Page:Popular Science Monthly Volume 56.djvu/374

360 hard, dense, even-grained cast iron, containing alloyed rather than combined carbon, have not been appreciably corroded. At Dayton, Ohio, on the other hand, seventy-seven thousand dollars' worth of damage has already resulted. One peculiarity of electrolyzed

 The collapsed appearance of the pipe is due entirely to the removal of the lead by electrolysis, the bore retaining its original shape. The dark spot on the upper surface of the pipe is the point of rupture. One third size.

cast iron is that the original shape is usually retained, the iron being eaten away and leaving a punky formation of pure or nearly pure graphite. In such a case a superficial examination detects nothing wrong, and it requires a mechanical scraping to show that the strength is not there. For this reason good photographs of cast-iron electrolysis are somewhat hard to obtain.

The reason for the comparative immunity of cast iron is not as yet definitely understood. It certainly does not lie particularly in the asphaltic varnish usually applied, for this varnish affords little or no protection when used upon wrought iron or other metals. Nor can it be accounted for by the composition of cast iron itself, inasmuch as a fractured or brightly scraped surface of cast iron shows approximately the same symptoms as other metals when acted upon by a given current for a given time. Whether the iron oxide is the saving feature, or whether the "skin" due to the process of casting acts as an insulator, is not yet settled.

When the trouble first appeared in Boston, in 1891, its cause was promptly identified. The electric-railway construction of those days was so crude, however, that many well-informed electricians fell into the error of assuming that heavier rails, more and larger return feeders, and better bonding (i. e., wire connections from rail to rail, around the joints, designed to decrease the resistance) would prove a panacea for all electrolytic ills. Indeed, this view is still held by a surprisingly large number of men versed in matters electrical.

I am of the opinion that it is impossible, from a financial standpoint, to provide so satisfactory a legitimate return that considerable electricity will not seek a path through pipes, cable covers, etc.; for, in order to confine the electric current to the rails, the resistance of the earth and its contained pipes would have to be infinitely great, and this condition can be realized only by making the resistance of the rail infinitely small as compared with that of