History
In 1850 Pierre Guitard found that when dusty air was electrified, the particles of dust would tend to attach themselves together in the form of strings. Again, in 1879, it was observed that drops of water from a small fountain, when exposed to the influence of a charged piece of sealing-wax, would not separate into small drops, but would cohere into large ones. Temistocle Calzecchi-Onesti is thought to have performed the first experiments with a predecessor of the coherer in 1884. These phenomena had been observed for many years, but it was not until 1890 that Edouard Branly made a practical application of the principle in the form of the filings coherer as it is now known. The invention of the device is usually credited to Branly.
He began his studies in this field in 1890, being led to undertake them by observing the anomalous change in the resistance of thin metallic films when exposed to electric sparks. Platinum deposited upon glass was first employed. 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 Branly to be due to the action of the electrical, or Hertzian, waves of which the spark was the source. The further developments from these experiments led to the coherer. Later this simple device was employed by British radio pioneer Oliver Lodge in his researches, and formed an important part of Guglielmo Marconi's successful system of wireless telegraphy.
The coherer was replaced in receivers by the simpler and more sensitive crystal detector around 1907, and became obsolete.
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