They are used in many applications including optics, metalworking, lifting, and robotics, to attach items to metal surfaces in a secure but temporary way.
[3] One type of magnetic switchable device is made from two blocks of iron, with a round cavity bored through the center.
The iron blocks act as keepers by bridging between both poles.
The field is effectively passing across an air gap (at the base and top).
If this gap is bridged with a piece of iron, it becomes part of the magnetic circuit and will be fixed to the structure.