, think of any one of the multiple effects
produced by the Hertzian waves. In many systems in use, and in the new
one of Marconi himself, the use of these tubes has been abandoned and
replaced by magnetic detectors.
Nevertheless, the first and the still most frequent successes are due
to radio-conductors, and public opinion has not erred in attributing
to the inventor of this ingenious apparatus a considerable and almost
preponderant part in the invention of wave telegraphy.
The history of the discovery of radio-conductors is short, but it
deserves, from its importance, a chapter to itself in the history of
wireless telegraphy. From a theoretical point of view, the phenomena
produced in those tubes should be set by the side of those studied by
Graham Bell, C.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., and that this
conductivity is easily destroyed; as, for instance, by turning the
tube over and over.
In several memoirs published in 1890 and 1891, M. Ed. Branly
independently pointed out similar phenomena, and made a much more
complete and systematic study of the question. He was the first to
note very clearly that the action described could be obtained by
simply making sparks pass in the neighbourhood of the radio-conductor,
and that their great resistance could be restored to the filings by
giving a slight shake to the tube or to its supports.
The idea of utilising such a very interesting phenome
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