Coherers
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Coherer as used by Marconi.
Coherers are temperamental electro-mechanical devices that work by arranging for a conduction path to be established by an RF signal which is then used to conduct DC. Marconi's version consisted of fine nickel and silver filings contained in a partial vacuum between two electrodes. The resistance between the electrodes is largely determined by the number and nature of inter-filing contact points and drops markedly if an RF signal is applied between the ends.
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Coherer as used by Marconi. Image courtesy Wikipedia.
The coherer does not reset itself and needs to be struck to shake the filings loose again. A relay could be connected and would close in the presence of a signal allowing for the reception of telegraphy.
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Drawing of Lodge-Muirhead Coherer. Image courtesy BVWS. Bulletin Vol 15 no. 2
Others coherers were used for telephony. One example is the Lodge Muirhead type which consisted of a bowl of mercury covered in a thin film of oil. A rotating sharp-edged iron disc was lowered into the mercury to almost - but not quite - break the oil layer. The RF signal would break the layer and allow conduction. Unlike the filings-based coherer, this device was self resetting.
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