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.
Alterations to image: cropped out surrounding heading text and signatures.