240 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
rail has about 700 insulators per mile, and if they are of
tolerably good material, the leakage current must neces-
sarily be small even in very wet weather. Testsshow that
thisis so. Inordinary weather the leakage is imperceptible,
and under the worst conditions it amounts to only a few
amperes per mile of track. This might be expected, for it
is certainly a poor insulator that, even when damp on the
surface, would let pass more thana few milliamperes under
a pressure of 600 volts. If the track is not actually sub-
merged, the insulation should remain fairly high. Snow
is a rather good insulator, and if the roadbed is well
drained, even melting snow will not cause much incon-
venience.
Such a third rail structure generally renders feeders
quite needless. For a road such as we have been investi-
gating a one hundred pound supply rail on each track would
give, when well bonded, a total equivalent conductivity of
just about 2,130,000 c.m., allowing one-third of the total
resistance to be in the bonding. This is almost precisely
the equivalent of the available copper shown in Fig. 121.
On a longer road, or with heavier service, supplementary
feeders would be necessary.
The cost of this third rail system is decidedly low. A
one-hundred pound rail weighs eighty-eight tons per mile,
costing at present prices not tar from $2300. Insulators,
placing and bonding should not exceed $700 per mile addi-
tional. On this basis the third rail system can be installed
rather more cheaply than the overhead system and is far
simpler tomaintain and operate.
A sectionalized third rail has been more than once
suggested as a remedy for leakage. Whatever may be its
merits for street work, it is disadvantageous in that it
virtually throws away the immense conductivity of the
supply rail and thus greatly increases the first cost of the
line. A fraction of the extra expense applied to careful
drainage of the roadbed and good insulation would render
sectionalization needless for this particular kind of work.
A copper third rail deserves consideration in connec-
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