An electromagnet, a magnet whose magnetic field is produced by the flow of electric current, works until the electric current ceases. The magnetic field in a simple electromagnet is created by a wire passing through it with an electric current. The strength of the magnet depends on the amount of current. By making the wire into a coil the magnetic field is concentrated. A straight tube coil is a solenoid. A stronger magnetic field can be produced by putting a ferromagnetic material, such as soft iron, inside the coil. This would be called a ferromagnetic core or iron-core electromagnet.
By controlling the amount of electric current electromagnets magnetic field can be rapidly manipulated over a wide range. This provides the electromagnet with an advantage over a permanent magnet. The core is made up of small regions called magnetic domains that act like tiny magnets. Before the current is turned on, the fields cancel each other out by pointing in random directions and therefore there is no large scale magnetic field. When a current activates the fields turn and align parallel to the magnetic field, thus adding there own magnetic fields to that of the wires. Larger currents allow more of the domains to align creating a stronger magnetic field. Once a certain amount of domains are lined up then further increases in current only cause slight increase in the magnetic field. This is called saturation.
Once the current is turned off most of the domains return to random; however, some remain aligned and cause the core to be a weak permanent magnet. This is called hysteresis and the remaining field is the remnant magnetism. Through degaussing the leftover magnetization can be removed.
Hans Christian Orsted discovered that electric currents create magnetic fields in 1820. In 1824, the electromagnet was invented. William Sturgeon was the first to create it and his first one was a horseshoe-shaped piece of iron that was wrapped with about 18 turns of bare copper wire. Once the iron in the magnet had a current pass through it, it attracted other pieces of iron. Although it only weighed seven ounces it could lift nine pounds when the current of a single cell-battery was applied. Sturgeon’s magnets were weak since the uninsulated wire he used could only be wrapped in a single spaced out layer around the core. Joseph Henry later used insulated wire, insulated with silk, to wrap multiple layers around the core creating much more powerful magnets, including one that could support 2063 pounds. Telegraph sounders were the first invention to make use of electromagnets.
Superconducting electromagnets are used when a magnetic field higher than the ferromagnetic limit of 1.6 T is needed. Superconducting windings cooled with liquid helium are used instead of ferromagnetic materials. The windings conduct current without electrical resistance. This allows an enormous current to flow which generates intense magnetic fields. Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting. Due to the need of refrigeration equipment and cryostat they are much more expensive than ordinary electromagnets. However, costs can be offset since after startup no power is required for the windings, since no energy is lost to ohmic heating. Superconducting magnets are used in particle accelerators, MRI machines, and research.