Popular Science Monthly/Volume 43/October 1893/Electricity at the World's Fair I
|ELECTRICITY AT THE WORLD'S FAIR.|
By CHARLES M. LUNGREN.
A PERIOD of but seventeen years separates the first great American exhibition from the second, yet what a vast difference between the two in the display of electrical appliances! The Centennial was not indeed without its electrical wonders, but these were unobtrusive and formed but isolated examples in an industrial domain which yet remained to be cultivated. Electricity had not then been brought home to the attention and interest of the thousands by multiplied daily use. It made no appeal to the imagination, and the immediate future that was to open for it was hardly dreamed of even by those in the vanguard of electrical discovery. The telephone here made its début; the quadruplex telegraph, but recently put into commercial service, was here shown for the first time; and the dynamos and arc lamps of Wallace were on exhibition. The Gramme machine, which was shortly to play such an important part in the commercial development of the electric light, and to prove such a stimulus to the inventors of electric apparatus, was also to be seen here, but beyond these electricity was very little in evidence at the earlier exposition. At the Columbian it is omnipresent. It is called upon to do the lighting of the great buildings and grounds, to the exclusion of all other means of illumination; to drive the trains of the intramural railway which winds through the exposition inclosure; to propel the graceful launches which glide through its waterways; to furnish the power distributed throughout the various buildings, and to make itself known in innumerable decorative effects. Grown too large to have a place merely, along with other industries, in a general building, it has a temple of its own, which is filled with the manifold applications of this strange and subtile agent to the arts and conveniences of life. And even this is inadequate to the demands it has made upon the space of the exposition, for what may rightly be considered two of the main exhibits—the great alternating lighting plant and the direct-current plant of the intramural railway—are without the inclosure of the Electricity Building, the one in Machinery Hall and the other in a structure by itself.
Complete and varied as the Columbian electricity exhibit is, it is not primarily an exhibition of novelties. It is rather a summing up of our progress to date—a slice taken from the far larger exhibit which everywhere surrounds us and is helping to do the daily work of the world in shop and factory and mine, on our streets and in our homes. Much of that to be seen is already familiar, but it is not on that account devoid of either interest or instruction. In the actual industrial world the processes and appliances of an art are scattered and not easily accessible, and it can only be studied piecemeal and with difficulty. A great exposition, on the other hand, gives an opportunity for studying an art in its entirety, and thus enables an observer to gain a clear conception both of the attained progress and the direction of future development. This opportunity is afforded by the Columbian in a marked degree. Illustrative examples are to be found in it of all the more notable steps of progress, and many of the exhibits are remarkably full and complete.
The visitor will find, for instance, an opportunity to study the telephone from its earlier form up to the present standard instruments, and to inspect and perhaps understand for the first time the central station system, by means of which he is daily put into communication with other subscribers. He will see in actual working what he will have but little opportunity to see elsewhere, and which, to judge by the crowds which throng about it, appeals strongly to the curiosity and interest of the average visitor—the delicate siphon recorder of Sir William Thomson, by which all the cable messages of the world are received. And he may perhaps wonder that any one should be able to interpret into intelligible signals the curious zigzag scrawl which the siphon leaves upon the moving band of paper. He will also see a set of quadruplex instruments and be able to understand by actual inspection much better than by mere description this most important of telegraphic appliances. He will also be able to see in the Western Union exhibit the original receiving instrument of Morse, made of a triangle of wood hinged at its apex to an artist's canvas frame, and carrying at the center of its lower side a pencil, with which a zigzag tracing can be made upon a moving band of paper beneath, as the triangle is swung to and fro under the impulses of an electro-magnet. The visitor will also have an opportunity to examine the new telautograph of Prof. Elisha Gray, by means of which the written word, it is promised us, may be transmitted to a distance with the same