52 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
doing poor work on his return circuit without even the ex- 
cuse of economy. 
We are now in a position to determine the quantity 
which was the ultimate object of this investigation into the 
details of the return circuit; i.e., its total net value as a 
conductor compared with the outgoing circuit. 
This is obviously not a fixed quantity in either abso- 
lute or relative value, for even neglecting joint resistances 
there is far less difference between the weights of the rail 
used in various systems than between the weights of over- 
head copper. An ordinary electric road uses perhaps a rail 
of seventy pounds per yard. A single track so constituted 
is, neglecting joints, of conductivity equal to 2,200,000 
c. m. of copper. If the rails were continuous it is clear 
enough that in a road of small or moderate size they would | 
be perhaps ten times as good a conductor as the overhead | 
system. ‘This would allow for a No. o trolley wire and a 
No. oo main feeder on the average all over the line. On 
the other hand, taking the resistance of bonds and joints as 
double that of the rail itself, the equivalent of the railin 
copper falls to, say, 733,000 c. m., which is less than four 
times the overhead system just assumed. If this system 
averaged a No. ooo feeder, plus the trolley wire, it 
would have almost exactly three times the resistance of the 
  
track circuit. 
In large systems the rails often run as high as ninety 
pounds per yard, so that a single track would be equal 
to 3,000,000 c. m. of copper. With continuous rails 
this full equivalent could be taken, but the fecder area 
plus a No. oo trolley wire would hardly be less than 750,- 
000 c. m., so that the resistance of the overhead wiring 
would be about four times that of the track. More com- 
monly, making the same allowance for bonds as before, the 
track equivalent would be 1,200,000 c¢. m. and the trolley 
and feeder copper would have only about one and a half 
times the track resistance. Not infrequently the bonding . 
is imperfect enough to reduce the track equivalent to goo,- 
000 c¢. m., which would frequently be equaled or ex-