It would also seem that although the general method of placing experimental lamps in multiple arc was known at this period, the idea of "drop" of electrical pressure was imperfectly understood, if, indeed, realized at all, as a most important item to be considered in attempting the solution of the problem. As a matter of fact, the investigators preceding Edison do not seem to have conceived the idea of a "system" at all; hence it is not surprising to find them far astray from the correct theory of subdivision of the electric current. It may easily be believed that the term "subdivision" was a misleading one to these early experimenters. For a very short time Edison also was thus misled, but as soon as he perceived that the problem was one involving the MULTIPLICATION OF CURRENT UNITS, his broad conception of a "system" was born.

Generally speaking, all conductors of electricity offer more or less resistance to the pa.s.sage of current through them and in the technical terminology of electrical science the word "drop" (when used in reference to a system of distribution) is used to indicate a fall or loss of initial electrical pressure arising from the resistance offered by the copper conductors leading from the source of energy to the lamps.

The result of this resistance is to convert or translate a portion of the electrical energy into another form--namely, heat, which in the conductors is USELESS and wasteful and to some extent inevitable in practice, but is to be avoided and remedied as far as possible.

It is true that in an electric-lighting system there is also a fall or loss of electrical pressure which occurs in overcoming the much greater resistance of the filament in an incandescent lamp. In this case there is also a translation of the energy, but here it accomplishes a USEFUL purpose, as the energy is converted into the form of light through the incandescence of the filament. Such a conversion is called "work"

as distinguished from "drop," although a fall of initial electrical pressure is involved in each case.

The percentage of "drop" varies according to the quant.i.ty of copper used in conductors, both as to cross-section and length. The smaller the cross-sectional area, the greater the percentage of drop. The practical effect of this drop would be a loss of illumination in the lamps as we go farther away from the source of energy. This may be ill.u.s.trated by a simple diagram in which G is a generator, or source of energy, furnishing current at a potential or electrical pressure of 110 volts; 1 and 2 are main conductors, from which 110-volt lamps, L, are taken in derived circuits. It will be understood that the circuits represented in Fig. 1 are theoretically supposed to extend over a large area. The main conductors are sufficiently large in cross-section to offer but little resistance in those parts which are comparatively near the generator, but as the current traverses their extended length there is a gradual increase of resistance to overcome, and consequently the drop increases, as shown by the figures. The result of the drop in such a case would be that while the two lamps, or groups, nearest the generator would be burning at their proper degree of illumination, those beyond would give lower and lower candle-power, successively, until the last lamp, or group, would be giving only about two-thirds the light of the first two.

In other words, a very slight drop in voltage means a disproportionately great loss in illumination. Hence, by using a primitive system of distribution, such as that shown by Fig. 1, the initial voltage would have to be so high, in order to obtain the proper candle-power at the end of the circuit, that the lamps nearest the generator would be dangerously overheated. It might be suggested as a solution of this problem that lamps of different voltages could be used. But, as we are considering systems of extended distribution employing vast numbers of lamps (as in New York City, where millions are in use), it will be seen that such a method would lead to inextricable confusion, and therefore be absolutely out of the question. Inasmuch as the percentage of drop decreases in proportion to the increased cross-section of the conductors, the only feasible plan would seem to be to increase their size to such dimensions as to eliminate the drop altogether, beginning with conductors of large cross-section and tapering off as necessary.

This would, indeed, obviate the trouble, but, on the other hand, would give rise to a much more serious difficulty--namely, the enormous outlay for copper; an outlay so great as to be absolutely prohibitory in considering the electric lighting of large districts, as now practiced.

Another diagram will probably make this more clear. The reference figures are used as before, except that the horizontal lines extending from square marked G represent the main conductors. As each lamp requires and takes its own proportion of the total current generated, it is obvious that the size of the conductors to carry the current for a number of lamps must be as large as the sum of ALL the separate conductors which would be required to carry the necessary amount of current to each lamp separately. Hence, in a primitive multiple-arc system, it was found that the system must have conductors of a size equal to the aggregate of the individual conductors necessary for every lamp. Such conductors might either be separate, as shown above (Fig.

2), or be bunched together, or made into a solid tapering conductor, as shown in the following figure:

The enormous ma.s.s of copper needed in such a system can be better appreciated by a concrete example. Some years ago Mr. W. J. Jenks made a comparative calculation which showed that such a system of conductors (known as the "Tree" system), to supply 8640 lamps in a territory extending over so small an area as nine city blocks, would require 803,250 pounds of copper, which at the then price of 25 cents per pound would cost $200,812.50!

