In the 19th century, surprising electrical properties were discovered in imperfect electrical connections between metals. A familiar example to the modern-day engineer is the intermittent faults caused by cold solder joints, other imperfect connections also include copper rods in loose contacts, loose springs, metal screws with a sharp tip, etc,. Initially, because the connection is loose, its electrical resistance is very high. But an one-time application of even a small voltage well under 1 V is enough to "fix it" temporarily and "turn it on" - causing its resistance to drop dramatically. Inversely, a mechanical shock loosens the connection again and resetting it to the initial high-resistance state.

Two oxidised copper rods lying in loose contact showed between them a resistance of 80,000 Ω, after the application of a voltage this fell to 7 Ω. Another early experimenter named Von Lang reported a change from infinity to 380 Ω. Many poor contacts exist in a loosely packed mass of metal filings, and these also exhibit a similar drop in resistance on application of a voltage.

Source: Early Radio Wave Detectors, Vivian J. Phillips, 1980, ISBN-13: 978-0906048245

In 1890, the French physicist Édouard Branly was investigating the same effect in devices containing metallic powders of various kinds, and found it also worked wirelessly. He confirmed that the electrical resistance of the powders often dramatically dropped by a factor of a million or so if an electrical spark was initiated in a nearby (but not directly wired together) spark-generating apparatus. The two devices were not directly connected electrically, nevertheless, the incoming radio wave can produce a voltage across the metal powders. Apparently, this tiny voltage is sufficient to make the powder 'cohere' (clump together), causing its resistance to drop. Branly called his device a “radio conductor".

In the beginning of 1900s, modified versions of his device - now dubbed 'coherers' - became the first generation of radio wave detectors and started the age of radio. They were in use in the very first long-range radio transmissions, in both early experiments and in later commercial service. For example, all early experiments and radio stations by Guglielmo Marconi used a coherer as the detector.

At the time, there was no accepted explanation of how or why these devices worked (including both "imperfect contact" and "metal powder" coherers). Several hypotheses were made, but none was universally accepted due to lack of experimental evidence. It's worth noting that there was no understanding at all of modern solid-state physics, and even the electron itself had not yet been discovered.

The first assumed that normally the filings or metal electrodes lie in loose contact with one another, but when a voltage is applied across them they experience electrostatic forces which cause them to come together and form chains. Each particle is now pressed into close contact with its neighbours thus establishing a good metal-to-metal contact. […] Others were very sceptical, arguing that it was impossible to conceive of the production of the large forces necessary to move filings around under the influence of the very small voltages to which the coherer was normally subjected.' Most coherers were, after all, able to operate quite successfully with voltages of only a few tenths of a volt.

The second theory to explain coherence was that the mechanical forces caused rupture of the very thin oxide film on the particles followed by welding together at the minute contact points thus created, a variant on this theme was that a tiny hole was formed in the insulating layer, followed by evaporation of a tiny amount of metal which coated the inside of the hole forming a conducting bridge across the insulation. The two theories mentioned so far are not incompatible in that breakdown of the insulation could occur after the formation of particles into chains.

The third theory was that the resistance drop was purely thermal in origin. Most metals have a positive coefficient of change of resistance with temperature — i.e. when you heat them their resistance rises. It was noticed that many of the metals which were satisfactory for use in coherers had oxides which possessed negative resistance coefficients. It was thought that localised heating at the points of contact led to a decrease in the resistance of the oxide film at those points. In certain cases this could lead to thermal runaway where decreasing resistance allowed more current to flow, which in turn decreased the resistance still further and so on, ending with a large flow of current which could weld the particles together.

Argument raged at the time as to which of these theories was correct, but other detectors came along and coherers fell into disuse before the matter could be finally resolved. There have been some modern investigations into the subject. Suffice it to say here that depending on the conditions in any particular case, and especially on the thickness of the oxide coating, all three mechanisms can play a part as the present author's own experiments have shown.

[ Source as above ]

This position remains largely the same today.

Although these electrical transport phenomena in metallic granular media were involved in the first wireless radio transmission near 1900, they still are not well understood. Several possible processes at the contact scale have been invoked without a clear verification: electrical breakdown of the oxide layers on grains, the modified tunnel effect through the metal-oxide/semiconductor-metal junction, the attraction of grains by molecular or electrostatic forces, and local welding of microcontacts by a Joule heating effect. A global process of percolation within the grain assembly also has been invoked"

Source : Electrical conductivity in granular media and Branly’s coherer: A simple experimentarXiv: Condensed Matter, 0407773v2, 2004

Although coherers no longer have any practical use, some researchers note that deep semiconductor physics, namely quantum tunneling, is possibly involved. Thomas Mark Cuff further speculated that a full understanding of the way they work might also shed light on the workings of some current-day devices, such as the Scanning Tunneling Microscopes (STMs), whose detector can be seen as a form of Metal-Oxide-Metal (MOM) diodes with unclear working mechanism.

As a result of the historical review, it became clear that the coherer evolved directly into the MOM (Metal-Oxide-Metal) ‘diode’ and, by only a slightly more circuitous route, it appeared as the forerunner to the STM (Scanning Tunneling Microscope). The MOM ‘diode’, besides being a progeny of the coherer, has something else in common with the coherer, no generally accepted explanation of how it works.

Source: Coherers, a review by Thomas Mark Cuff (master thesis), 1993.