Marconi Induction Coil, circa 1910

An induction coil creates a high-voltage, pulsating current which arcs across a "spark gap." The spark creates electromagnetic waves, which can then be detected by a radio receiver. Induction coils were essential to the first decades of wireless and radio. Used from the late 1880s until the 1920s, spark gap powered radios were replaced as vacuum tube transmitters became available.