In 1820, the Danish chemist and physicist Hans Christian Oersted performed a celebrated experiment demonstrating that an electric current passing through a wire could deflect a magnetic needle.

The experiment can be reproduced by means of the following apparatus. A wooden base carries a magnetic compass. A brass frame above the compass holds a thick copper wire with a small clip at each end. A pinion is used to raise and lower the wire above the compass needle to determine the relationship between its deflection and its distance from the wire, through which an electric current flows. Three leveling screws ensure that the system is perfectly horizontal.

After arranging the device so that the magnetic needle is parallel to the conducting wire, the two ends of the wire are connected to the electrodes of an array of batteries, and a current is sent through it. We observe that the needle deflects toward a position at a right angle to the direction of the wire. When the direction of the current flow is reversed, the needle still tends to deflect toward a perpendicular position relative to the wire, but this time its poles are reversed from the previous position.

In a similar experiment, Oersted demonstrated that a continuous electrical current flowing through a conductor generates a magnetic field. He was thus the first to prove the interdependence of electricity and magnetism.