File:Rivelatore di Righi con riflettore parabolico - Museo scienza tecnologia Milano 09831 1.jpg

English: A radio wave sensor invented by Italian physicist Augusto Righi in his 1894 microwave experiments. It consists of a cylindrical metal parabolic reflector mounted on an adjustable stand. Mounted along the focus of the reflector is a vertical strip of glass with a metal coating evaporated on it, which acted as a dipole receiving antenna. The reflector and antenna is mounted on an axle through a hole through the wooden support stand and can be rotated, with a pointer attached at the rear over a circular scale indicating the inclination of the antenna in degrees. The antenna can be rotated to various angles to measure the polarization of the radio waves.

English: FUNCTION
This was part of the apparatus used by Italian physicist Augusto Righi in his landmark 1894 Hertzian wave experiments. Righi (along with Jagadish Chandra Bose) was one of the first researchers to generate microwaves; he produced radio waves as short as 2.5 centimeters (a frequency of 12 GHz) with a metal ball spark oscillator powered by an induction coil. 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 electromagnetic theory that radio waves and light are both electromagnetic waves, differing only in frequency. This device is what he used to receive the microwaves. At the focus of the parabolic reflector is a strip of glass which had a layer of silver a few thousands of a millimeter thick evaporated onto it. 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. The mirror concentrated the microwaves on the antenna. By rotating the receiving antenna to various angles with respect to the transmitting antenna, he showed that the strongest response occurred when the antennas were parallel, while the received power got weaker as the angle between them got larger until no waves were received when the antennas were perpendicular. This showed that radio waves were polarized, like light.