In 1820 Hans Christian Oersted discovered that a magnetic needle deviates when it is near an electric current. Shortly afterwards, the French physicist Dominique François Jean Arago discovered that also a fast rotating disc made of non-magnetic material caused a deviation of a magnetic needle. This phenomenon can be demonstrated with this instrument. The disc rotates just beneath a glass plate on which a magnetised needle is placed. The position of the needle deviates from the magnetic meridian and above a certain speed of rotation the needle also rotates. Eventually, it was Michael Faraday who found an explanation for this phenomenon. The behaviour of the needle is a result of the fact that so-called induction currents (currents generated by a magnetic field) are formed in the disc. Arago's disc is applied, for example, in the drive of electric clocks, and the construction of electromotors.