facility that the spoken word now is by telephone. Turning from this lighter and more delicate form of apparatus, the visitor will find a very complete display of the class of applications that has brought electricity into such close contact with the daily life of the masses in recent years. From the great Westinghouse lighting installation and from the power plant of the intramural he will get some adequate idea of a modern central-station equipment, while from the illustration of long-distance power transmission he will be able to comprehend one of the directions in which electricity holds out the greatest promise for the future. In the exhibits of electric welding and forging he will learn of the help the electric current is giving to the metal worker, and in that of cooking and heating the attempts that are being made to displace with electrical appliances the kitchen range and the hot-air furnace.The most prominent exhibit of electricity at the fair is undoubtedly the lighting of the Exposition itself. This is carried out along lines already well established, and is remarkable chiefly for the great scale upon which it is planned and executed. Nearly five thousand arc lamps and a hundred thousand incandescents have been called into requisition for the illumination of the grounds and buildings. The placing of these required, no doubt, a great deal of detail work and called for nice discrimination in adapting means to ends, but involved no electrical problems of especial novelty. The lighting of the big Manufactures Building, with its thirty acres of main floor space and ten acres of galleries, presented the most difficult problem to the Exposition authorities, but this has been successfully solved by the use of the arc lamp hung from immense coronas along the central line of the building, supplemented by individual lamps in the corridors, galleries, and separate rooms. The coronas are hung a hundred and forty feet from the floor and sixty feet from the crown of the great arched roof which spans the structure, and are of colossal size, the central one being seventy-five feet in diameter and the two which flank it on either side sixty feet. Something over four hundred lamps are disposed of in this way, while to these are added some twelve hundred more to complete the lighting of this great inclosure. The incandescent, so flexible in the hands of the decorator, has been used very effectively to outline the buildings and the waterways of the Exposition, in addition to their use in interior illumination.
Fig. 1.-Westinghouse Ten-thousand Light Alternator. (From the Electrical Engineeer.)
The power and machinery which give vitality to this vast array of lights are to be found in Machinery Hall, and constitute one of the chief electrical exhibits. The most striking feature of this exhibit is the great Westinghouse alternating plant, which supplies the current for the incandescent lamps. It consists of twelve enormous alternating-current generators, each having a capacity of ten thousand sixteen candle-power lamps and requiring a thousand horse power apiece to drive them. They are arranged in two groups, the first six of which are coupled direct to Westinghouse upright engines. Of the remaining six, four are driven separately by different makes of engines, and two are belt-driven in tandem fashion by an Allis-Corliss cross-compound engine nominally rated at two thousand horse power, but which may be worked up to three thousand horse power upon occasion. This engine is one of two of the same type and by the same maker, the other one being stationed in the power house of the intramural railway, and is regarded as a very fine example of modern steam engineering. The alternating generators themselves are of a type only recently devised, in which there is a double row of field poles, and a double set of armature coils, by means of which the machines can supply two separate circuits for the requirements of incandescent lighting, or furnish what is known as a two-phase current for use with alternating-current motors. The current as generated has a pressure of two thousand volts, which is reduced down, at the point of consumption by means of converters, to fifty or a hundred volts.
Besides the "alternators," as these machines are technically termed, there are a large number of direct-current machines in this building supplying the currents to the arc lamps, and the motors scattered through the various buildings. The plan adopted by the Exposition authorities has been to confine the engines and boilers to Machinery Hall, so that all the power required in the Exposition except that for the intramural railway, is generated here and transmitted by electricity through underground conduits to the place where it is to be used. The exhibition is therefore an illustration of the electric transmission of power upon a large scale, and should furnish a basis for the collection of instructive data.
