Horseshoe magnet
Magnet in the shape of a horseshoe
From Wikipedia, the free encyclopedia
A horseshoe magnet is either a permanent magnet or an electromagnet made in the shape of a horseshoe (in other words, in a U-shape). The permanent kind has become the most widely recognized symbol for magnets.[1]: 2 It is usually depicted as red and marked with 'North' and 'South' poles.[1]: 3 Although rendered obsolete in the 1950s by squat, cylindrical magnets made of modern materials,[1]: 3,467 horseshoe magnets are still regularly shown in elementary school textbooks.[1]: 3 Historically, they were a solution to the problem of making a compact magnet that does not destroy itself in its own demagnetizing field.[1]: 2 [2]
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History
The first recorded instance of a horseshoe magnet was the invention of Daniel Bernoulli in 1743.[3][4]
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 practical electromagnet.[5]
This was 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.[5][6] 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.[6] This would lay the groundwork for development of the electrical telegraph and the future of world-wide telecommunications for the next century and more.[6]
Shape
The shape of the magnet was originally created as a replacement for the bar magnet as it makes the magnetic field stronger for a magnet of comparable strength.[7] 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.[8]
The shape of the horseshoe magnet also drastically reduces its demagnetization over time.[9] This is due to coercivity also known as the "staying magnetized" ability of a given magnet.[9] Coercivity is weaker in disc or ring shapes, slightly stronger in cylinder or bar shapes, and strongest in horseshoe shapes.[8][9] To increase the coercivity of horseshoe magnets, steel keepers or magnet keepers are used.[9] 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.[10] The nearness of the horseshoe magnet’s poles facilitates the ability to use these magnet keepers more easily than other types of magnets.[10]
A horseshoe magnet made of AlNiCo, an iron alloy. The attached iron bar is a magnet keeper used to prevent demagnetization.
Magnetic field of a horseshoe magnet. The field is greatest where the lines are densest, around the poles (lower)
Alnico horseshoe magnet used in a magnetron tube in an early microwave oven. About 3 in (8 cm) long.
Assortment of AlNiCo horseshoe magnet shapes available from a manufacturer in 1956.
A rectangular horseshoe magnet.
