[Electric engineering.] . Conductor EDISON UNDERGROUND-TUBE SYSTEM. 29. The Edison underground-tube system differsfrom the conduits previously described in that the con-ductors are placed in iron tubes that are buried in theground. The conductors are,therefore, not removable. Thisarrangement has been used ex-tensively by illuminating andpower companies in the largercities. The conductors them-selves are usually in the shapeof round copper rods; the maintubes are designed for use on the three-wire system and are,therefore, provided with three rods, as shown in the section ?/ron Pipe -Insulating
[Electric engineering.] . Conductor EDISON UNDERGROUND-TUBE SYSTEM. 29. The Edison underground-tube system differsfrom the conduits previously described in that the con-ductors are placed in iron tubes that are buried in theground. The conductors are,therefore, not removable. Thisarrangement has been used ex-tensively by illuminating andpower companies in the largercities. The conductors them-selves are usually in the shapeof round copper rods; the maintubes are designed for use on the three-wire system and are,therefore, provided with three rods, as shown in the section ?/ron Pipe -Insulating CompoundFig. 30. in Fig. 30. Each rod is wound with an open spiral of ropethat serves to keep the rods separated in case the insulating 26 ELECTRIC TRANSMISSION. §15 material in the tubes should become soft. After the rodshave been provided with the rope spiral, they are boundtogether by means of a wrapping of rope and inserted inthe iron pipe, the rods projecting for a short distance at each end. The whole tube is thenfilled with an insulating compoundthat becomes hard when cold. Thetubes are made in 20-foot lengthsand are laid in the ground about30 inches below the surface of thepavement. They are joined to-gether by means of the couplingboxes shown in Fig. 31 (a) and (b).Fig. 31 (a) shows the lower half ofthe box only, with the main tubes entering each end. Theconductors are connected together by means of short, flexi-ble, copper cables c, c, c, provided with lugs b, b, that fitover the rods and are soldered in place. A cover d similarto the lower half e is then placed in position and the two KALE
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