The feature of the Exposition which will command the most interest of any of those in which light plays a prominent part will probably be the electric fountains. Fountains of this character have been features of a number of exhibitions since 188-i, when the first one, designed by Sir Francis Bolton, was shown at the Healtheries in London, but those at Chicago are upon a much greater scale than any heretofore attempted. The principle of operation is that of throwing a powerful beam of light from below upward along the axis of the water jet, the lamps being placed in a chamber under the fountain provided with a transparent roof. Color effects are produced by the interposition of glass screens in the path of the beam. In the present fountains, which rise from basins sixty feet in diameter, the underground chamber is built upon piling, a construction rendered necessary by the shifting sand foundation. The piling is of unequal length, the shorter piles supporting the floor structure, and the longer, which project through and are seen as pillars in the room, the roof. The water nozzles are grouped to form nineteen composite jets, and as many powerful
Fig. 2.—Electric Fountains.
reflectors are arranged to throw a beam of light along the axis of each group. It is estimated that the beam of these powerful lights has a luminous intensity of two hundred and fifty thousand candles. The size of the fountains may be appreciated by the fact that they require a twenty-four-inch supply main conveying water at a hundred pounds pressure, and have a consumption of nearly twenty-one million gallons per twenty-four hours. The central jet or grand geyser formed by a two-inch stream rises to a height of a hundred and fifty feet. The color screens are in the shape of fan blades arranged to rotate horizontally, and are grouped so as to be capable of producing an almost unlimited combination of color effects.
If any demonstration were needed of the capacity of the electric motor to take the place of steam on such roads as the elevated in New York and Chicago, or of the enormous superiority of electric traction in the matter of cleanliness, comfort, and freedom from noise, the intramural would furnish it to the satisfaction of any impartial observer. This road is a double-track elevated structure something over three miles in length, which forms the highway of communication between the different buildings. It is purposely laid out with many an unnecessary curve, to accentuate the conditions of actual travel, and demonstrate the ability of electric traction to do its work satisfactorily under extreme conditions. The trains are made up of a motor car and three trailers, all four cars being arranged to seat passengers, the space occupied by the motorman at the extreme front end of the motor car being no greater than that of the ordinary trolley car. The cars are open, with the seats extending clear across the car body, each pair facing upon the entrance aisles. These aisles are closed by sliding gates, which are connected so that all those on one side of the car may be opened or closed at the same time by the movement of a lever at the end of the car. This construction might be very readily adapted to a closed car, and would seem to be admirably suited to cars having the phenomenally heavy traffic of those on the elevated roads of New York. A very noticeable feature of the cars is the perfection of the lighting. Too often, when electricity has been called upon for the lighting of public conveyances, there has been but little improvement over former results, due both to the bad habit of placing the lights in the aisle spaces and stinting in the candle power. In the intramural cars particular attention has been paid to securing abundant light, the lamps being up to candle power and placed in the most effective position along the sides near the car roof.
The electrical equipment of the motor car consists of four motors having a combined capacity of over five hundred horse power. These are geared to the axles by a single reduction gear, and take their current from side rails through the medium of sliding shoes. The side rail was adopted in preference to a central one on account of the greater simplicity of the switching arrangements, the facility in getting at the contact shoes, and the very limited space between the motor and road bed in which to make a satisfactory rail contact. The return path for the current is through the traffic rails and iron girders of the elevated structure, the rails being copper-banded at the joints and joined by bands of the same material to the girders. Feeder rails extend from the power house for three fifths of the length of the line and are cross connected to the supply rails at every rail joint. The train equipment of the road consists of eighteen trains, weighing when loaded about ninety-six tons each, the motor car accounting for thirty tons of this weight and the other cars for twenty-two tons each.The central figure of the power-house equipment is the great two-thousand-horse-power generator from the shops of the General Electric Company, said to be the largest machine yet built. It occupies the middle space of the power house and is driven by an Allis-Corliss cross-compound engine, which is a duplicate of the one in the Westinghouse plant in Machinery Hall. It is a direct-current machine of what is known as the multipolar type. This is a type of machine which has been developed in recent years in response to the increasing demands of railway power and central lighting stations for larger units of power. In machines of the power desired slow speed becomes essential, and this requirement has resulted in radically transforming the design of the dynamos. The two-pole field magnet, common in all machines a few years back, has