Faraday's induction coil. This is an iron ring, around which are wounds two insulated coils of copper wire on each side. It was made by Michael Faraday in 1831. Passing a current through one wire induces a voltage in the other by a process called mutual inductance. This is because the initial current creates a magnetic field in the ring. A change in magnetic field within the second coil creates a transient current - a burst when the initial current is switched on and another when it is switched off. This coil is now on display at the Royal Institution, London.