THE RETURN CIRCUIT. 41 
accordingly. In reality the bonding was then so generally 
inefficient, that perhaps even the earth may have improved 
the general conductivity. FExperience has shown however 
that the view here presented is the correct one, and the 
realization of it has done much to improve general prac- 
tice. Possibly interference with telephone circuits did 
much to prolong faith in the earth as a conductor, but the 
telephone deals with millionths of amperes, which are quite 
insufficient for operating street cars. 
Recurring to Fig. 29, and granting the conditions to 
be such that a current flows from track to pipe at some 
point in the system, that current must leave the pipe and 
either pass back to a part of the track having a lower po- 
tential or to some other conductor by which it may work 
its way back towards the station. . 
Now wherever an electric current leaves a metallic con- 
ductor for one which owes its conductivity, as does the 
earth, to the presence of liquid, the surface of the former 
is corroded—gnawed away by the chemical action set up 
by the current. Hence the pipe under consideration would 
soon show a surface pitted with rust, and eventually the 
corrosion would extend through to the inner surface of the 
pipe and start a leak. Similarly the rails are corroded 
from the exit of the current, but the result is not of much 
consequence. 
This matter of electrolytic corrosion of water pipes, 
gas pipes and other buried conductors is serious in very 
many electric railway systems, so serious that it is worth 
detailed study as one of the gravest factors bearing on the 
design of the return circuit. One would naturally suppose 
that the actual amount of damage done by the compara- 
tively small currents distributed over a large space, would 
be rather slight. So it would be if it were intermittent, 
but when the electrolytic process goes steadily on week 
after week and month after month, the aggregate result is 
somewhat formidable. One ampere flowing steadily from 
an iron surface will eat away very nearly twenty pounds of 
metal per year. So, in the case of conduction to a pipe