Its a really nice idea though. Its stories like this that make me wish i paid a bit more attention in science when i was younger.
It would be trivial to put some circuitry in there to rectify the voltage so it works in either direction. No separate version required.
(changing ambient temperature is the obvious answer to my joke, btw)
Yes, I realize this was meant as a joke, but I'm still curious if current can be reversed by just changing the gradient. Does it still work but maybe it is less efficient? Is there no difference at all?