U 
THE RETURN CIRCUIT, 43 
thence to the pipes and cables, emerging from them at vari- 
ous points in the system. The corrosion was thus widely 
distributed, but from local conditions of conductivity was 
most apparent in spots. Careful measurements of the 
potential between the track and the cables were made in a 
large number of places with the result shown in the map 
(Fig. 31). Near the power stationsthe flow was from track 
to cables, but over the main area of the city it was from 
cables to "track, giving a large area in which corrosion 
might be expected. Differences of potential as high as 
  
  
  
  
  
five volts were observed, while experiments in other cities 
have shown as much as twenty-five volts. It is interest- 
ing to note that one of the first experiments tried to re- 
lieve this electrolytic action was to sink in the earth 
ground plates connected to the cables in the hope that 
the current flow would take place mainly through them. 
The potential differences even at points quite near these 
plates were practically unchanged, showing very plainly the 
intense badness of the earth as a conductor, which has 
already been pointed out. 
The method of treatment which proved most effective 
in reducing the electrolytic effects; was first to locate the