https://patentimages.storage.googleapis.com/de/5d/14/a57ffad14ccd94/US600457.pdf
It is not free energy, but it's so close that you should stop complaining and just build one. It won't last forever and you will have to re-build it every couple of years and buy new wire.
Basically, you make an extremely high iductance coil where the windings themselves are the electrodes in a galvanic battery.
When you tap off of the outer iron to the inner copper or the outer copper to the inner iron, this lets current flow through the battery and due to the fact that it's wound as a soleloid it produces a magnetic field.
The galvanic reaction occurs over the entire length of the wires so the voltage is produced along the entire length of the wire. This means that you don't really observe a delay in power delivery because the votlage reaches saturation immediately even though it's a very high inductance coil.
The votlage you are provided at the taps is delivered to the user as if it was a battery.
Nathan Stublefield used this to transmit radio. He would control the current draw from the battery and the battery would act as an amplifying radio transmitter.
By wrapping a secondary winding around the battery, you can step-down the "effective voltage" that drives the solenoid-battery. The higher the switching frequency on the battery, the higher the "effective voltage" being applied to the solenoid part is.
Nathan used this phenomena to get actual useful power you can use to power you house or farm.
It's not infinite energy, it will wear out.
But it's a lot more energy than you would normally expect if you were only looking at the galvanic reactions.
The reason it's "optimal" isn't because it's robust it's optimal exclusively in terms of energy production per unit mass.
You also don't need copper, two dissimilar metals will do.
The large surface area of the wire allows for a lot of galvanic action to happen all at once, it's just not accessible at the terminals, you have to access it through the magnetic field.
It have relatively low energy per unit mass because windings are heavy.
Yes, but it could be even more expensive and there will be more losses, because only aluminium have similar to copper conductivity, but have little difference in potential with iron. So, you will need some other metal instead of iron and it will be more expensive than copper. Just check electrochemical potential of different metals.
Surface area in modern batteries is orders of magnitude larger, because of using porous materials for electrodes. So, again, this battery is much worse than modern ones.
Surface of electrodes determine maximum possible current battery could give out. Really it is not directly connected with total energy stored a battery.