Such, in brief, was the state of the art, generally speaking, at the period above named (1878-79). As early in the art as the latter end of the year 1878, Edison had developed his ideas sufficiently to determine that the problem of electric illumination by small units could be solved by using incandescent lamps of high resistance and small radiating surface, and by distributing currents of constant potential thereto in multiple arc by means of a ramification of conductors, starting from a central source and branching therefrom in every direction. This was an equivalent of the method ill.u.s.trated in Fig. 3, known as the "Tree"

system, and was, in fact, the system used by Edison in the first and famous exhibition of his electric light at Menlo Park around the Christmas period of 1879. He realized, however, that the enormous investment for copper would militate against the commercial adoption of electric lighting on an extended scale. His next inventive step covered the division of a large city district into a number of small sub-stations supplying current through an interconnected network of conductors, thus reducing expenditure for copper to some extent, because each distribution unit was small and limited the drop.

His next development was the radical advancement of the state of the art to the feeder system, covered by the patent now under discussion.

This invention swept away the tree and other systems, and at one bound brought into being the possibility of effectively distributing large currents over extended areas with a commercially reasonable investment for copper.

The fundamental principles of this invention were, first, to sever entirely any direct connection of the main conductors with the source of energy; and, second, to feed current at a constant potential to central points in such main conductors by means of other conductors, called "feeders," which were to be connected directly with the source of energy at the central station. This idea will be made more clear by reference to the following simple diagram, in which the same letters are used as before, with additions:

In further elucidation of the diagram, it may be considered that the mains are laid in the street along a city block, more or less distant from the station, while the feeders are connected at one end with the source of energy at the station, their other extremities being connected to the mains at central points of distribution. Of course, this system was intended to be applied in every part of a district to be supplied with current, separate sets of feeders running out from the station to the various centres. The distribution mains were to be of sufficiently large size that between their most extreme points the loss would not be more than 3 volts. Such a slight difference would not make an appreciable variation in the candle-power of the lamps.

By the application of these principles, the inevitable but useless loss, or "drop," required by economy might be incurred, but was LOCALIZED IN THE FEEDERS, where it would not affect the uniformity of illumination of the lamps in any of the circuits, whether near to or remote from the station, because any variations of loss in the feeders would not give rise to similar fluctuations in any lamp circuit. The feeders might be operated at any desired percentage of loss that would realize economy in copper, so long as they delivered current to the main conductors at the potential represented by the average voltage of the lamps.

Thus the feeders could be made comparatively small in cross-section. It will be at once appreciated that, inasmuch as the mains required to be laid ONLY along the blocks to be lighted, and were not required to be run all the way to the central station (which might be half a mile or more away), the saving of copper by Edison's feeder system was enormous.

Indeed, the comparative calculation of Mr. Jenks, above referred to, shows that to operate the same number of lights in the same extended area of territory, the feeder system would require only 128,739 pounds of copper, which, at the then price of 25 cents per pound, would cost only $39,185, or A SAVING of $168,627.50 for copper in this very small district of only nine blocks.

An additional ill.u.s.tration, appealing to the eye, is presented in the following sketch, in which the comparative ma.s.ses of copper of the tree and feeder systems for carrying the same current are shown side by side:

XII. THE THREE-WIRE SYSTEM

THIS invention is covered by United States Patent No. 274,290, issued to Edison on March 20, 1883. The object of the invention was to provide for increased economy in the quant.i.ty of copper employed for the main conductors in electric light and power installations of considerable extent at the same time preserving separate and independent control of each lamp, motor, or other translating device, upon any one of the various distribution circuits.

Immediately prior to this invention the highest state of the art of electrical distribution was represented by Edison's feeder system, which has already been described as a straight parallel or multiple-arc system wherein economy of copper was obtained by using separate sets of conductors--minus load--feeding current at standard potential or electrical pressure into the mains at centres of distribution.

It should be borne in mind that the incandescent lamp which was accepted at the time as a standard (and has so remained to the present day) was a lamp of 110 volts or thereabouts. In using the word "standard,"

therefore, it is intended that the same shall apply to lamps of about that voltage, as well as to electrical circuits of the approximate potential to operate them.

Briefly stated, the principle involved in the three-wire system is to provide main circuits of double the standard potential, so as to operate standard lamps, or other translating devices, in multiple series of two to each series; and for the purpose of securing independent, individual control of each unit, to divide each main circuit into any desired number of derived circuits of standard potential (properly balanced) by means of a central compensating conductor which would be normally neutral, but designed to carry any minor excess of current that might flow by reason of any temporary unbalancing of either side of the main circuit.

Reference to the following diagrams will elucidate this principle more clearly than words alone can do. For the purpose of increased lucidity we will first show a plain multiple-series system.

