Alright, as a chemist, I can't stand this anymore.

Iron corrodes by becoming the anode of an electrochemical cell. However if we connect the iron to a more easily oxidized metal such as zinc or Mg,, then this metal becomes the anode of a cell and it corrodes rather than the iron. The iron becomes the cathode at which oxygen is reduced to water.

For example, if a Mg or Zn rod is connected to an underground tank or steel pipe, then the Mg or Zn is oxidized instead of the pipe. The more easily oxidized metal is called a sacrificial anode.

Above paraphrased from "Chemistry" - Gillespie, Humphreys, Baird and Robinson. Page 620.

So, It doesn't matter if it thirty feet away or three, as long as there is path for electricity to follow - it works. Like someone else said - we've been using this on boats for years. I know my boat has Zn sacrificial anodes.

If you want, I can give a somewhat quick explanation of standard reduction potentials.

TimR