Coherer

Classic "tapping" form of the "filings coherer".

A coherer is a type of radio detector that was used in the early days of radio development, beginning around 1890. Its defining characteristic is a "light-contact" segment, that is normally a poor electrical conductor, but which undergoes a sudden change in conductivity — usually a marked increase — in the presence of the increased voltage induced by a received radio signal.

Coherers, in conjunction with spark-gap transmitters, were the first devices to make radio communication practical. But, although hailed at the time as a "marvelous electric eye", they could not be used for full audio reception of radio signals, and were limited to receiving simple on-off transmissions such as the telegraphic dots-and-dashes of Morse code. In addition, their relative insensitivity and unreliability quickly led to their replacement by more sophisticated detectors, so the device that helped create a communications revolution rapidly disappeared from commercial use by around 1905.

Initial development

The basic phenomenon on which the coherer is based was discovered independently by a number of investigators prior to the development of radio. In the mid-1800s, various experimenters noticed that lightly touching objects, as well as loosely compacted particles, often clung together in reaction to nearby sparks and strong electrical currents, although when physically jarred they instantly separated and returned to their original state. It was also found that the light-contact configuration normally had a high electrical resistance, but after coherence occurred the resistance decreased dramatically, and the connection became highly conductive. In 1852, anticipating the relay coherer (see below), S. A. Varley used this effect to invent a lightning arrester to protect telegraph wires. His design consisted of a wire running from the telegraph line to the ground, in series with a segment containing loosely packed carbon granules in a wooden box. Under normal conditions, no electrical current could flow though the carbon granules because of their high resistance. But the increased current produced by a lightning strike caused the granules to cohere and form a highly conductive electrical path, providing a route for lighting bolt to flow into the ground, protecting the telegraph line, and its operators, from injury.

In 1879, David Edward Hughes heard crackling sounds in a carbon-contact telephone receiver, whenever sparks were generated by nearby electrical equipment. This effect was found to occur for distances up to about 450 meters (500 yards). In retrospect it is apparent that the telephone receiver was acting as a form of microphonic coherer (see below), however, at the time radio waves were not yet known to exist, and Hughes did not publicize his findings, so the discovery was lost until it was re-encountered by radio experimenters.

Radio usage

In the 1880s, Heinrich Hertz became the first person to demonstrate the existence of radio waves, at the time commonly known as "Hertzian waves" or just "wireless". However, the receiver used in his experiments was a simple wire loop, which included a small gap where a spark was visible when a radio signal was received. This loop receiver proved impractical outside of laboratory experiments, as the sparks were so faint that they were difficult to see unless the room was darkened, and the device was so insensitive it could only be used for transmission distances of a few meters. Thus, there was a need for a more practical receiver, which led to the development of the coherer as the primary radio detector during the early days of radio research.

The first person to demonstrate radio wave detection using a coherer was Edouard Branley of France, who studied the properties of a wide variety of light-contact connections. In 1890 he developed a coherer consisting of metallic filings that were placed between electrodes in an enclosed glass tube, which he called a radio-conductor. Other experimenters followed, including the Russian Alexander Popov, who in 1894 used his version of the device to detect the radio signals produced by lightning discharges. In 1894, Oliver Lodge of the United Kingdom became the first to apply the term "coherer" to the detector. The next year, Italian Guglielmo Marconi developed the first practical system of radiotelegraphy, which included an improved version of the Branley filings coherer that was more compact and reliable than previous designs.

Most coherers, after switching to the conductive state, remained that way until they were physically jarred, which caused the joined elements to "decohere" and become non-conductive. A variety of techniques, including tapping, shaking and axial rotation, were used to trigger the decohering. The most common coherer receiver design was known as the "tapping filings coherer", where the coherer consisted of a glass tube filled with minute metal filings, and decoherence was effected by lightly tapping the tube with a tapper similar to a bell clapper. A variety of substances was used for the filings, most commonly mixtures of lightly oxidized metallic particles.

Relay coherers

There were two main types of coherer circuits used in early radio development. The most common arrangement used the coherer in a relay circuit: an electrical current provided by a battery was initially blocked by the high resistance of the light-contact segment, until coherence — triggered by the increased voltage due to the reception of a radio signal — occurred, which allowed the battery current to now flow through the light-contact segment and operate an indicating device, such as a bell or telegraph register. Decoherence returned the coherer to its high resistance state, again blocking the battery current, thus shutting off the indicating device.

One advantage of relay coherers was they could be easily adapted to record Morse code messages on paper tape, providing a permanent record. In addition, the receiver could be left unattended, to be triggered by a radio transmission, with the incoming message recorded telegraphically for later reading. However, it was soon discovered that relay coherers had many drawbacks for practical commercial use. Even under the best conditions they could not operate at speeds faster than about 15-words-per-minute, perhaps one-third the speed of standard landline telegraph practice. Sensitivity was quite variable and often unreliable — sometimes overly sensitive, and at other times unresponsive. Because relay coherers merely recorded that they were receiving a signal, it was difficult to differentiate between the signals that were of interest from interfering ones from other radio transmitters, or mere static.

Microphonic coherers

A second coherer circuit arrangement, often referred to as "microphonic", connected a telephone receiver to the coherer circuit, in order to provide audio reception. In this case, changes in the electrical current, due to the variations in coherer conductivity, resulted in changes in the sounds being heard in the telephone receiver, so a radio transmission, broken up into intermittent short-and-long pulses, could be received audibly as Morse code. Although these devices were not capable of full-audio reception, in some cases the characteristics of different transmitted signals could be identified. The first commercial radiotelegraph use of a microphonic coherer appears to have been a link between the California coast and the island of Santa Catalina, set up in 1902 by the American Wireless Telephone and Telegraph Company, which used a light contact between steel and an oxide of iron. Other versions were employed by a number of early developers in the United States, including Walter Massie, Thomas Clark and Joseph Murgas. Although microphonic coherers lacked the relay coherers capability of providing a permanent record, they could receive telegraphed signals much faster, and working speeds approached landline standards. But even they were not very sensitive, and prone to getting out of adjustment when used in unstable environments such as on shipboard.

Later usage and replacement

The original coherers sufferer from the complexity of having to use a manual means, such as tapping, for decoherence, which led to the later refinement of coherer designs that automatically returned to their non-conductive state as soon as the received radio signal ceased. However, these incremental improvements were not enough to make coherers reliable or sensitive enough to compete with other more sophisticated detectors that were being developed, including electrolyic, crystal, magnetic and vacuum-tube detectors.