In this diagram G<1s> and G<2s> represent two generators, each producing current at a potential of 110 volts. By connecting them in series this potential is doubled, thus providing a main circuit (P and N) of 220 volts. The figures marked L represent eight lamps of 110 volts each, in multiple series of two, in four derived circuits. The arrows indicate the flow of current. By this method each pair of lamps takes, together, only the same quant.i.ty or volume of current required by a single lamp in a simple multiple-arc system; and, as the cross-section of a conductor depends upon the quant.i.ty of current carried, such an arrangement as the above would allow the use of conductors of only one-fourth the cross-section that would be otherwise required. From the standpoint of economy of investment such an arrangement would be highly desirable, but considered commercially it is impracticable because the principle of independent control of each unit would be lost, as the turning out of a lamp in any series would mean the extinguishment of its companion also.

By referring to the diagram it will be seen that each series of two forms one continuous path between the main conductors, and if this path be broken at any one point current will immediately cease to flow in that particular series.

Edison, by his invention of the three-wire system, overcame this difficulty entirely, and at the same time conserved approximately, the saving of copper, as will be apparent from the following ill.u.s.tration of that system, in its simplest form.

The reference figures are similar to those in the preceding diagram, and all conditions are also alike except that a central compensating, or balancing, conductor, PN, is here introduced. This is technically termed the "neutral" wire, and in the discharge of its functions lies the solution of the problem of economical distribution. Theoretically, a three-wire installation is evenly balanced by wiring for an equal number of lamps on both sides. If all these lamps were always lighted, burned, and extinguished simultaneously the central conductor would, in fact, remain neutral, as there would be no current pa.s.sing through it, except from lamp to lamp. In practice, however, no such perfect conditions can obtain, hence the necessity of the provision for balancing in order to maintain the principle of independent control of each unit.

It will be apparent that the arrangement shown in Fig. 2 comprises practically two circuits combined in one system, in which the central conductor, PN, in case of emergency, serves in two capacities--namely, as negative to generator G<1s> or as positive to generator G<2s>, although normally neutral. There are two sides to the system, the positive side being represented by the conductors P and PN, and the negative side by the conductors PN and N. Each side, if considered separately, has a potential of about 110 volts, yet the potential of the two outside conductors, P and N, is 220 volts. The lamps are 110 volts.

In practical use the operation of the system is as follows: If all the lamps were lighted the current would flow along P and through each pair of lamps to N, and so back to the source of energy. In this case the balance is preserved and the central wire remains neutral, as no return current flows through it to the source of energy. But let us suppose that one lamp on the positive side is extinguished. None of the other lamps is affected thereby, but the system is immediately thrown out of balance, and on the positive side there is an excess of current to this extent which flows along or through the central conductor and returns to the generator, the central conductor thus becoming the negative of that side of the system for the time being. If the lamp extinguished had been one of those on the negative side of the system results of a similar nature would obtain, except that the central conductor would for the time being become the positive of that side, and the excess of current would flow through the negative, N, back to the source of energy. Thus it will be seen that a three-wire system, considered as a whole, is elastic in that it may operate as one when in balance and as two when unbalanced, but in either event giving independent control of each unit.

For simplicity of ill.u.s.tration a limited number of circuits, shown in Fig. 2, has been employed. In practice, however, where great numbers of lamps are in use (as, for instance, in New York City, where about 7,000,000 lamps are operated from various central stations), there is constantly occurring more or less change in the balance of many circuits extending over considerable distances, but of course there is a net result which is always on one side of the system or the other for the time being, and this is met by proper adjustment at the appropriate generator in the station.

In order to make the explanation complete, there is presented another diagram showing a three-wire system unbalanced:

The reference figures are used as before, but in this case the vertical lines represent branches taken from the main conductors into buildings or other s.p.a.ces to be lighted, and the loops between these branch wires represent lamps in operation. It will be seen from this sketch that there are ten lamps on the positive side and twelve on the negative side. Hence, the net result is an excess of current equal to that required by two lamps flowing through the central or compensating conductor, which is now acting as positive to generator G<2s> The arrows show the a.s.sumed direction of flow of current throughout the system, and the small figures at the arrow-heads the volume of that current expressed in the number of lamps which it supplies.

The commercial value of this invention may be appreciated from the fact that by the application of its principles there is effected a saving of 62 1/2 per cent. of the amount of copper over that which would be required for conductors in any previously devised two-wire system carrying the same load. This arises from the fact that by the doubling of potential the two outside mains are reduced to one-quarter the cross-section otherwise necessary. A saving of 75 per cent. would thus be a.s.sured, but the addition of a third, or compensating, conductor of the same cross-section as one of the outside mains reduces the total saving to 62 1/2 per cent.

