many yards when large masses of iron are
polarised, so that the derangement of compasses at sea from moving iron
objects aboard ship, or from ferric ores underlying a sea-coast, is a
constant peril to the mariner.

Electrical conductors behave much like magnetic masses. A current
conveyed by a conductor induces a counter-current in all surrounding
bodies, and in a degree proportioned to their conductive power. This
effect is, of course, greatest upon the bodies nearest at hand, and we
have already remarked its serious retarding effect in ocean telegraphy.
When the original current is of high intensity, it can induce a
perceptible current in another wire at a distance of several miles. In
1842 Henry remarked that electric waves had this quality, but in that
early day of electrical interpretation the full significance of the fact
eluded him. In the top room of his house he produced a spark an inch
long, which induced currents in wires stretched in his cellar, through
two thick floors and two rooms which came between. Induction of this
sort causes the annoyance, familiar in single telephonic circuits, of
being obliged to overhear other subscribers, whose wires are often far
away from our own.

The first practical use of induced currents in telegraphy was when Mr.
Edison, in 1885, enabled the trains on a line of the Staten Island
Railroad to be kept in constant communication with a telegraphic wire,
suspended in the ordinary way beside the track. The roof of a car was of
insulated metal, and every tap of an operator's key within the walls
electrified the roof just long enough to induce a brief pulse through
the telegraphic circuit. In sending a message to the car this wire was,
moment by moment, electrified, inducing a response first in the car
roof, and next in the "sounder" beneath it. This remarkable apparatus,
afterward used on the Lehigh Valley Railroad, was discontinued from lack
of commercial support, although it would seem to be advantageous to
maintain such a service on other than commercial grounds. In case of
chance obstructions on the track, or other peril, to be able to
communicate at any moment with a train as it speeds along might mean
safety instead of disaster. The chief item in the cost of this system is
the large outlay for a special telegraphic wire.

The next electrician to employ induced currents in telegraphy was Mr.
(now Sir) William H. Preece, the engineer then at the head of the
British telegrap