In order to prevent a train passing a danger signal during a fog or
snowstorm without being seen by the engineer, the Southern Railway
Company of France have attached to the locomotive a steam whistle, which
is controlled by the signal. The whistle is connected with an insulated
metallic brush placed under the engine. Between the rails there is a
projecting contact bar, faced with copper, which is swept by the brush
when the train passes. This contact piece is connected with the
positive pole of a voltaic battery, the negative pole of which is in
communication with a commutator on the signal post, from which a wire
leads to the ground. When the signal is "line clear" the passage of the
brush over the fixed contact produces no result; but when the signal
marks "danger," the commutator brings the negative pole of the battery
in direct communication with the ground, and when the brush passes over
the contact the completion of the electric current causes the whistle to
be sounded, so as to alarm the driver.--_L'Ingen. Univ._
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