A. Brown, and Summer Tainter, from the year 1878
onward. The variations to which luminous waves give rise in the
resistance of selenium and other substances are, doubtless, not
unconnected with those which the electric waves produce in filings. A
connection can also be established between this effect of the waves
and the variations of contact resistance which enabled Hughes to
construct the microphone, that admirable instrument which is one of
the essential organs of telephony.
More directly, as an antecedent to the discovery, should be quoted the
remark made by Varley in 1870, that coal-dust changes in conductivity
when the electromotive force of the current which passes through it is
made to vary. But it was in 1884 that an Italian professor, Signor
Calzecchi-Onesti, demonstrated in a series of remarkable experiments
that the metallic filings contained in a tube of insulating material,
into which two metallic electrodes are inserted, acquire a notable
conductivity under different influences such as extra currents,
induced currents, sonorous vibrations, etc.
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