A filings coherer changes from high to low electrical resistance following electrical excitation. A local battery supplies the readout electric current; a tap resets the detector. It records events rather than the incident radio wave waveform.
Marconi described nickel and silver filings between silver plugs, with a cell and sensitive telegraph relay controlling a tapper and recorder.[ref]
Wireless Telegraphic Communication by Guglielmo Marconi. 1909 lecture, printed p. 197, Fig. 1 and following paragraph: coherer construction and local readout
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