46 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
an ampere in a space of one square foot. Suppose the pipe to 
be 54 in. thick, therefore weighing about twenty-five pounds 
per square foot of surface. If the electrolytic action were 
perfectly uniform the pipe would be reduced to an unsub- 
stantial shell in a single year, and since the corrosion al- 
ways shows irregular pits the pipe would almost infallibly 
be perforated in six months. 
  
Fic. 33. 
It is worth while to note that surface protection of 
pipes by painting with asphalt and thelike has been shown 
by the Boston experience to be practicaily worthless, as 
  
F1c. 34. 
the corrosion seems to work under the film which can never 
be made really insulating to any useful extent. 
In spite of the quite perceptible assistance that may 
be rendered by underground pipes to the general conduc- 
tivity of the return system, every effort should be made to 
avoid it. For, even if the various lines of pipe are protect- 
ed by the supplementary wire method described, there