Magnetic switchable device

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.

A magnetic switchable device with a post attached, as used in the engineering metaltrades.
Base and magnet assembly. Shown in the inverted position.
Magnet in Off position. Iron blocks act as keepers.
Magnet in On position. Iron blocks act as extension of magnet.