Image: Bose 60 GHz microwave spark oscillator 1894

Description: Spark ball oscillator used in 1894 - 1900 by Jagadish Chandra Bose to generate 60 GHz (5 millimeter) microwaves during his historic electromagnetic wave experiments. It consists of a 3 millimeter metal ball on a nonconductive post, with two smaller spark balls on either side to apply high voltage. When pulses of high voltage from an induction coil were applied between the side electrodes, sparks jumped to the center ball, exciting standing waves of current in the ball, which radiated microwaves. The oscillator was mounted inside a shielded box, with a short waveguide on the side containing a lens made of sulfur which emitted a beam of microwaves. The concept behind this design was that isolating the center ball from the rest of the circuit allowed standing wave oscillations to build up in the ball, generating waves at the resonant frequency of the ball, which was around 60 GHz. In contrast an ordinary spark gap produced a very wide bandwidth of waves.
Usage Terms: Public domain