Horseshoe magnet

In 1819, it was discovered that passing electric current through a piece of metal deflected a compass needle. Following this discovery, many other experiments surrounding magnetism were attempted. These experiments culminated in William Sturgeon wrapping wire around a horseshoe-shaped piece of iron and running electric current through the wires creating the first horseshoe magnet.[1]

This was also the first practical electromagnet and the first magnet that could lift more mass than the magnet itself when the seven-ounce magnet was able to lift nine pounds of iron.[1][2] Sturgeon showed that he could regulate the magnetic field of his horseshoe magnet by increasing or decreasing the amount of current being run through the wires.[2] This would lay the groundwork for development of the electrical telegraph and the future of world-wide telecommunications for the next century and more.[2]

The shape of the magnet was originally created as a replacement for the bar magnet as it makes the magnet stronger.[3] Over time it became the universal symbol for all magnets.[3] A horseshoe magnet is stronger because both poles of the magnet are closer to each other and in the same plane which allows the magnetic lines of flux to flow along a more direct path between the poles and concentrates the magnetic field.[4]

The shape of the horseshoe magnet also drastically reduces its demagnetization over time.[5] This is due to coercivity also known as the "staying magnetized" ability of a given magnet.[5] Coercivity is weaker in disc or ring shapes, slightly stronger in cylinder or bar shapes, and strongest in horseshoe shapes.[4][5] To increase the coercivity of horseshoe magnets, steel keepers or magnet keepers are used.[5] A magnetic field holds its strength best when the entire magnetic field is given the ability to loop through a ferromagnetic substance instead of air.[6] The nearness of the horseshoe magnet’s poles facilitates the ability to use these magnet keepers way more easily than other types of magnets.[6]

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  A horseshoe magnet made of AlNiCo, an iron alloy. The attached iron bar is a magnet keeper used to prevent demagnetization.
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  Magnetic field of a horseshoe magnet. The field is greatest where the lines are densest, around the poles (lower)
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  Alnico horseshoe magnet used in a magnetron tube in an early microwave oven. About 3 in (8 cm) long.
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  Assortment of AlNiCo horseshoe magnet shapes available from a manufacturer in 1956.
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  A rectangular horseshoe magnet.