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- 
  
  
| R TR