given place to a multipolar one, generally made in the form of a ring-shaped yoke with inwardly protruding pole pieces, though this construction has been reversed in some large generators constructed by Siemens, in which the field poles radiate from a central hub, and the armature, made in the form of a flattened ring or band, is placed on the outside, its outer surface constituting the commutator upon which the brushes bear. A fine example of this machine coupled direct to a thousand-horse-power triple-expansion upright engine is to be seen in Machinery Hall. In the intramural generator the field consists of two massive semicircles of cast steel, bolted together, the lower of which is provided with supporting feet. This yoke is fifteen feet in diameter and three broad and with its twelve poles weighs over forty tons. The armature is what is known as the ironclad type, and is ten feet and a half in diameter, and weighs complete about thirty-five tons. The ironclad type of armature now used upon all railway motors and large generators is a comparatively recent development, and possesses marked advantages both mechanically and electrically. Its characteristic feature is the imbedding of the coils in the laminated iron core, either by forming tubular passages through this core near the edge or making it with open slots narrowed at the mouth to securely hold the
Fig. 3.—Intramural railway generator
coils in place. It has the mechanical advantage of presenting a smooth exterior surface which can be turned true, and of holding the winding in such a way that it can not become displaced, as is possible with coils wound over the core and bound in place by a wrapping of wire. Electrically it has the advantage of materially diminishing the air gap—the space between the face of the armature and the field poles—and hence allowing the coils to move in an intenser magnetic field. The armature core is carried by a cast-iron spider weighing over fifteen tons which is keyed directly to the shaft of the driving engine. The brush holders, of which there are twelve sets, corresponding to the number of field poles, are mounted upon a yoke supported at one side of the field magnet frame. They are moved into position by means of a shifting gear operated by a hand wheel and are readily accessible from a stairway passing over the shaft. The machine is designed to run at seventy-five revolutions a minute and furnish a current under a pressure of six hundred volts. It has an electrical capacity of fifteen hundred kilowatts, and is claimed to have an efficiency of ninety-six per cent. This ponderous machine was found to be much too large and heavy to be shipped in its complete form, and was accordingly forwarded from the factory in parts and assembled upon its present foundation.
An appreciation of its size and capacity may be gained by remembering what the standards of size were only ten years ago when the Edison "Jumbo" was put to work in the first New York Central station. This machine, which created a veritable sensation at the Paris Exposition of 1881 on account of its immense size, required only a hundred and twenty-five horse power to drive it when working at its normal load. It had a capacity of less than one hundred kilowatts, which is but a fifteenth of that of the present "Jumbo," and weighed very much more in proportion to its output. It is to be seen in the exhibit of the General Electric Company, where it is rightly given a place of honor as the precursor of the race of modern direct-connected dynamos.
While a motor car will answer admirably for the lighter forms of electric traction, the invasion of the domain of the steam railroad, which electricians are already contemplating, will necessitate the design and construction of special electric locomotives. These have already been used quite largely in mine work, and a number of electrical constructors have designed and built such machines of moderate power, but the first one of any considerable size and designed for high speed is one built at the Lynn shops of the General Electric Company and shown in the Transportation Building at the Fair. It is a thirty-ton locomotive intended for a normal speed of thirty miles per hour, and is of sufficient power for light passenger and freight traffic. It is mounted on four forty-four-inch wheels and is propelled by two gearless motors suspended in such a way as to leave the wheels free to adjust themselves to the irregularities of the roadbed. This method of suspension consists in mounting the motors upon spiral springs resting on the side frames of the locomotive truck, and the armatures upon hollow shafts through which the axles of the wheels pass, the connection between the two being made by universal
Fig. 4.—General Electric Thirty-ton Electric Locomotive.
couplings. The commodious cab is constructed of sheet iron, finished in the interior in hard woods, and is given a curved shape to diminish as far as possible the air resistance. The braking power is furnished by compressed air supplied by a special electrical air compressor, and the whistle is operated by the same means. The use of the electric locomotive is not yet practicable on long lines on account of the great cost of long feeders, but this bar to its employment is certain to be overcome in time. Wherever traffic is dense and the distance to be traversed not too great, the conditions are already present for the advent of this form of locomotive; and when we recall the rapidity with which city and suburban railways have spread, we can not doubt that once the problems of electric railway engineering are worked out, and the necessary preliminary work of demonstration gone through with, we will witness an equally rapid extension of electric traction to the steam highways of the world.
