From a tiny bit of experience, I know that industrial electromagnets (think of the ones they use to pick up steel at a scrap yard) often have an iron/steel hous

From a tiny bit of experience, I know that industrial electromagnets (think of the ones they use to pick up steel at a scrap yard) often have an iron/steel housing that is used as a "return path" for the field. The front face of the magnet has a field that projects out into the air, but the field on the back face gets channeled into the housing.