Wind power--what's generated by earth's magnetic field?

Part of a wind power generating device consists of metal parts (a conductor) moving through the earth's magnetic field. By doing so, as I recall, an electric current is generated in the conductor. Now I'm sure no such device captures this current, since it must be minuscule. My question is, just how much power would this add up to if it could be captured? For number of wind power generators, I'm thinking that it would be the number of such devices currently online in the U.S.