Unbuilding.
The loss of its charge or excitation by a self-exciting dynamo. It is
the reverse of building-up. The latter indicates the exciting of the
field by the action of the machine itself; the former the spontaneous
loss of charge on open circuit or from other cause.
Underground Conductor.
An electric conductor insulated and placed under the surface of the
earth, as distinguished from aerial conductors.
Underground Electric Subway.
A subway for the enclosing of electric telegraph and other conductors
under the surface, generally in the line of streets, to do away with
telegraph poles and aerial lines of wire. Many systems have been
devised. The general type includes tubes called ducts in sets, called
conduits, bedded in concrete or otherwise protected. Every two or three
hundred feet the sets lead into a cistern-like cavity called a manhole.
The insulated wires or cables, generally sheathed with a lead alloy are
introduced into the tubes through the man-holes. A rope is first fed
through the tube. To do this short rods which screw together are
generally employed. One by one they are introduced, and each end one is
screwed to the series of rods already in the duct. When the end of the
duct is reached the rope is fastened to the last rod, and the rods are
then drawn through, unscrewed one by one and removed, the rope following
them. By means of the rope a windlass or capstan may be applied to draw
the cable into the duct. At least at every second man-hole the cables
have to be spliced.
Each cable may contain a large number of conductors of small size for
telephoning, or a smaller number for electric light and power. The
tendency is now to separate the different classes of wires in important
lines, placing the heavier wires on one side of the street and the
telephone and telegraph wires on the other. This of course necessitates
two separate conduits.
The advantage of underground distribution affects not only the
appearance of streets in doing away with unsightly telegraph poles, but
it also removes an element of danger at fires. Aerial wires interfere
greatly with the handling of ladders at fires, and expose the firemen
who attempt to cut them to danger to their lives from shock.