43

I.—la.

Specimens number 8, 9, and 10 are pieces of lead pipe, and numbers 13 and 14 pieces of iron gas-pipe, which were destroyed by the street-car currents. Specimen number 14 is especially interesting, as it shows very clearly what must often take place at points of high resistance in buried conductors. In this piece of gas-pipe the high resistance at the joint was probably due to the red-lead and oil used for the purpose of sealing. Owing to this resistance, a portion of the current which the pipe was carrying was shunted through the

Destruction of Telephone Cables by Electric Railway Currents.

surrounding moist earth, either to the coupling or to the section beyond, thereby causing the local destruction of the pipe. Corrosion occurred on both sides of the coupling, owing to reversals of street-car currents. Specimen number 15 is a piece of iron gas-pipe which, while partially buried in moist clayey earth, was subjected to the action of a current of 0-3 of an ampere for two weeks. When the action of the currents was first noticed, th» experiment of grounding the cables to lead plates buried in manholes, was tried on quite an extensive scale, but was soon abandoned as being impracticable. The quantity of electricity to be dealt with was so enormous that the buried plates effected no appreciable protection to the cables. If such a system were feasible, the expense for the constant renewal of plates would be very large. The severity of the action may be diminished to a certain limited extent by so arranging the direction of the current used for the street railways as to make it pass out over the trolly wires and back through the ground. In this way the direction of the current would be quite generally from the earth to the cables, thus diminishing in certain places the corrosive action. This method would not be a complete protection from the corrosive action, as there would be places where the current would still flow from the cables. Even though the current is uniformly to the earth from the cables, there is the possibility of an action caused by alkaline substances formed from a cable due to the decomposition by the current from the street cars of the soluble salts contained in the surrounding earth. These alkaline substances are capable, under certain conditions, of dissolving the lead when the currents are shut off or much reduced, as would be the case at night. As there has been some misunderstanding in regard to the potential measurements made in connection with the numerous corrosion investigations, it should, perhaps, be impressed upon those who are about to carry on similar investigations, that the potential measurements between the cables or pipes and the materials surrounding them should, in the majority of cases, only be looked upon as indicating the direction in which the currents tends to flow. To say that the pipes and cables are even practically safe from corrosion when the measurements are below a specified figure would be extremely misleading. These measurements are, in a certain sense, like the measurements which might be made in an electrolytic cell, between one of the electrodes and various portions of the electrolyte, so that it is possible to conceive of almost zero potentials in the immediate vicinity of the most violent electrolytic corrosive actions. The fact should be clearly borne in mind that whenever we have a current passing from an easily oxidizable metal to the liquid, such as would be encountered in the earth, corrosion is bound to occur. A large number of electrolytic experiments have been carried out showing the extremely