A high-frequency air-core transformer.
In 1892, Nicola Tesla used a form of induction coil, consisting of two magnetically coupled coils wound on an air core—a transformer—and now known as a Tesla coil, to obtain currents of very high frequencies and high voltages. The primary winding is made of a few turns of heavy wire, while the secondary consists of fine wire with a very large number of turns. By circulating a high-frequency current in the primary, very high voltage is induced in the secondary. The Tesla coil is used in radio and television receivers as well as for producing high discharge voltages.