The casing placed around the ferrite (ceramic) magnetic pot focuses the strength of the magnets to the exposed surface thus providing a much higher holding strength than a same sized magnet without the steel cup. The magnetic circuit closes through the holding surface so the magnetic field leak is minimized. The steel cup provides a protection on the ferrite (ceramic) magnet which increases the lifespan of the pot magnet.
The magnetic pot is also known as cup magnet. Ferrite pot magnets are great for science projects, craft projects & are a ceramic ring magnet in an attractive protected casing with all of its magnetic energy focused in one direction.