This measurer of the current was infinitely more sensitive
than any which preceded it, and enables the electrician to detect the
slightest flaw in the core of a cable during its manufacture and
submersion. Moreover, it proved the best apparatus for receiving the
messages through a long cable. The Morse and other instruments,
however suitable for land lines and short cables, were all but useless
on the Atlantic line, owing to the retardation of the signals; but the
mirror instrument sprang out of Thomson's study of this phenomenon, and
was designed to match it. Hence this instrument, through being the
fittest for the purpose, drove the others from the field, and allowed
the first Atlantic cables to be worked on a profitable basis.
The cable consisted of a strand of seven copper wires, one weighing 107
pounds a nautical mile or knot, covered with three coats of gutta-
percha, weighing 261 pounds a knot, and wound with tarred hemp, over
which a sheath of eighteen strands, each of seven iron wires, was laid
in a close spiral. It weighed nearly a ton to the mile, was flexible as
a rope, and able to withstand a pull of several tons. It was made
conjointly by Messrs. Glass, Elliot & Co., of Greenwich, and Messrs. R.
S. Newall & Co.
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