You may have only seen horseshoe magnets in cartoons, but this invention was pivotal to the development of many modern devices, especially in telecommunications.
William Sturgeon created the first horseshoe magnet in 1825, shortly after it was discovered that an electric current passing through a conductor caused nearby compass needles to move.
By wrapping a U-shaped piece of iron in copper wire and passing a current through it, he had created the first electromagnet—a ubiquitous device today—and also the first magnet that could lift more than its own mass. Sturgeon’s 7 oz. magnet could lift 20 times its weight.
The shape of these magnets allows the north and south poles to be much closer together, which results in a highly concentrated magnetic field. This concentration makes the magnetic force stronger, which also keeps them magnetized much longer. Despite their name, most permanent magnets are prone to demagnetization over time due to the influence of external magnetic fields.
The metal bar connecting the poles, called a magnet keeper, further reduces demagnetization by providing a conductive path for the field to travel through when not in use.
Shown in the last photo with magnetic viewing film, available here.