Coherer
[2] In 1835 Swedish scientist Peter Samuel Munk[3] noticed a change of resistance in a mixture of metal filings in the presence of spark discharge from a Leyden jar.
[4] In 1850 Pierre Guitard found that when dusty air was electrified, the particles would tend to collect in the form of strings.
He also found that other types of metal filings would have the same reaction to electric sparks occurring at a distance, a phenomenon that he thought could be used for detecting lightning strikes.
Branly's filings tube came to light in 1892 in Great Britain when it was described by Dr. Dawson Turner at a meeting of the British Association in Edinburgh.
George Minchin noticed the Branly tube might be reacting to Hertzian waves the same way his solar cell did and wrote the paper "The Action of Electromagnetic Radiation on Films containing Metallic Powders".
[6][7] These papers were read by English physicist Oliver Lodge who saw this as a way to build a much improved Hertzian wave detector.
On 1 June 1894, a few months after the death of Heinrich Hertz, Oliver Lodge delivered a memorial lecture on Hertz where he demonstrated the properties of "Hertzian waves" (radio), including transmitting them over a short distance, using an improved version of Branly's filings tube, which Lodge had named the "coherer", as a detector.
That same year, Italian inventor Guglielmo Marconi demonstrated a wireless telegraphy system using Hertzian waves (radio), based on a coherer.
The coherer's electrodes were also attached to a DC circuit powered by a battery that created a "click" sound in earphones or a telegraph sounder, or a mark on a paper tape, to record the signal.
If a dash is being transmitted then the radio frequency is still being received when the tap happens, and the coherer immediately becomes conductive again and the whole process repeats for another mark on the tape.
In some coherers, the electrodes were slanted so the width of the gap occupied by the filings could be varied by rotating the tube about its long axis, thus adjusting its sensitivity to the prevailing conditions.
In later practical receivers the decoherer was a clapper similar to an electric bell, operated by an electromagnet powered by the coherer current itself.
An automatic braking system for rail locomotives, patented in 1907, used a coherer to detect electrical oscillations in a continuous aerial running along the track.
If the block ahead of the train were occupied the oscillations were interrupted and the coherer, acting through a relay, showed a warning and applied the brakes.
By means of an adjusting screw the lower edge of the disc is made to touch the oil-covered mercury with a pressure small enough not to puncture the film of oil.
The action of detection occurs when the radio frequency signal somehow breaks down the insulating film of oil, allowing the device to conduct, operating the receiving sounder wired in series.
In 1899, Bose announced the development of an "iron-mercury-iron coherer with telephone detector" in a paper presented at the Royal Society, London.
As a simple switch that registered the presence or absence of radio waves, the coherer could detect the on-off keying of wireless telegraphy transmitters, but it could not rectify nor demodulate the waveforms of AM radiotelephone signals, which began to be experimented with in the first years of the 20th century.
This problem was solved by the rectification capability of the hot wire barretter and electrolytic detector, developed by Reginald Fessenden around 1902.
