ is the mean-free path) varies from 1 to 10, the coefficient of
proportionality between $\lambda^{-2}(0)$ and $T_c (d\lambda^{-2}/dT)_{T_c}$
changes from 0.43 to 0.38. Combining this relation with the Rutgers
thermodynamic identity, one can express $\lambda (0)$ in terms of the slope
$(dH_{c2}/dT)_{T_c}$ and the density of states.
