Ok, it seems like a lot of websites and people who try to explain DC motors are unable to explain why an armature should be made out of metal. See if you follow my thinking. Current passes through the windings around the armature. That produces the e-field that fights the magnets that makes the whole thing turn. So what part does an armature made out of metal play? If anything, the e-field in the windings will create currents in the armature, and that in turn will create a back field that fights the direction-change of the current in the wire. If anything, it seems like a conductive armature would make a motor less efficient, not more efficient. In fact, they make armature-less motors. Most all small motors (think phone size and pager size) are armature-less. Give me a little help: explain why an armature needs to be conductive metal (other than for heat-transfer).