Apart from the unsightly appearance of overhead wires, there are many
reasons why any extended system of supply of electricity should be
carried out by underground cables. It is true that there have been no
accidents in this country due to electric light wires falling, owing to
the care bestowed on their insulation and erection; on account of the
heat generated by the passage of the current through the leads no snow
can accumulate on them, and therefore they are not subjected to the
extra weight which destroyed so many of the telegraph and telephone
wires in the last snow storm. Overhead electric light wires are
exclusively used by the largest electric-supply company in London, and
it is probable that, without further legislation takes place, other
companies will shirk the expense of an underground system; and even
a more dangerous method of running cables than that which has been
condemned in the principal cities of the United States will become
not the exception but the rule. In the city of New York the process
of conversion of the present overhead to an underground system is a
fact about to be accomplished to a very great extent at least, in the
near future. Since July, 1887, the Western Union Telegraph Company
have occupied the conduits, which have been constructed and laid with
some 500 miles of wire; also the Metropolitan Telephone and Telegraph
Company have 1000 miles of wire in the subways; and the Edison
Illuminating Company, whose conductors were laid in the trench at the
time of construction, has more than 1000 miles of underground cable.
The plan adopted is to build conduits of section, as in Fig. 30, which
shows the subway in course of construction, with man-hole opening and
exposed ends of conduits. The single tube at top is for distribution
between man-holes, and some wires are shown entering the vault on the
right from the service box in the foreground. The conduits are of
various types; creosoted wooden tubes are placed in creosoted wooden
casings; wrought-iron pipes are sometimes laid in asphaltic concrete
with creosoted wooden box; another arrangement is to be of composition
blocks on concrete, and cover them with brick—or wrought-iron pipe
is lined with cement, and laid in hydraulic cement concrete and cased
with creosoted plank. About 85 per cent. of all the conduits have been
constructed on this plan, the interior diameter of the pipes being 2½
inches.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
Fig. 31 shows how the street arc lighting wires are taken, also a
branch for house use, out of the man-holes, which are placed at each
street crossing. For the cleaning purposes and for drawing the cable
through the conduits, these must be laid practically straight.
[Illustration: FIG. 32.]
Fig. 32 illustrates a method proposed by Mr. Kenneth Mackenzie, which
is somewhat similar to the system of conduit which, used at Tours
for the past two years, has been found most efficient for the high
potential supply mains to the transformers. The troughs would be about
4 ft. long and 15 in. deep, having spigot and socket joints at the ends
like ordinary water pipes. Transverse pieces of wood, or preferably
slate, would rest upon projections, and would support the mains, and
a cover recessed as shown would make the conduit fairly water-tight;
drain holes would be provided, and the branches to houses led off
through glands in the side of troughs. The American plan is, doubtless,
the best, as there is no space for moisture to collect in the conduits;
but Mr. Mackenzie’s system is well worth trying, and has the advantage
of being much cheaper in first cost.
The Edison plan is to place two solid conductors in a tube which is
filled up solid with an insulating material, suitable bends and offsets
being supplied, so that the tube containing the two conductors can be
buried in the ground like a gas-pipe. The system is very largely used
both in the United States and in Continental cities; but it is doubtful
whether the protection would suffice in our towns, where the streets
are already at the mercy of the gas and water companies, whose workmen,
with a single blow of a pick, might perforate the tube, and cause a
dangerous short circuit.