machine. When a car or train of cars approaches one of these interurban substations the voltage of the trolley falls and when it has reached a certain point the substation automatically starts up and carries the load while the train is in its vicinity. As the car recedes from the substation the demand for current decreases and when the load has reached a predetermined minimum the substation shuts down.

This type of substation with small converters has been successfully introduced in some cities, the most notable installation being that at Des Moines, Iowa, where six substations were distributed throughout the city to replace one centrally located power supply station.

The characteristics of large city loads are different from those on interurban lines. The movement of a single car produces but slight fall in the trolley potential and the starting and stopping of the substation is governed by the demand for power during the morning and evening rush hours. A few substations with large converters have been provided for such city service and are now in experimental operation. Remote control substations are also being developed for city service where they are required to operate continuously throughout the load period of the day or during the morning and evening peaks.

Semi-automatic equipment, consisting of re-closing circuit breakers, time switches, and protective devices have been installed in a number of railway substations at a very much smaller cost than would be required for full automatic operation. The circuit breakers in the positive feeders automatically re-close after a definite time interval provided the short circuit or overload has been removed. The synchronous converter has to be started by hand and may be shut down either by a time switch or by hand. Otherwise it operates in a manner similar to those provided with full automatic control.

The first cost of automatic substations is often justified by the saving in operating labor and feeder losses and the recovery of existing feeding copper. Minor savings arise from the elimination of light load losses and the station heating. A further benefit also to be derived from their general use is better voltage conditions and therefore faster car schedules.

The total amount of substation equipment now operated automatically is in excess of 50,000 k.w., and much of the equipment being installed is intended for automatic operation or remote control. The increased savings attending this development will undoubtedly increase the number of substations which can economically be installed on both interurban and city systems, and if full advantage is taken of these economics, the feeding distances will be reduced to such an extent as to greatly reduce stray currents generally.

(f) Location of Supply Stations. As pipes and other underground structures become increasingly positive to the earth as they approach street railway supply stations or the low potential points on the track system, it is obvious that if stations were located away from pipe networks trouble from electrolysis would seldom occur. As a rule other

considerations will determine the location of supply stations in cities. However, on interurban lines the protection of piping systems in small towns against electrolytic corrosion often presents a grave problem because of the long feeding distances and the difficulty of employing the measures of mitigation ordinarily used in city systems. Under such conditions the location of the supply station at a distance from the city and away from the underground structures may be the most satisfactory way of insuring their protection. This is particularly true of automatic substations which require no regular attendants.

The character of the earth in the vicinity of supply stations naturally has an important effect on the magnitude of stray currents. It is, therefore, desirable to avoid connecting negative feeders to tracks in unusually wet locations.

(g) Alternating Current Systems. When the first alternating current railways were proposed, the question of possible electrolytic effects received special investigation. Considerable work was done upon a laboratory scale, in which it was established that alternating currents could produce corrosion on electrodes of the metals commonly used underground, such as lead and iron, but that the effects were very much less in magnitude than those produced by equivalent direct currents, usually less than 1 per cent. and in most cases negligible. See Fig. 16. The objections to the substitution of alternating current for direct current in the case of systems already installed in large cities are so well known and so serious that the question needs no discussion.

5. Interconnection of Tracks.

Electrical interconnection between parallel tracks in close proximity, or of tracks, one of which passes over the other, belonging to the same or different railway systems is usually a necessity in order to prevent wide fluctuations of voltage between the tracks. Such interconnections tend to equalize the potentials of the tracks so connected and thus tend to prevent the flow of current from the track of high potential through earth and intervening metallic subsurface structures to the track of low potential. In general such interconnections also afford a saving in track losses.

Whether parallel tracks should be connected naturally depends upon the distance between tracks, location of supply stations, leakage characteristics of the roadbeds and other local considerations.

Interconnection generally reduces the track voltage drop by providing more metallic paths for the current. It has also the same general effect as cross-bonding between rails of the same tracks, in that if one track circuit should be accidentally opened the current would be shunted around through the interconnection to the other trask. As a rule interconnection of tracks will improve electrolysis conditions but may be detrimental to one locality while improving conditions in another. A failure of one of the companies to maintain its bonding would naturally tend to increase the current on the better bonded track.

Interconnection of tracks has been found to be particularly advantageous where two or more lines of electric railways operating in one locality and belonging to the same or to different systems are supplied from two or more power stations located in different parts of the city. By interconnecting the tracks of such lines in the neighborhood of the power stations and also at several intermediate points a reduction in

the resistance of the return circuit can be brought about whereby the drop formerly existing in one track can be balanced by the drop in the opposite direction in the other track. The rail drop in each track is greatly reduced and all high potential gradients between tracks eliminated.

Where the tracks of the two independent railway systems are parallel and a short distance apart, and fed by power supply stations in opposite directions, the potential profiles of the rails will be as shown in Fig. 8 in which, for simplicity, the negative buses at the two stations have been assumed to be at the same potential. In the figure are also indicated the potential profiles of the pipes adjacent and parallel to the two sets of tracks.

If then gas or water pipes extending from the parallel mains cross under two sets of tracks at different locations where the tracks are at a considerable difference of potential, as at RB, Fig. 8, then the pipes may be negative to one track and positive to the other. At the crossings where the pipes are positive to the tracks electrolysis will be liable to occur.

If now the rails of the two systems are interconnected at points near the two stations and also at intermediate points the potential profile along the rails after such interconnection will be as shown by the curve OYP. It will be noted that this interconnection results in a very considerable reduction of the potential drop in the return circuit, and the resulting reduction in the losses will in many cases be alone sufficient to warrant the cost of the interconnections.

Railway systems employing track circuit signals must insulate their rails used for signal circuits from other systems in order that other currents may not be introduced in the signal circuits and for this reason cannot avail themselves of the advantages of interconnection. This applies only to rails used for signal circuits.

B. FEATURES OF RAILWAY CONSTRUCTION AND OPERATION EMPLOYED FOR ELECTROLYSIS MITGATION

1. Insulated Negative Feeder System.

Of the various methods of railway construction and operation employed to improve electrolysis conditions, the insulated negative feeder system has been most widely used. While it has been generally thought that such a system is necessary in connection with a large supply station if underground structures are to receive adequate protection, the present tendency to greatly increase the number of railway supply stations, and particularly the development of the automatic substation, makes the extensive use of insulated negative feeders less important. An increase in the number of track drainage points is often more economically attained by the use of more substations than by the use of insulated negative feeders. The tendency is now in the direction of a relatively few short insulated negative feeders and a large number of substations, rather than an extensive use of insulated feeders from a few large supply stations. (a) Description. In the insulated negative feeder system, instead of tying the tracks directly to the negative bus and depending on the tracks and such copper conductors as may be in parallel with them to return the current to the supply station, the connection at the station is either removed or a suitable resistance is inserted and insulated feeders