English: Four ball Righi spark gap, invented by physicist Augusto Righi in 1894 for use in spark-gap radio transmitters. Righi was one of the first scientists to investigate microwaves, generating waves up to 12 GHz (2.5 cm wavelength) using resonator ball spark gaps like this. In use the outer balls were connected to the terminals of a induction coil which applied pulses of high voltage electricity, causing sparks to jump across the 3 spark gaps. The concept was that the outer balls were "charging" balls, which charged the two inner balls with opposite voltage, and the spark discharge between the inner "resonator" balls generated standing waves of current in the metal balls, which radiated the energy as microwaves. The two inner balls were often enclosed in a container of vaseline to allow the balls to be located closer together, to minimize the length of spark and thus the thermal energy lost during oscillation. Righi was the mentor of Guglielmo Marconi who designed the first practical radio communication systems. Marconi used Righi spark gaps like this in his first transmitters.