= Free-electron count from a Hall coefficient
{title2=$z=M/(e|R_H|\rho_mN_A)$}
In a one-carrier <Drude model>, the <Hall coefficient> is $R_H=1/(nq)$. Combining its inferred <electron density> with atomic density $\rho_mN_A/M$ gives the displayed number of mobile <electrons> per atom. The <molar mass> and <mass density> must be supplied independently. Multiple bands, anisotropic carrier motion and Hall factors limit this simple interpretation; field reversal helps distinguish the transverse response from voltage offsets.
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