Abstract: A stringer comprises a base portion and first and second webs extending outwardly from the base portion. The orientation of at least one of the first and second webs may transition from a first angle to a second angle within an angle transition zone.

Abstract: A collapsible bladder has flexible outer walls and at least one strut inside the enclosure. The strut is connected to the outer walls and controls the shape into which the enclosure may be collapsed.

Abstract: When a floating battery feed circuit is used to power telephone lines, it is desirable to provide an anti-corrosion biasing circuit to keep the voltage on the tip and ring conductors from becoming positive. This can be done using an anit-corrosion circuit that sinks a dynamic amount of current from the tip conductor. A line circuit including such biasing is, however, incompatible with special lines such as PBX lines that rely on a grounded tip conductor in the talk state. In accordance with the present invention, a constant current is supplied from a ground reference circuit to the ring conductor of such special lines only when loop current is flowing. The magnitude of this supplied current is such that the anti-corrosion circuit holds the tip conductor sufficiently close to ground potential to allow the proper operation of circuits connected to the special lines.