tive generator terminal, the other end hung convenient to the feeder switch, the generator built up ready for use, the feeder opened and attached to the cable, and finally the circuit breaker and negative generator switch closed.

If it is desired to extend this system to booster working, it is necessary to have a second cable leading from the positive bus-bar and capable of connection to the negative terminal of the booster. It is best to include a circuit breaker in this cable to protect the booster, as the regular generator circuit breaker will be cut out.

At best, however, cables are troublesome appliances and allow only comparatively slow transfers. Moreover, it is impracticable to arrange cables so that two generators may be run in parallel on the special high voltage service. Where a new board is being installed, and where the circumstances justify the alterations in an existing board, such an one as is shown in Fig. 1 meets the required conditions. The two equalizing bars make it possible to run two or more machines each on the regular and high voltage at one time. With this arrangement, any two combinations of feeders and generators desired may be had. When running the "Special Generator," the negative switches are thrown up, and the positive and equalizing switches on the special machines thrown down. The switches on the feeders needing the increased voltage are likewise thrown down. If it is desired to use the auxiliary bus for booster purposes, the equalizing switch is left open, and the adjustable field shunt applied, the negative generator switch is thrown down, after closing the booster circuit breaker, and the feeder is transferred down to the auxiliary bus. This puts the booster generator in series with the particular feeder. One very important thing in this connection is to make sure that the shunt field circuit is open on the boosting generator. It might be advisable to interlock the field switch with the negative generator switch in such a way that the latter could not be thrown down except with the former open.

The drawing of the board shows a watt-meter and an ammeter in each bar. By providing a double throw switch connected in such a way as to open the lower watt-meter from the feeder board, and connect it to the upper bus-bar on the generator side of the ammeter shunt, the four instruments will be in series between the generators, all running on the lower bus, and the feeders all running on the upper bus. This offers convenient means for checking meters, care being taken to see that current passing through the diagonal bar, on the back of the instrument panel, does not introduce an error into the watt-meter indications.

In a modern station of few large units, it is improbable, were the necessity for booster service to exist, that one of the large generators could be spared or would be desirable for the purpose.

In such stations, however, it is usual to install a smaller machine to carry the all night and other light loads. This machine might be relied

upon for the booster work. If the engine driving it have its governor arranged to adjust for a large variety of speeds, both above and below normal, and the unit be designed to run safely at the increased speeds, the combination will be found very convenient for meeting all classes of demand. The speed may be dropped for booster working, and raised when the generator is run at high voltage on a particular feeder. With this arrangement of dependence upon an individual unit for the special service, it is, of course, unnecessary to equip the whole switchboard with the extra negative bus. The double positive bus will be found convenient in any station, however.

The "Three Wire System," although very useful in lighting work, has been of little service in railroad distribution. Its value for the latter purpose has been considerably overestimated, although there are places where its use has resulted in the maintenance of satisfactory potential on sections of track, using the same copper, when the usual system could not approach doing it.

The advantage of the three wire system is based on the fact that it enables the use of double the usual potential for distribution, thus in the perfect case requiring but one-fourth the copper to transmit a given current at a given loss.

This law, that the amount of copper varies inversely as the square of the potential, is due to the fact that doubling the voltage halves the current for a given amount of energy. The same percentage of loss allows a loss of twice the number of volts, and this, with but half the current, admits of the use of copper having four times the resistance, hence one-fourth the weight. The "perfect case" just cited is one in which no third or "neutral" wire is necessary. Such an one does not exist in practice, and this neutral wire has to be supplied to conduct the excess of current used on one side of the system over that used on the other, between the generators and the motors. In railroad work the rail is used as the neutral conductor. It is seldom that a whole road is operated on the system, the common practice being to apply it to only those parts on which the ordinary system is insufficient. Where there are but one or two sections to operate with the reversed potential, the switchboard described may care for them satisfactorily. If one generator is made to build up with reversed polarity, and is coupled into the auxiliary bus, it will handle such feeders as may be thrown upon it as the negative side of a three wire system. The generator may have its residual charge reversed in a number of ways, one of which is to have the shunt field wired through a double throw switch on the machine, arranged to disconnect the field coils from the armature and connect them in the reverse direction to a pair of charging wires fed from the switchboard. The machine may have its residual charge brought back to the usual direction by the commonly used means of throwing in the positive and equalizing switches, with the negative open, and allowing the current shunted from the series coils of the other machines to

energize its series coil. Where three pole switches are used on the generators, the shunt field double throw switches may be omitted, and the field reversed through the series coil by using an X-shaped pair of contact pieces insulated from each other at the point of intersection. These may be pressed against the switch jaws in such a way as to reverse the current shunted from the other series coils through that of the machine to be reversed. Where the three wire system is the regular system of operation, and the sections are divided equally between the two sides of the system, a regular three wire board is preferable. For a less general use of the system, the method described answers as well. The arrangement then becomes the same as that of the "Special Generator," with the polarity reversed, and the potential on the machine may be raised above normal, and the advantage of greater allowable drop in the copper added to that of the saving in track drop, due to the reversed current flow.

