Now at two other points
touch the water with the terminals of a circuit containing a telephone. A
sound will be heard, except when the two telephone terminals touch the
water at points where the potential is the same. In this way the
equipotential lines can easily be picked out. Now to apply this to the
case of a ship at sea: Suppose one ship to be provided with a dynamo
machine generating a powerful current, and let one terminal enter the
water at the prow of the ship, and the other to be carefully insulated,
except at its end, and be trailed behind the ship, making connection with
the sea at a considerable distance from the vessel; and suppose the
current be rapidly made and broken by an interrupter; then the observer on
a second vessel provided with similar terminal conductors to the first,
but having a telephone instead of a dynamo, will be able to detect the
presence of the other vessel even at a considerable distance; and by
suitable modifications the direction of the other vessel may be found.
This conception Professor Bell has actually tried on the Potomac River
with two small boats, and found that at a mile and a quarter, the furthest
distance experimented upon, the sound due to the action of the interrupter
in one boat was distinctly audible in the other.
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