Tarnished Silver as a Crystal Radio Detector

A fully tarnished, shiny black silver artefact was tried out as a detector in place of the germanium transistor in my series-tuned, shunt-fed crystal radio.

Series-tuned Shunt-fed Crystal Radio - Schematic
Fortunately the presence of an untarnished spot facilitated the metal to metal connection.

A length of pencil 'lead' was used to probe the tarnished portion to locate the 'sweet spot'.

The local 612 kHz, 200 kW AM broadcast station, just 20 km away, was received quite well using a 60' wire antenna and sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, was 300 μA as compared to 750 μA with the germanium transistor and 50 μA with a razor blade as the detector. 

It was a confirmation of the shiny black tarnish being silver sulphide (a semiconductor).

A silver coin, stored in a plastic pouch and tarnished a dull grey, in its failure to detect, indicated that its tarnish was silver oxide (a good conductor of electricity).
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