File:Rivelatore di Righi con riflettore parabolico - Museo scienza tecnologia Milano 09831.jpg

English: Photo of the apparatus used by Italian physicist Augusto Righi in his landmark 1894 Hertzian wave experiments. Righi (along with Jagadish Chandra Bose) was the first researcher to generate microwaves; he produced radio waves as short as 2.5 centimeters (a frequency of 12 GHz) with the metal ball spark oscillator shown here. With this equipment he duplicated classic optics experiments with microwaves, using quasioptical components such as prisms and lenses of sulfur and paraffin and wire diffraction gratings to refract and diffract the microwaves, confirming the prediction of James Clerk Maxwell's 1873 electromagnetic theory that radio waves and light are both electromagnetic waves, differing only in frequency.

His 12 GHz microwave spark oscillator is on the left. It consisted of 4 metal balls at the focus of a metal parabolic reflector. When pulses of high voltage from the induction coil (far left) were applied, sparks between the balls caused oscillating standing waves in the center balls which produced pulses of microwaves, which were focused into a beam by the reflector. The two center resonator balls are immersed in a cylindrical container of vaseline oil to reduce the energy wasted in the spark. The bottom spark ball appears bent out of alignment. His microwave receiver shown at right consisted of a strip of glass which had a layer of silver a few thousands of a millimeter thick evaporated onto it, at the focus of a second parabolic reflector. The silver had a razor blade incision a few thousandths of a millimeter wide across its center, dividing the silvering into two conductive strips, which acted as a dipole antenna. When the microwaves struck it they created oscillating currents which caused tiny sparks to jump across the gap, which rang a bell. The reflector concentrated the microwaves on the antenna.