Hans Christian Oersted predicted in 1813 that a connection would be found between electricity and magnetism. In 1819 he placed a compass near a current-carrying wire and observed that the compass needle was deflected. This discovery demonstrated that electric currents produce magnetic fields. As shown here, the magnetic field lines circle around the current-carrying wire.

Iron filings arrange themselves along the lines of a magnetic force, making the lines visible. Magnets are surrounded by a magnetic field, and the magnetic lines of force run from one pole of the magnet to the other.

Liquid oxygen becomes trapped in an electromagnet’s magnetic field because oxygen (O₂) is paramagnetic. Oxygen has two unpaired electrons whose magnetic moments align with external magnetic field lines. When this occurs, the O₂ molecules themselves behave like tiny magnets, and become trapped between the poles of the electromagnet.