= Cross-sectional-area current
{title2=$J_\sigma$}
The <geometric cross-section> passing a fixed point of a steady orbital wire per unit time is $J_\sigma=\lambda_\sigma v$, where $\lambda_\sigma=d\sigma/ds$. Conservation of fragments requires $J_\sigma$ constant and $\lambda_\sigma\propto1/v$. For total area $\sigma_{\rm tot}$ and <orbital period> $P$,
$$
J_\sigma=\frac{\sigma_{\rm tot}}P.
$$
With the <fractional luminosity of a phase-mixed eccentric wire>, this becomes $J_\sigma=2fh$. The current describes transported area, not the swept area enclosed by the <orbit>.
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