= Solution
Measure the reversible cell potential at several temperatures while maintaining standard-state thermodynamic activities and the same phases. From $\Delta_rG^\circ=-nFE^\circ$ and $\Delta_rS^\circ=-(\partial\Delta_rG^\circ/\partial T)_p$,
$$
\boxed{\Delta_rS^\circ=nF\left(\frac{\partial E^\circ}{\partial T}\right)_p}.
$$
The sign of the slope gives the sign of the standard reaction <entropy>. Measuring a loaded cell voltage without correcting polarization would not give this reversible thermodynamic derivative.
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