The three-wire system is in universal use throughout the world at the present day.

XIII. EDISON'S ELECTRIC RAILWAY

AS narrated in Chapter XVIII, there were two electric railroads installed by Edison at Menlo Park--one in 1880, originally a third of a mile long, but subsequently increased to about a mile in length, and the other in 1882, about three miles long. As the 1880 road was built very soon after Edison's notable improvements in dynamo machines, and as the art of operating them to the best advantage was then being developed, this early road was somewhat crude as compared with the railroad of 1882; but both were practicable and serviceable for the purpose of hauling pa.s.sengers and freight. The scope of the present article will be confined to a description of the technical details of these two installations.

The ill.u.s.tration opposite page 454 of the preceding narrative shows the first Edison locomotive and train of 1880 at Menlo Park.

For the locomotive a four-wheel iron truck was used, and upon it was mounted one of the long "Z" type 110-volt Edison dynamos, with a capacity of 75 amperes, which was to be used as a motor. This machine was laid on its side, its armature being horizontal and located toward the front of the locomotive.

We now quote from an article by Mr. E. W. Hammer, published in the Electrical World, New York, June 10, 1899, and afterward elaborated and reprinted in a volume ent.i.tled Edisonia, compiled and published under the auspices of a committee of the a.s.sociation of Edison Illuminating Companies, in 1904: "The gearing originally employed consisted of a friction-pulley upon the armature shaft, another friction-pulley upon the driven axle, and a third friction-pulley which could be brought in contact with the other two by a suitable lever. Each wheel of the locomotive was made with metallic rim and a centre portion made of wood or papier-mache. A three-legged spider connected the metal rim of each front wheel to a bra.s.s hub, upon which rested a collecting brush.

The other wheels were subsequently so equipped. It was the intention, therefore, that the current should enter the locomotive wheels at one side, and after pa.s.sing through the metal spiders, collecting brushes and motor, would pa.s.s out through the corresponding brushes, spiders, and wheels to the other rail."

As to the road: "The rails were light and were spiked to ordinary sleepers, with a gauge of about three and one-half feet. The sleepers were laid upon the natural grade, and there was comparatively no effort made to ballast the road. . . . No special precautions were taken to insulate the rails from the earth or from each other."

The road started about fifty feet away from the generating station, which in this case was the machine shop. Two of the "Z" type dynamos were used for generating the current, which was conveyed to the two rails of the road by underground conductors.

On Thursday, May 13, 1880, at 4 o'clock in the afternoon, this historic locomotive made its first trip, packed with as many of the "boys" as could possibly find a place to hang on. "Everything worked to a charm, until, in starting up at one end of the road, the friction gearing was brought into action too suddenly and it was wrecked. This accident demonstrated that some other method of connecting the armature with the driven axle should be arranged.

"As thus originally operated, the motor had its field circuit in permanent connection as a shunt across the rails, and this field circuit was protected by a safety-catch made by turning up two bare ends of the wire in its circuit and winding a piece of fine copper wire across from one bare end to the other. The armature circuit had a switch in it which permitted the locomotive to be reversed by reversing the direction of current flow through the armature.

"After some consideration of the gearing question, it was decided to employ belts instead of the friction-pulleys." Accordingly, Edison installed on the locomotive a system of belting, including an idler-pulley which was used by means of a lever to tighten the main driving-belt, and thus power was applied to the driven axle. This involved some slipping and consequent burning of belts; also, if the belt were prematurely tightened, the burning-out of the armature.

This latter event happened a number of times, "and proved to be such a serious annoyance that resistance-boxes were brought out from the laboratory and placed upon the locomotive in series with the armature.

This solved the difficulty. The locomotive would be started with these resistance-boxes in circuit, and after reaching full speed the operator could plug the various boxes out of circuit, and in that way increase the speed." To stop, the armature circuit was opened by the main switch and the brake applied.

This arrangement was generally satisfactory, but the resistance-boxes scattered about the platform and foot-rests being in the way, Edison directed that some No. 8 B. & S. copper wire be wound on the lower leg of the motor field-magnet. "By doing this the resistance was put where it would take up the least room, and where it would serve as an additional field-coil when starting the motor, and it replaced all the resistance-boxes which had heretofore been in plain sight. The boxes under the seat were still retained in service. The coil of coa.r.s.e wire was in series with the armature, just as the resistance-boxes had been, and could be plugged in or out of circuit at the will of the locomotive driver. The general arrangement thus secured was operated as long as this road was in commission."