A stretch of overlapping railroad track, with one rail of one track being between the two rails of another track; used over narrow bridges and passes.


Related Terms

POLE-DIPOLE ARRAY

An electrode array used in a lateral search conducted during a resistivity or induced polarization survey, or in drill hole logging, in which one current electrode is placed at infinity while another current electrode and two potential electrodes in proximity are moved across the structure to be investigated.

POLARITY EFFECT

An effect for which the breakdown voltage across a vacuum separating two electrodes, one of which is pointed, is much higher when the pointed electrode is the anode.

POINT OF INTERSECTION

The point at which two straight sections or tangents to a road curve or rail curve meet when extended.

PNPN DIODE

A semiconductor device consisting of four alternate layers of p-type and n-type semiconductor material, with terminal connections to the two outer layers. Also known as npnp diode.

PN JUNCTION

The interface between two regions in a semiconductor crystal which have been treated so that one is a p-type semiconductor and the other is an n-type semiconductor; it contains a permanent dipole charge layer.

PLUNGE

To set the horizontal cross hair of a theodolite in the direction of a grade when establishing a grade between two points of known level.

PLATE GIRDER BRIDGE

A fixed bridge consisting, in its simplest form, of two flange plates welded to a web plate in the overall shape of an I.

PLANE STRESS

A state of stress in which two of the principal stresses are always parallel to a given plane and are constant in the normal direction.

PLANAR LINKAGE

A linkage that involves motion in only two dimensions.

PITCHED ROOF

1. A roof that has one or more surfaces with a slope greater than 10 . 2. A roof that has two slopes meeting at a central ridge.

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