A button horseshoe magnet is a cylindrical horseshoe magnet with the two magnetic poles at one end of the cylinder, separated by an air gap.
A button horseshoe magnet can be used as an indicator base and tool holder, due to the clearance hole in the top. It is important for an indicator base to have a button magnet on the bottom to hold it in place on any ferromagnetic surface whilst it is being used to align different tools and work pieces. e.g. when inspecting a car engine.
It can also be used for holding pieces of ferromagnetic material together for sign making, if you do not have a weld clamp magnet.