A core, in fact, is an attenuated Leyden
jar; the wire of the core, its insulating jacket, and the soil or water
around it stand respectively for the inner tinfoil, the glass, and the
outer tinfoil of the jar. When the wire is charged from a battery, the
electricity induces an opposite charge in the water as it travels along,
and as the two charges attract each other, the exciting charge is
restrained. The speed of a signal through the conductor of a submarine
cable is thus diminished by a drag of its own making. The nature of the
phenomenon was clear, but the laws which governed it were still a
mystery. It became a serious question whether, on a long cable such as
that required for the Atlantic, the signals might not be so sluggish
that the work would hardly pay. Faraday had said to Mr. Field that a
signal would take 'about a second,' and the American was satisfied; but
Professor Thomson enunciated the law of retardation, and cleared up the
whole matter. He showed that the velocity of a signal through a given
core was inversely proportional to the square of the length of the core.
That is to say, in any particular cable the speed of a signal is
diminished to one-fourth if the length is doubled, to one-ninth if it is
trebled, to one-sixteenth if it is quadrupled, and so on.
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