An electromagnet is a type of man-made magnet. Though it is made out of materials that are not themselves magnetic, when power is applied to its circuit, it acts just like a natural magnet, except it can be turned on and off. An electromagnet is essentially just a battery attached to a coil of wire wrapped around a metal core. The metal most often used for this is iron.
The Discovery of the Electromagnetic Effect
In 1820, the Danish physicist Hans Christian Oersted made an important discovery. While conducting electricity experiments in his lab, he discovered electric currents were capable of deflecting a compass needle. This meant in some way the electric current was generating a magnetic field. This ability of electrical currents to produce magnetic fields resulted in many technological innovations, including the electromagnet.
The Source of Natural Magnetism
All matter is made of atoms. All atoms are made of protons, neutrons and electrons. The protons and neutrons are bunched together in the center of the atoms, with the electrons surrounding them. As these electrons spin and move around their atoms, they produce tiny magnetic fields, because moving electrons are electrical current.
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In most materials these tiny magnetic fields, called “domains,” point every which way and cancel out each other's effects. In some materials, like magnetite, these domains naturally line up and work with each other instead. This makes the whole object one big magnet, with a large magnetic field. Other materials can be magnetized. When an external magnetic field is applied to them, their magnetic domains line up, and they can sometimes keep them lined up even when the external field is removed. Iron is one of these substances.
Every material responds to magnetic fields differently. A measure of this response is an object's “relative permeability.” This number is the ratio of the magnetic field induced in an object to the magnetic field applied to the object. The higher this ratio, the more magnetizable a substance is. If a substance is nonmagnetic, the relative permeability is 1. This means the magnetic field inside the substance is just the same as the field applied to it: the substance adds no magnetic field of its own. The higher the permeability, the more powerful the field inside the substance is compared to the field applied to it.
The relative permeability of iron can be as high as 200,000 if it is pure enough. This is hundreds or even thousands of times higher than most other metals, though specialized scientifically-created alloys have a higher permeability. This is why iron is almost always used for electromagnet cores. When the current in the wire generates its magnetic field, it also induces a field in the iron, which makes the magnetic field of the wire thousands of times stronger.