First of all, your going to need an electrode with solid sodium and iron electrode in NaOH and CuOH or something (if you want to check voltage and hook it up to a galvanometer or voltmeter)
In your example, I'm guessing that it is just an iron nail in NaOH solution. Look at the reaction
Oxidation:Na(s) -> Na^+ and an electron. Eox = 2.71v
Reduction: Fe^2+ and 2 electrons -> Fe(s) Ered = -0.44
Overall : 2Na(s) + Fe^2+ -> Fe(s) + 2Na^+(aq) Ecell = 2.27
See the reactants needed? You need sodium solid not aqueous. Reaction should work in an electrolytic cell since the reaction reverses.