In the wake of the considerable theoretical advances in electromagnetism, Michael Faraday discovered in 1831 a phenomenon whose applications had a scope still unsuspected at the time: electromagnetic induction.
Faraday‘s experiment calls for two coils – windings of conductive wire. The first coil is connected to a battery, which charges it with electric current. The second, larger coil is connected to a galvanometer, which measures its electricity level. Moving the first coil into the cavity of the second one will create a current in the latter, without the wires touching at any point. This is the phenomenon of induction: the magnetic field generated by the first coil generates an electric voltage in the second. More precisely, it is during the variations in this magnetic field that we observe an electric tension.
This principle was later applied to the design of electric motors, dynamos and electromagnets.