Any disturbance of this delicate balance, however, say by the movement
of a coil or a metallic body in the neighbourhood of the apparatus, will
be at once reported by the induction currents in the telephone. Being
sensitive to the presence of minute masses of metal, the apparatus was
applied by Professor Graham Bell to indicate the whereabouts of the
missing bullet in the frame of President Garfield, as already mentioned,
and also by Captain McEvoy to detect the position of submerged
torpedoes or lost anchors. Professor Roberts-Austen, the Chemist to
the Mint, has also employed it with success in analysing the purity and
temper of coins; for, strange to say, the induction is affected as well
by the molecular quality as the quantity of the disturbing metal.
Professor Hughes himself has modified it for the purpose of sonometry,
and the measurement of the hearing powers.
To the same year, 1879, belong his laborious investigations on current
induction, and some ingenious plans for eliminating its effects on
telegraph and telephone circuits.
Soon after his discovery of the microphone he was invited to become a
Fellow of the Royal Society, and a few years later, in 1885 he received
the Royal Medal of the Society for his experiments, and especially
those of the microphone.
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