40 YOWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
ing here only with the average result. Suppose the ma- 
terial has a specific resistance of a thousand ohms per cubic 
centimetre, then the resistance of one element would be 
200,000 ohms, but the whole mass would have a resistance 
of only one-fifth of an ohm; hence if there should be between 
track and pipe an average difference of potential of ten volts, 
an amount sometimes exceeded in real cases, there would be 
within the distance considered a flow of fifty amperes be- 
tween track and pipe. 
As large pipes may weigh several hundred pounds per 
yard, it is clear that their conductivity cannot be neglected, 
although in most cases it has no mnoticeable effect on 
the resistance of the system. In any case, these extra- 
neous metallic conductors cannot properly be counted as a 
A B 
  
     
  
" *Street .R’a'il‘way Journa 
FIG. 29. 
part of the circuit, except under very unusual conditions, 
since flow of current to them is highly objectionable, as 
will presently be shown. 
To sum up the matter of earth return, properly so 
called, the earth, so far from being a body of high con- 
ductivity, useful for eking out the carrying power of the 
rail return, is, for most useful purposes, to be regarded al- 
most as a non-conductor. Its specific resistance is so high 
and irregular that it is of no value as part of the return 
circuit, while its conducting power in great areas comes 
into play only in an unpleasant and troublesome way. For 
all long lines of railroad and for many small street railway 
systems, the earth may be left entirely out of account, and 
in large street railway systems it is generally a source of 
anxiety. In the early days of electric railroading quite 
the opposite view was often held and roads were constructed