An excellent reference on "coherers."
A set of good pictures.
In 1890, Edouard Branly, Physics Professor at the Catholic University of Paris, (good reference) found that a nearby electromagnetic disturbance (spark) can lower the resistance of a thin layer of platinum deposited upon glass and he is, thus, credited as the inventor of the coherer wireless detector. The effect was at first attributed to the influence of the ultraviolet light of the spark. The variations in the resistance of metals in a finely divided state were even more striking, and they were shown by Dr. Branly to be due to the action of the Hertzian, waves of which the spark was the source.
However, in 1879 David E. Hughes (1831-1900), a British-born Professor of Music at the college in Bardstown Kentucky and inventor of the loose-contact carbon microphone, discovered that a tube of iron filings becomes conductive by action at distance by electrical sparks, he makes a signal audible on a headphone on a distance of 500 metres, he stopped his experiments when Sir George Stokes professed that the appearances concerned just ordinairy induction, he did not publish about his discovery. (reference)
In 1894, Oliver Joseph Lodge delivered before the Royal Institute a series of seminal lectures entitled "The Work of Hertz and Some of His Successors." In particular, he emphasized that Branly's powders were "The most astonishingly sensitive detector of Hertz waves" and coined the term "coherer." Lodge's device quickly became the standard detector in early wireless telegraph receivers. It was outmoded the following decade by magnetic, electrolytic, and crystal detectors. (excellent reference)
Professor A. S. Popov's Receiver (July 1892)
"…The date was May 7, 1895 and the occasion was a meeting of the Russian Physical and Chemical Society held in the (then) capital city of St. Petersburg. On this day, Alexander Popov presented a demonstration which would become recognized as an historic achievement. This demonstration, together with another by Popov which reportedly took place the following year, eventually would produce controversy among historians concerning whether the credit for "inventing" radio should be given to Marconi or to Popov.
"Those in attendance for Popov's May 7 presentation were very much impressed when he demonstrated a receiver which could detect the electromagnetic waves produced by lightning discharges in the atmosphere many miles away. The value this instrument could have in weather forecasting was obvious.
"Only seven years earlier, Heinrich Hertz had conducted laboratory experiments in Germany which demonstrated conclusively that the electromagnetic waves predicted by James Clerk Maxwell in 1865 actually do exist. Prior to Popov's work, however, few practical uses for these electromagnetic or "Hertzian" waves had been found.
"Popov's receiver consisted of a metal filings coherer he had developed as the detector element together with an antenna, a relay, and a bell. The relay was used to activate the bell which both announced the occurrence of a lightning discharge and served as a 'decoherer' (tapper) to ready the coherer to detect the next lightning discharge." (source)