These facts seemed to indicate that the brush discharge from the wires carrying the 75,000-volt current developed nitric acid from the oxygen and nitrogen of the air, and that this acid attacked the wire.
One of the above-mentioned pins used on the Electra line was much charred and burned away at a point a little below the threads. The charred path of the current could also be traced down the side of the pin to the cross-arm, but this path was not as badly burned as the spot near the top of the pin.
A composite pin from a 33,000-volt line, probably a part of the transmission system between the Santa Ana River and Los Angeles, was burned through its wooden threads to the central iron bolt, along a narrow strip at one side. Every pin burned on this line was said to show the effects of the current in the way just described, but no cross-arms were burned and very few insulators punctured.
The composite pin was made up of a central iron bolt 10-5/8 inches long, 1/2-inch in diameter, and with a thin head above the wooden threads, a sleeve of wood 2-5/8 inches long and 1 inch in diameter in its threaded portion, and a sleeve of porcelain 3-1/8 inches long and 1-1/4 inches in diameter at its upper and 2-11/16 inches at its lower end. The sleeves of wood and porcelain were slipped over the central iron bolt so that the portions of the pin above the cross-arm measured 5-7/8 inches. In this case the path of the leakage current seems to have been over both the exterior and interior surface of the insulator and then through the wooden sleeve to the central bolt and the cross-arm.
The facts just outlined certainly indicate a serious menace to the permanence and reliability of long, high-voltage transmission lines supported by insulators on wooden pins. If such results have been encountered on the lines above named, where some of the largest and best designs of insulators are employed, it is only fair to assume that similar destructive effects of leakage currents are taking place on many other lines that operate at high voltages.
It seems at least doubtful whether any enlargement or improvement of the insulators themselves will entirely avoid the destruction of their wooden pins in one of the ways mentioned. It is probable, but not certain, that further extension of distances through air and over insulator surfaces, both exterior and interior, between line wires and wooden pins, will prevent charring and burning of the latter by leakage currents. Much has already been done in the way of covering most of the pin above its cross-arm with the insulator parts, but even those portions of the pin that are best protected in this way are not free from burning.