Ever since Faure started electricians on the quest of an economical storage battery, the peculiar fitness of such batteries as a source of power for pleasure boats has been recognized, and they have frequently been used for such purpose. The slow development
Fig. 5.—Electric Launch.
of this type of battery into an efficient instrument, the absence of any means of getting the batteries recharged, and the much greater cost of this method of propulsion, have heretofore acted to effectually prevent its adoption by the owners of such craft. But after riding in the launches of the exhibition one can not help but wish for the early dawn of the day in which this ideal method of water propulsion becomes generally available. The exhibition launches are of a very graceful model, about thirty-six feet long and six feet breadth of beam. They are designed to carry thirty passengers, and have motors capable of exerting four horse power. The batteries are placed beneath the seats and flooring, and as the motor is also beneath the flooring the cockpit is clear of any obstruction. Each launch carries seventy-eight battery cells, which, by appropriate connections, may be grouped in various combinations. For the regular operation of the boats the cells are grouped in three divisions containing twenty-six cells each, arranged in series.
The batteries are charged for a run of ten to twelve hours, and are then recharged at the power station of the fleet in from five to seven hours. The launches run over a course of about three miles, at a speed of six miles an hour, and make landings at the principal buildings, all of which front upon the waterways.
To the engineer and to those who desire to know the trend of electrical development, the most interesting exhibit at the Fair will doubtless be the apparatus designed to show the long-distance transmission of power. Almost at the beginning of the modern electrical era, dreams were indulged in of the command which electricity was to give us of the natural sources of power. Marcel Deprez, at the Paris Exposition of 1881, had in operation a system of power transmission, and similar attempts have been made at every important exposition since, the most elaborate having been that at the Frankfort Exposition of two years ago. Of the importance of the economic transmission of power over long distances there can not be two opinions. The modern world has come to rest down upon an abundant and cheap supply of power in such a measure that without it civilization itself would go by the board. Statisticians have frequently shown that the coal supply, while large and ample for present needs, is not only exhaustible, but is being encroached upon at such a rate as to make its conservation a matter of grave concern. Electric transmission of power, by opening up to civilization the enormous supply of power of the waterfalls and running streams of the earth, will be able to postpone indefinitely the evil day that would be ushered in by the failure or material decrease of our fuel supply. To be of avail, however, such transmission must be economical, not only in the percentage of utilizable power sent through the line, but in the investment which must be made to realize it. So long as we were dependent upon the direct current, but little progress could be expected in this important problem. It has only been, therefore, in the last few years that the art was ripe for the taking up of this subject in a serious spirit, and with any hope of a real solution. The direct-current dynamo, handicapped with the commutator, is necessarily limited to supplying currents of relatively low voltage; the economic transmission of power demands the use of currents of small volume and very high pressure. This means small line conductors, and hence a relatively small investment. It means also a small loss in heating the line, since the heating power of the current varies as the square of the volume transmitted.
