Electrical inductance. Engraving of a public dem- onstration of an electrical induction coil during the 19th century. An induction coil consists of a primary coil of wire with only a few turns wound on an iron core, surrounded by a secondary coil with many turns. When an electrical current in the primary coil is interrupted, a large voltage is induced in the secondary coil. This is because the magnetic field generated by the electric current in the primary coil is changed. Image of an induction coil demonstrated by Professor Pepper at the London Polytechnic Institute, UK, in 1869. It was 3.3 metres (m) long and 0.6 m wide, and could produce sparks up to 76 centimetres long.