English: Spark oscillator ball transmitter Oliver Lodge used to generate 1.2 GHz microwaves during an early demonstration of radio wave transmission in 1894. It consists of a 5 inch oscillator ball flanked on either side by smaller spark balls. When high voltage pulses from an induction coil were applied between the small balls, sparks jumped to the big ball, exciting standing waves of current in the big ball, which radiated microwaves. Lodge demonstrated it 1 June 1894 during a memorial lecture on the work of Heinrich Hertz before the British Association for the Advancement of Science. He transmitted microwave signals up to a distance of 180 feet (55 m), using a coherer to receive them. This was one of the first demonstrations of the generation of microwaves.

The idea behind this design was to narrow the bandwidth of the oscillator by isolating the resonator ball from the rest of the circuit, allowing it to oscillate freely. The brief high voltage sparks would charge opposite sides of the ball, exciting standing waves in the ball at its resonant frequency that persisted after the charging sparks ended, so the ball would radiate microwaves in a narrow band around the resonant frequency. The resonant frequency of the 5 inch (12.7 cm) ball was roughly 1.2 GHz. In conventional spark gaps, with the spark balls attached to the rest of the circuit, the oscillations only lasted as long as the spark did, a few oscillations, until the charge was dissipated. This created a wideband radio signal with power spread over the radio spectrum.