It is only by the alternating system of distribution that we can realize this essential condition of economy. We have here no such limit to the electrical pressure in the generating apparatus as in the direct-current system, and through the medium of the converter it becomes possible to vary the two elements of electrical energy—current volume and pressure—to suit the most widely differing applications. It is this latter feature of the system which gives it its great range and flexibility, and its consequent economic value. It enables us, for instance, to generate a current of a certain voltage at the machine, then to raise this to ten, twenty, or fifty times the original pressure for transmission through the line, and then at the far end to step down to as low a pressure as we may want—a pressure suitable for entering dwellings, offices, and shops, and safe in the hands of the consumer. These successive transformations and retransformations, it should be noted, are effected in the simplest kind of a way. They involve no machinery with moving parts, but simply coils of wire placed in such relation to each other that the currents passing in one induce similar currents in the other. The practical value of this system arose with the discovery that the induction coil, like the dynamo, is reversible. This coil had long been used to transform a current of considerable volume and low pressure into one of very great pressure and small volume. The construction which enabled this to be done consisted in making the primary coil with a few turns of stout wire; and the secondary—that on which the induced current was produced—of a great many turns of fine wire. It was presently discovered, however, that this mode of operation might be reversed, and that, by passing a high-tension current of small volume through many turns of wire, a current of large volume and low pressure could be induced in a secondary circuit of few turns, and that the pressure and volume of the induced current in relation to that of the primary one depended only on the relative number of wire turns in the two circuits. If, for instance, the primary and secondary coils contained the same number of turns, the pressure and volume of the induced current would be precisely the same as the primary one. If, on the other hand, the induced circuit contained ten times the number of coils of the primary, the current in it would have a tenth of the volume and ten times the pressure of the primary one, while if the relation of the two circuits were reversed the induced current would have its volume increased to ten times and its pressure reduced to one tenth of that flowing in the primary.
In the field of lighting this method of electric distribution has taken a leading place, and it is no longer questioned that it is destined to displace entirely all methods of direct-current supply. It has heretofore found but little application to power transmission, because it has lacked the prime requisite for such a use—a satisfactory motor. This missing link in the chain of appliances necessary to render the system complete has in recent years been supplied by the discoveries and inventions of Mr. Nikola Tesla, whose remarkable experiments with alternating currents of great tension and enormous frequencies have excited such widespread interest among scientific men. To understand the solution given to the alternating-current motor problem by Mr. Tesla it will be necessary to consider briefly the principle of the electric motor and the cause of the rotation of an armature in a magnetic field. If we take a loop of wire forming a closed circuit and place it between the poles of a magnet it will tend, when a current is flowing through it, to set itself so as to inclose the greatest number of lines of force—that is, in a plane at right angles to the line joining the magnetic poles. If the mechanical inertia of the moving loop carry it slightly past its position of equilibrium, and at the same moment the current through the loop be reversed, it will be pulled around by the attraction of the magnetic poles to a new position of equilibrium; and if at each of these positions there takes place a reversal of the current, continuous rotation of the loop will be produced. Where there are many loops, as in actual machines, the pull upon the moving system of coils tending to rotate it will be continuous and equal at all points of the rotation, as, while some coils are approaching and passing through the position of equilibrium, others are in position to have exerted upon them the maximum strain. The pull of the field magnets upon the moving conductors is greatly increased if these be wound over an iron center, as in this case each loop tends to set up magnetic poles in this core in a position at right angles to its plane. Two magnetic poles attract each other when of different polarity and repel each other when of the same polarity. The poles of the iron core are consequently repelled and attracted by the field poles with each change of the direction of the current, and this occurs in exact synchronism with the changing forces acting upon the wire circuits. It must, of course, be understood that with a continuous current the direction of the current in space is always the same. The alternating current impulses set up in the armature coils of the direct-current dynamo are through the device of the commutator made to follow each other in the same direction through the line. Arriving at the motor, these impulses pursue a continuous course through the armature always in the same direction, the positive current always flowing in by one brush and the negative out by the other. The armature coils, however, by reason of their rotation, present their two ends in succession to the positive and negative brushes, and hence are alternately traversed by the current in reverse directions. If now the commutator be suppressed on both generator and motor, it is evident that the armature coils of the motor will be traversed by successive positive and negative electrical impulses at just the right time, if the armature rotates in unison with that of the generator, as both armatures then pass through like portions of their magnetic fields during the same current phase. If these alternating current impulses are not, however, properly timed, they will interfere with each other and the motor armature will not rotate. It is possible, then, to utilize the alternating-current dynamo as a motor, but only on the condition that it runs synchronously with the generator. Evidently it must first be brought up to the speed of the generator before the conditions are realized that will keep it in motion. As a practical motor it has therefore the fatal defect that it will not start of itself, and it has the further one that it is readily thrown out of synchronism by a slight excess of load, and is then speedily brought to a standstill.
