"[Henry's magnetic force] totally eclipses every other in the whole annals of magnetism."
William Sturgeon on Joseph Henry
Englishman William Sturgeon invented the simple electromagnet in 1825; five years later, American Joseph Henry (1797-1878) successfully improved its magnetic field by a factor of about 400.
The most important characteristic of modern electromagnets is the speed with which their magnetic fields can be manipulated. Basically an electromagnet is just a corkscrewlike coil of current- carrying wire—a solenoid. If a direct current is passed through the wire, the strength of the magnetic field is directly proportional to the amplitude of the current. A solenoid has only air in its core. If a stronger field is required, the core is filled with soft iron or any other suitable paramagnetic of ferromagnetic material. A disadvantage with this system is that remnant magnetism can persist after the electric current has been switched off. This has to be removed by applying a decreasing alternating current to the coil.
Direct-current electromagnets are used to pick up iron objects or hold switches in one position, on or off. Alternating-current electromagnets are used in scanning devices such as television tubes. Television tube yokes have two sets of electromagnets mounted perpendicular to each other. Current flowing through the coils controls the beam of electrons that are focused on the screen. This beam can be made to trace out a raster (scan pattern) of horizontal lines that are scanned from the top to the bottom of the screen.
Loudspeakers also rely on electromagnets. Here the central iron core is attached to the paper cone of the speaker. Alternating current through the coil causes the speaker to vibrate and emit sound.
