File:Hertzian wave apparatus - Augusto Righi 1897.png

English: Microwave spectrometer apparatus built by Italian physicist Augusto Righi in 1894 with which he generated 12 GHz microwaves. Radio waves were called Hertzian waves at the time. Righi (along with Jagadish Chandra Bose) was the first person to generate microwaves. He used this apparatus to perform classic optics experiments demonstrating refraction, diffraction and polarization of these waves, providing confirmation of Maxwell's theory that light and radio waves were both electromagnetic waves, differing only in frequency.

The apparatus consists of a metal ball spark oscillator (F) at the focal point of a metal parabolic reflector (ACBD) to which pulses of high voltage (10-20 kV) were applied from an induction coil (not shown). The high voltage created sparks between the balls. Each spark excited standing wave electrical oscillations which caused the balls to radiate a brief pulse of microwaves, which were focused into a beam by the reflector. The microwaves were detected by a coherer (not shown) at the focus of the second parabolic reflector (L). The coherer consisted of a strip of glass with a thin metal coating evaporated onto its surface, divided into two equal conductors by a thin razor blade score. The two conductors acted as a dipole antenna. The microwaves excited standing waves in the antenna, causing microscopic sparks to jump across the razor blade cut. This was detected by a circuit with a battery and bell.

Righi put prisms and lenses of paraffin on the platform and showed that they caused the microwave beam to be bent by refraction. By rotating the receiving antenna he showed that the received waves were strongest when the antennas were parallel and zero when the antennas were perpendicular, demonstrating the polarization of microwaves.