= Superconducting carrier density from London screening
{title2=$n_s=m/(\mu_0e^2\lambda_L^2)$}
In a local <London equation> description, the measured <London penetration depth> satisfies $\lambda_L^2=m/(\mu_0n_se^2)$ when $n_s$ counts superconducting <Electrons> and $m$ is their <effective mass>. Thus screening measures $n_s/m$. If instead one counts <Cooper pairs>, use pair <mass> $2m$ and pair <electric charge> $-2e$, giving pair density $n_s/2$ for the same screening length. Identifying this superconducting carrier density with the total conduction-electron density requires the additional assumption that the relevant carriers all participate in screening at the measurement <temperature>.
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