lead sheathed. In emergency, as, for example, in the case of the
destruction of a number of the cables in a manhole, they are,
therefore, interchangeable. The connections are such as to minimize
"track drop," as will be evident upon examination of the diagram. The
electrical separation of the several contact rails and the positive
feeders connected thereto secures a further important advantage in
permitting the use at sub-stations of direct-current circuit-breakers
of moderate size and capacity, which can be set to open automatically
at much lower currents than would be practicable were all contact
rails electrically connected, thus reducing the limiting current and
consequently the intensity of the arcs which might occur in the subway
in case of short-circuit between contact rail and earth.

The contact rail itself is of special soft steel, to secure high
conductivity. Its composition, as shown by tests, is as follows:
Carbon, .08 to .15; silicon, .05; phosphorus, .10; manganese, .50 to
.70; and sulphur, .05. Its resistance is not more than eight times the
resistance of pure copper of equal cross-section. The section chosen
weighs 75 pounds per yard. The length used in general is 60 feet, but
in some cases 40 feet lengths are substituted. The contact rails are
bounded by four bonds, aggregating 1,200,000 c. m. section. The bonds
are of flexible copper and their terminals are riveted to the steel by
hydraulic presses, producing a pressure of 35 tons. The bonds when in
use are covered by special malleable iron fish-plates which insure
alignment of rail. Each length of rail is anchored at its middle point
and a small clearance is allowed between ends of adjacent rails for
expansion and contraction, which in the subway, owing to the
relatively small change of temperature, will be reduced to a minimum.
The photographs on pages 110 and 111 illustrate the method of
bonding the rail, and show the bonded joint completed by the addition
of the fish-plates.

The contact rail is carried upon block insulators supported upon
malleable iron castings. Castings of the same material are used to
secure the contact rail in position upon the insulators. A photograph
of the insulator with its castings is shown on page 113.

[Sidenote: _Track
Bonding_]

The track rails are 33 feet long, of Standard American Society Civil
Engineers' section, weighing 100 pounds a yard. As has been stated,
one rail in each track is used for signal purpose