The writer has said that the value of the three wire system in railroad service has been considerably overestimated. This statement probably needs explanation. In lighting work, the three wire system saves 621⁄2 per cent. of the copper. Each wire has to be but onefourth as large as with the two wire system, but it is usual to make the neutral of the same size as the other wires, thus requiring three wires, each of one-fourth the size of those of a two wire system, hence the use in the three wire system of three-eighths, or 371⁄2 per cent., of the copper necessary in the two wire system. Because this is true of lighting work, it is often assumed to be true of railway work. It is not, however, and in no case, where the track renders any appreciable service as a conductor, can the saving from the adoption of the three wire system approach the above figures. In general, its use will make a gross saving of the loss in track (assuming that there is no copper paralleling the track that may be used as a positive feeder). The net saving will be this gross loss in track less the track losses due to the passage of current from car to car, and of the excess of current due to lack of balance, back to the power station, and the increased overhead loss due to the greater drops in the divided feeders than in them when combined. The sketch, Fig. 2, and table, Fig. 3, may present this more intelligibly. It illustrates the simple case of an unbalanced three wire system, and shows that its losses aggregate 4,100 watts under the particular load. If this system were changed to a two wire one by throwing the two feeders in parallel, it would save 400 watts in the feeders, and lose 800 more in the track, leaving a net loss of 400 watts, or about 10 per cent. of the total loss. Such a difference as is shown by this hypothetical case would not justify the complication introduced by the system. The only conditions where the method seems to be applicable, are those of excessive track losses with fair opportunities for balanced load. Such a case existed on the Lowell and Suburban road. They have a heavy business over a double track line to their park, some six miles from the power station. By

changing to positive feeders those formerly used as overhead returns, and operating the line as a three wire one, they very greatly improved the efficiency of the distribution system.

In the selection from the various systems of distribution, of the one best adapted to a particular instance, there is always a large number of variable factors to consider, and usually more or less speculation as to the values of certain ones on which no accurate data are accessible. This makes it impossible to work from fixed laws, and demands individual consideration of each case. For these reasons there is considerable room for divergence of opinions in the matter. The writer has given his ideas as to the comparative infrequency of cases in which the three wire system is desirable. The booster has a very much more general application, and is suited to a greater number of cases than is any one of the other systems. Nevertheless, wherever work can be satisfactorily handled by a special generator, it should be, unless the 2 okrus

20 amperes per cav

FIG. 2.-DIAGRAM OF UNBALANCED THREE WIRE SYSTEM.

change involves abandoning existing apparatus. The last method saves considerable over the booster, and is preferable for its simplicity. The only limiting condition to its use is satisfactory regulation.

The alternating current system, with rotary converter sub-stations, competes with the booster and special generator in cases where the load factor (the ratio of average to maximum load) is high, and where the regular 500 volt transmission will suffice during only a few of the twentyfour hours. In such a case, the copper losses, because of the length of time during which they are excessive, are so great that they may often exceed the entire expense of an alternating transmission system. The latter system also enters into competition with the independent power station for that class of business that is too remote from an existing station to admit of satisfactory handling by the other transmission methods, and too small to furnish load for such a station as may approach in economy the station from which the alternating transmission

would be operated. Cases of each kind arise with city roads, in their needs for transmission to suburban centers of load, and with interurban roads.

The loads which the city station receives from the suburban centers have usually a very low factor. At morning and night they are excessive, and through the remaining hours comparatively light. Were an alternating transmission system installed for such use, it would probably lie idle a large part of the time, its copper being used to transmit direct current. During the heavy hours it would be called into service. This same service might be rendered by raising the voltage on the direct current feeders by use of the "Special Generators," and allowing the loss, which has been from 5 to 10 per cent. during the light load, to become 25 or 30 per cent. In this way the line may carry three or four times as much current, and maintain the voltage at the distant end.

In comparing the systems it is necessary to compute the annual cost of each, including the items in the following list:

ALTERNATING CURRENT.

Fixed Charges and Repairs on Main Station, Static Transformers, if any, and on Sub-Station Static and Rotary Transformers, and Accessories.

Fixed Charges on Sub-Station Land and Buildings.

Sub-Station Labor.

DIRECT CURRENT.

Interest, Depreciation, and Repairs on such increased cost of Engines and Generators, as is occasioned by their being fitted to furnish increased voltage.

Cost of fuel to produce output representing difference between losses in direct current feeders and those in alternating current feeders and transformers.