"Magnet electromotor", after Christian Ernst Neeff (1839)

In 1831 Michael Faraday discovered that the change in a magnetic field can generate an electric current in a nearby conductor, for example a copper wire. In 1839, the German physicist Christian Ernst Neeff devised this apparatus, so that the electromagnetic field could also be studied in air at reduced pressure. It consists of a base with a coil and a contact breaker, on which a glass bell-jar fits. A coil behaves like a magnet as soon as a current runs through it. The current is regularly interrupted by means of the contact breaker. Each time, this causes an electric impulse, which discharges with a spark. Because of the high frequency of the contact breaker, the spark discharges take place so fast, that you cannot see them separately, but you see them as a continuous arc of light. When the air in the glass bell-jar is sucked away for a large part, this arc of light appears to become six to eight times as large.