Horseshoe magnet

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Horseshoe magnet with computed magnetic field lines. The two magnetic poles are in close vicinity, which concentrates the field lines and creates a strong magnetic field.

Magnetic fields of a horseshoe magnet visualized using iron filings.

A horseshoe magnet is a magnet made in the shape of a horseshoe or a U shape and has become the most widely recognized symbol for magnets. It was invented by William Sturgeon in 1825. This type of magnet can be either a permanent magnet or an electromagnet. The main advantage of a horseshoe magnet over other types of magnets is that the magnetic poles are close together creating a much stronger magnetic field.

History[]

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]

Shape[]

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.

References[]

External links[]

• Media related to Horseshoe magnets at Wikimedia Commons