At a receiving station the current coming in from the cable has simply
to be passed through the coil of the 'speaker' before it is sent into
the ground, and the wandering light spot on the screen faithfully
represents all its variations to the clerk, who, looking on, interprets
these, and cries out the message word by word.
The small weight of the mirror and magnets which form the moving part of
this instrument, and the range to which the minute motions of the mirror
can be magnified on the screen by the reflected beam of light, which
acts as a long impalpable hand or pointer, render the mirror
galvanometer marvellously sensitive to the current, especially when
compared with other forms of receiving instruments. Messages have been
sent from England to America through one Atlantic cable and back again
to England through another, and there received on the mirror
galvanometer, the electric current used being that from a toy battery
made out of a lady's silver thimble, a grain of zinc, and a drop of
acidulated water.
The practical advantage of this extreme delicacy is, that the signal
waves of the current may follow each other so closely as almost entirely
to coalesce, leaving only a very slight rise and fall of their crests,
like ripples on the surface of a flowing stream, and yet the light spot
will respond to each.
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