It follows, that with a receiving instrument set to indicate a
particular strength of current, the rate of signalling would be very
slow on long cables compared to land lines; and that a different form of
instrument is required for cable work. This fact stood greatly in the
way of early cable enterprise. Sir William (then Professor) Thomson
first solved the difficulty by his invention of the 'mirror
galvanometer,' and rendered at the same time the first Atlantic cable
company a commercial success. The merit of this receiving instrument
is, that it indicates with extreme sensibility all the variations of the
current in the cable, so that, instead of having to wait until each
signal wave sent into the cable has travelled to the receiving end
before sending another, a series of waves may be sent after each other
in rapid succession. These waves, encroaching upon each other, will
coalesce at their bases; but if the crests remain separate, the delicate
decipherer at the other end will take cognisance of them and make them
known to the eye as the distinct signals of the message.
The mirror galvanometer is at once beautifully simple and exquisitely
scientific. It consists of a very long fine coil of silk-covered copper
wire, and in the heart of the coil, within a little air-chamber, a small
round mirror, having four tiny magnets cemented to its back, is hung, by
a single fibre of floss silk no thicker than a spider's line.
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