Fig. 159.
Fig. 160.
In large buildings, pairs of wires, called risers, branch out from the service wires and carry the current up through the building. These have still other branches—floor mains, etc., that pass through halls, etc., smaller branches finally reaching the lamps. The sizes of all of these wires depend upon how much current has to pass through them. The mains in large cities are usually placed underground. In some places they are carried on poles.
Fig. 161.
140. Electric Conduits are underground passages for electric wires, cables, etc. There are several ways of insulating the conductors. Sometimes they are placed in earthenware or iron tubes, or in wood that has been treated to make it water-proof. At short distances are placed man-holes, where the different lengths are joined, and where branches are attached.
Fig. 162.