Clearly an apparatus so sensitive as this could not be relied upon for commercial work nor expected to stand as a solution of
Fig. 6.—Diagram illustrating Principle of Tesla Motor.
the alternating-current motor problem. When Mr. Tesla took up the question he sought for a new principle of action and found it in what has since come to be known as the multiphase current. He conceived that by providing the armature of his generator and the field of his motor with two more sets of coils, connected so as to form distinct circuits, he would be able to produce a progressive shifting of the magnetic poles of the motor field, and thus drag around an armature capable of magnetic induction and placed within the sphere of influence of his rotating field. This method of operation will be clearly understood from the diagrammatic sketch A (Fig. 6) and the illustration (Fig. 7) showing a diagram of the connections of the motor and generator circuits. Considering the latter first, M is the motor and G the generator. The armature A of the generator is wound with two sets of coils, B and B', brought out through the shaft and connected with the contact rings b b and b' b'. The field magnet of the motor consists of the iron ring R, also wound with two sets of coils, C C
Fig. 1.—Diagram of Tesla Motor Connections.
and C' C', the diametrically opposite coils being connected together in series. The generator coils B and the motor coils C' C' it will be seen are included in one circuit L, and the remaining generator coils B' and the motor coils C C in another circuit L'. The armature of the motor consists simply of a disk of iron cut away at the sides, which becomes a magnet by induction when the motor field is energized. Turning to Fig. 6, B and B' represent the coils of the generator armature and C and C' those of the motor field as in Fig. 7. When the generator coils are in the position shown in the first diagram the coil B is generating no current and B' is generating its maximum amount. The coils C of the motor field, which are included in the circuit of B', are therefore traversed by their greatest current and produce magnetic poles in the iron ring R at N and S. As the generator armature revolves, B is brought to a position in which it is generating current, and when this movement amounts to one eighth of a revolution the circle will be in the position shown in the second diagram of the figure. Each of the pair of coils C and C' will now tend to set up poles in the ring R of the motor ninety degrees from each other, and as their action is equal and opposite, the position of the poles will be determined by the resultant of the magnetic forces acting on the ring, and the poles will therefore be shifted around the ring an eighth of a revolution. They will be shifted another eighth when the generator armature reaches the position shown in the last diagram, and will be successively displaced around the ring R as this armature revolves until a complete revolution has been made, when the parts are in their original position and ready to repeat the same cycle of operations.
The principle of the rotation of the magnetic poles has been applied by Mr. Tesla to a great variety of constructions. He has designed machines in which the field magnetism remains fixed and that of the armature is shifted, and others again in which there is a progressive shifting of the magnetic poles of both the field and armature in opposite directions. He has also found that the motor armature may consist of sets of closed coils, currents being developed in them by induction, and by making the induced portion of the generator stationary and the field revolving he has been able to produce apparatus free from all movable electrical contacts. In operating motors of this character Mr. Tesla usually employed a generator with multiple armature circuits as described above; but in the course of his experiments he discovered that the ordinary continuous or direct current machine could by slight alterations be made to furnish an alternating multiphase current as well as and in addition to the direct current. To accomplish this he found it was only necessary to add to the machine a pair of collector rings for each circuit of the multiphase current, and connect them with the proper armature coils. If, for instance, he desired to produce a two-phase current requiring two circuits from his generator to his motor, one circuit would include a set of coils in the armature of the generator that were passing through the position in which the maximum current was being produced, and the other a set of coils in which at the same time the minimum current was being generated. The phases of the current would then follow each other in the same order as in the previous machines with distinct circuits on the armature. With this form of machine a multiple phase alternating current, it will be seen, can be taken off from the collector rings, while a direct current can be taken from the commutator, and a part or the whole of this direct current be sent through the field coils to energize them and then put to any use for which such currents are suitable.
