English: A galena semiconductor crystal detector for microwaves invented by Indian physicist Jagadish Chandra Bose during his landmark millimeter wave optics experiments 1894-1900. It consists of a tiny crystal of the semiconducting mineral galena (lead sulfide, PbS) (4) with a metal point (5) making electrical contact with it, at the focus of a lens (13) which focuses microwaves on it. A battery (18) passes direct current through the contact and a galvanometer (19) measures the current. When microwaves strike the crystal they reduce the resistivity of the contact, and the galvanometer registers an increase in current. The pressure of the point can be adjusted with a thumbscrew (7). Bose used it to detect microwaves in the frequency range from 12 to 60 GHz generated by a spark transmitter in the first historic experiments with millimeter waves, in which he duplicated classical optics experiments, demonstrating refraction, diffraction, polarization, and standing waves, proving that microwaves are electromagnetic waves predicted by James Clerk Maxwell in 1873. The detector is deliberately built to resemble and function similarly to a human eye; due to Maxwell's theory, physicists of this era regarded radio waves as "invisible light". Bose called his detector an artificial retina. This was the first crystal detector (semiconductor diode) and is often considered the first patent on a semiconductor device.

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