The coherer was a very early radio wave detector. It was used for detecting the signals for many early wire-less transmissions.
The coherer was used with the spark transmitters of the day: these transmitters used a high voltage, often created by an induction coil arrangement, and this was then used to create a spark, and the resultant signal was radiated by an antenna connected to the system.
Although not very sensitive, the coherer was often the best way of detecting these wireless signals and being able to read the radio communications transmission being sent.
As such the coherer history forms an essential element within the whole development of radio communications technology and was a key enabler within many major firsts.
Early coherer history
There are a number of early observations that have been recorded that set the foundations for the developemtn of the coherer.
In 1850 Pierre Guitard found that when dusty air was electrified, the particles of dust would tend to link together in strings.
In another observation, but this time with an application two British inventors, C and S A Varley took out a patent in 1866 for a lightning arrestor. In this patent application, the arrestor had two copper points that were almost touching as in a normal arrestor. However the box that contained the arrestor had carbon power. The carbon would then provide a conduction path in the event of discharge that was not sufficient to jump across the spark gap, thereby removing any residual charge.
This was an important discovery in the overall history of the coherer as it set out the fundamental principle of the coherer.
The next major step in the development of the coherer was the discovery in 1878 by a researcher named Hughes that a glass tube containing loose zinc and silver filings was sensitive to electric sparks at a distance. The filings cohered in the presence of a nearby spark.
In a further development another researcher named Onesti is believed to have experimented with a predecessor to the coherer in 1884.
However the person to whom the credit for the invention of the coherer is generally given is a Frenchman called Edouard Branly.
In 1890 Branly published details of his findings. He had confirmed the observations made previously by Hughes and also investigated the way in which an electric spark in the vicinity of the filings of power could change the conductivity of these powdered conductors as they "cohered".
He observed a change in the resistance of thin metallic films that were exposed to electric sparks. First he used platinum deposited on glass and he later discovered that the variations in the resistance of metals in a finely divided state were even more marked. He then went on to show that these changes were due to the presence of an electrical spark, and as a result of the newly discovered Hertzian waves (i.e. radio waves).
He then conducted some investigations into the use of different types of filings and powered conductors to discover which produced the optimum effect.
Input from Oliver Lodge
Sir Oliver Lodge had been working independently in Britain and came up with similar findings to those of Branly.
However, it appears that he saw the findings of Branly at some stage. He improved the design taking the ideas a stage further forward. Lodge also called this improved detector a "coherer" and thus it was he who named the device.
Lodge presented some of his findings to the Institution of Electrical Engineers, IEE, in London in 1890. Then in 1894 he devised and exhibited an adjustable point device. This had an end with a fine spiral spring that gently touched an aluminium plate.
Marconi & the coherer
Marconi was one of the major early developers in the field of wireless or radio communications systems. He came over to Britain in 1896 because he was not able to gain any interest in his native Italy for commercial applications for wireless / radio communications systems.
He had been introduced to Oliver Lodge and most likely as a result he started to use the coherer for his radio systems.
Marconi was a great experimenter but not a theorist.
Marconi tried many different methods, perpetually trying new ideas to see if they made any improvements. He was then very careful not to allow his competition to gain the secrets of his developments. There were stories that his engineers took home the mixtures of filings over the weekend in case there was a break in at the offices by any of the competition.
Marconi's favoured coherer format was to have a sealed glass tube. This had two silver plugs which were attached to platinum wires. The inner ends of the plugs were cut at an angle, and brought within two millimetres of each other. The space between was generally filled with nickel and silver filings in the correct proportions. Occasionally other mixtures were used according to the experiments that had been undertaken.
Marconi used his coherers in many of his commercial systems. These were generally installed on ships where they enabled messages to be sent back to land based stations.
End of the coherer
The coherer was never a very satisfactory form of radio detector. In the early days of radio, the detector was the main factor limiting the performance and achievable range of a system. In addition to this the coherer was expensive to produce, especially as significant amounts of silver were often used in their manufacture.
As a result, much effort was placed into finding a better replacement for the coherer. In 1904 Ambrose Fleming invented the diode valve (vacuum tube). These devices were expensive to make and run. As a result thermionic technology was not widely used until later.
Around 1907 the first viable alternative was found in the form of the cat's whisker diode rectifier. Although a little temperamental, their performance was far superior and the cohere soon fell out of use.
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