This machine was later developed into what has come to be known as a rotary transformer. Instead of being driven by power it is driven by one of the forms of current which it is capable of furnishing, the other current being taken off and utilized. For example, if a multiple-phase current is passed into the machine by the collector rings it will be driven as a motor and generate direct or continuous currents. If, on the other hand, it be supplied with direct currents, it will also run as a motor, and deliver multiphase alternating currents This apparatus promises to hold an important place, if not an indispensable one, in any complete system of electric distribution. For many purposes, such as electroplating and electrotyping and all forms of electro-decomposition, the continuous current is essential, and for other uses, in the present state of the art, it can not well be dispensed with. One of these uses is the operation of electric railways. The alternating-current motor though answering many of the requirements of a commercial motor, has one disadvantage in comparison with the motor driven by direct or continuous currents. It has a less powerful starting torque-that is, the pull upon the armature tending to rotate it is much less at the start than in the direct-current machine In railway work a powerful starting torque is of the greatest importance, as a motor is frequently called upon to exert four or five times the power in starting that is needed to keep the cars in motion. Whether the direct-current motor will continue to be essential for railway work or not, it is evident that a device which enables either direct or alternating currents to be supplied to the consumer at will must add much to the flexibility and completeness of any system of distribution. With the apparatus as at present worked out it is possible to place a generating dynamo at the source of power, say a waterfall twenty miles away, produce with this multiphase alternating currents, raise the potential of these to any desired amount by means of a step-up converter pass them through the line, and then at the distribution end reduce them through the medium of a step-down converter to any suitable pressure. These reduced currents may then be used direct for operating alternating-current motors, for running in candescent or arc lamps, and, through the medium of the rotary transformer direct currents may be obtained for operating street railways and other continuous-current motors, both classes of lights and all kinds of chemical decomposition apparatus. It might be supposed that the multiphase system of alternating currents was a departure away from the direction of line economy so necessary a consideration in long-distance transmission since this system requires two or more circuits. This, however,'is not the case. It was early discovered by Mr. Tesla that the multiple circuits could have a common return wire, and it appears that the amount of copper in the combined circuits is actually less Than in the single circuit required for the ordinary single-phase current.
The value of departure in alternating apparatus made by Mr. Tesa has been very generally appreciated in the electrical world, and electric companies, both in this country and abroad have set themselves the task of working out complete systems of apparatus along the lines laid down by him. The Westinghouse Company, which early secured control in this country of Mr. Tesla's inventions, has developed a system using a two-phase current, while the other considerable American company, the "General Electric" has worked out a system employing a three-phase current, which form of current has also been adopted by the Allgemeine Elektricitäts Gesellschaft of Berlin. All these companies make an exhibit of this class of apparatus at the exposition, arranged to show the system in operation. The exhibits of the two chief American companies are substantially the same, differing mainly in the character of current used. Each shows the generation of multiphase currents, their transmission to the point of distribution, and their utilization in alternating and direct current apparatus.
How completely the problem of the distribution of electrical power over long distances has been solved by this system, and to what extent we may expect to see it pass into commercial use, experience alone can determine. Disregarding its future utility, when we will perforce be driven to the utilization of natural powers, and looking only to the immediate present, it is not difficult to' see that its adoption will be primarily determined by the cost of operating local steam plants. Where fuel is abundant, and hence cheap, there will be little inducement to resort to sources of power at a distance, but in all situations in which this condition does not obtain, and water power is to be had within a reasonable distance, electric power transmission will find a field, and one which will constantly widen with experience. While the utilization of water powers is the most obvious use for electric power transmissions, and certainly its most immediate one, it is quite possible that it will not prove to be the only one. As is well known, a large part of the cost of coal to the consumer is the expense of hauling it from the mines. It has been often pointed out that if the coal could be burned at the pit's mouth and its energy transmitted to the place of use there might result a great saving, but any economical method of doing this has heretofore been wanting. The suggestion has many times been made to convert the coal into gas and distribute this, but the cost of piping has heretofore rendered this method of eliminating the cost of railroad carriage impracticable. It would seem, however, to be quite within the range of practical possibilities to find in electric transmission an efficient and economic method.
[To be concluded.]