THE RETURN CIRCUIT. 39 
Thus it is that while ground plates along the track 
according to early usage are insignificant in modifying the 
conductivity of the return circuit, there may be, if the 
rails are poorly connected, very perceptible flux of cur- 
rent from the track to, for instance, a water main running 
parallel to it and but a few feet away. Fig. 29 shows this 
state of things. Let A B be the track and CD a water 
main half a dozen feet below the level of the track. The 
resistance between any particular points of A Band CD is 
at all times large, owing to the high specific resistance of 
the material between them, but the area between A B and 
CD in a long stretch of track is so great that if the 
   
    
  
  
  
  
       
  
    
  
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FIG. 28. 
fall in potential in A B is not very slight indeed, there will 
be a considerable flow of current into and along CD. To 
take a concrete example, let A B be twenty rods long, and 
suppose C D to be a foot in diameter and six feet distant 
from A B. The total area of material in direct circuit 
would probably be a strip 100 metres long and not less than 
a metre wide. Such a strip would contain a million square 
centimetres area and we then have to compute the resist- 
ance of a block of bad conductor a million square centi- 
metres in section and perhaps averaging 200 cm. long. 
‘I'his we can regard as built up of amillion strips, each one 
centimeter square and 200 cm. long, connected in parallel. 
T'he total resistance would then be the resistance of one such 
strip divided by 1,000,000. In fact the resistances of 
these elements would be very various and the currenfs 
would flow in all sorts of irregular lines, but we are deal.