A word now as to the instruments used for the transmission of
messages. Those for cables are of two kinds, the mirror galvanometer
and the siphon recorder, both the product of Sir Wm. Thomson's great
inventive genius.
When the Calais-Dover and other short cables were first worked, it was
found that the ordinary needle instrument in use on land lines was not
sufficiently sensitive to be affected trustworthily by the ordinary
current it was possible to send through a cable. Either the current
must be increased in strength or the instruments used must be more
sensitive. The latter alternative was chosen, and the mirror
galvanometer was the result.
The principle on which this instrument works may be briefly described
thus: the transmitted current of electricity causes the deflection of
a small magnet, to which is attached a mirror about three-eighths of
an inch in diameter, a beam of light is reflected from a properly
arranged lamp, by the mirror, on to a paper scale. The dots and dashes
of the Morse code are indicated by the motions of the spot of light to
the right and left respectively of the center of the scale.
The mirror galvanometer is now almost entirely superseded by the
siphon recorder. This is a somewhat complicated apparatus, with the
details of which we need not trouble our readers. Suffice it for us to
explain that a suspended coil is made to communicate its motions, by
means of fine silk fibers, to a very fine glass siphon, one end of
which dips into an insulated metallic vessel containing ink, while the
other extremity rests, when no current is passing, just over the
center of a paper ribbon.
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