Popular Science Monthly/Volume 45/September 1894/Commercial Power Development at Niagara
|COMMERCIAL POWER DEVELOPMENT AT NIAGARA.|
AS many of the readers of The Popular Science Monthly are aware, there is a great engineering project on foot at Niagara Falls, looking to the development of a part of the water power at present running to waste over the gigantic cataract. A company, or rather an association of companies, working for a common end, is at present occupied at the falls with the object in view of utilizing the power commercially.
That this situation is the finest in the world for developing mechanical power has long been realized, but the local demands at Niagara were comparatively trifling, and only lately have our facilities for transmitting power over distances become sufficiently developed to warrant such an undertaking as is now in hand. The power company does not, however, look entirely to distant points for consumers of their output; on the contrary, a very large amount will be used almost on the spot by manufactures which are now moving to Niagara. The variety of purposes to which this power will be put may be gathered from the fact that they are as diverse as the manufacture of "mechanical" wood pulp and the smelting of aluminum.
There are already at the falls a few establishments using power developed by turbines, and which have been quietly at work for years. There is a canal known as the Hydraulic Canal on the American side, skirting the city of Niagara Falls, and terminating on the cliffs, half a mile below the cataract. There are a number of mills here which, for the most part, however, utilize only a fraction of the total fall available, probably for the reason that when they were built there were not in existence the high-grade water wheels suitable for great head that are on the market to-day.
People in general have the idea that the Niagara water power is inexhaustible, and so it probably is, so far as human requirements go. There are, however, some tolerably close data on which to figure the total horse power. The Lake Survey Board and Mr. R. C. Reid, examining the matter independently, have come to a very fair agreement in their conclusions on this point. From their figures it would appear that the average flow is about 270,000 cubic feet per second, and this is almost exactly the same as the almost unthinkable quantity of 1,000,000,000 pounds per minute. A horse power of work is the equivalent of 33,000 foot pounds per minute, and as the weight above mentioned falls 161 feet, the horse power of the total is expressed as follows: close on five million.
Owing to the lack in full efficiency of even the best commercial turbine wheels, we may take the limit of power that could be developed as about 4,000,000 horse power.
The average power is not departed from to any great extent at different sea,sons, as is the case with other water powers, because the spring thaws and summer droughts affect hardly at all the level of Lake Erie, from which the falls get their supply.
The system of Great Lakes above Ontario would require a year in order to have their level reduced by three feet and a half by even the enormous drain of a thousand million pounds of water per minute above referred to, supposing the system to be entirely cut off from its normal supply. A paper by Mr. R. C. Reid before the Royal Scottish Society of Arts in March, 1885, gives the foiling data: Total water-shed area down to Niagara, 290,000 square miles; total lake surface, 92,000 square miles; average rainfall in the lake district, thirty-six inches—and that we may assume twenty inches annually of evaporation and absorption, leaving sixteen inches over the whole area finding its way to the lakes. From the lake surface proper, there occurs evaporation to the extent of twenty-four inches per annum. Further, in reference to the enormous storage capacity of the system, he shows
that "it would take six months for the full effect of a flood in Lake Superior to be spent at Niagara Falls." It is easy, therefore, to understand how little fluctuation of level there can be due to seasonal variation in rainfall. Thus we see that quite apart from the fact of the vast volume and head available, and of there being no necessity for building a dam to back up the water, the situation is peculiarly favorable to the development of a constant power all the year round.
In spite of the generally equable level of Lake Erie, there are sometimes very considerable fluctuations, not of volume, but of distribution, due to high winds sweeping the length of the lake and causing a considerable banking of water at the end blown into. Sometimes such storms have lasted for days, and have had a very noticeable effect in increasing or diminishing the volume going over the fall. A more serious cause of low water is an ice jam at the head of the Niagara River. It is on record that in March, 1847, the water practically ceased to flow, "not enough, going over to turn a grindstone," as a local paper had it at the time. These two circumstances do not, however, affect the evenness of flow to any extent worth mentioning compared with the seasonal variations in rivers in general.
The total fall between Lakes Erie and Ontario is three hundred and twenty-nine feet, and is made up as follows: From Lake Erie to the head of the falls, seventy feet; the falls, one hundred and sixty-one feet, and below to Lake Ontario, ninety-eight feet. Consequently, the total power running to waste is more than double the five million horse power on the falls. An idea of the proportion that this total bears to what may be called the world's consumption of power may be had from the fact that it is computed to be equal to the total of all the steam-generated power in the world.
The geographical situation of the falls with respect to nearness to the at present great power-consuming centers is, as hinted above, not quite all that could be desired; but there are, nevertheless, several cities within reach, electrically speaking, which will use an enormous amount. Buffalo may be said to be next door, and Rochester is within easy reach*. In the not too distant future we may expect to see the great electrical manufacturing works in Schenectady operated, as is meet, by electrical power from Niagara.
The power company has, however, made branch track connections between the territory owned by it and three important railway lines which all pass within a few miles of the property. These connections and the good freight rates which have been contracted for in various directions, together with the cheapness of power, will in all likelihood attract to the spot manufactures besides those which have already undertaken to go there, to an extent that will make it the foremost power-consuming center in the world.
The chief piece of work in connection with the power installation has been the construction of what, in almost any other situation, would be termed the tailrace. In this case the head utilized is so great that what is ordinarily understood by a tailrace would be an artificial chasm of abysmal proportions that would almost require illumination other than the natural to be visible to the bottom at midday. Instead, a tunnel has been excavated, of which
the dimensions are so remarkable as to make it unique among engineering exploits of the kind.
The location of the power house, on account of difficulty in acquiring sufficient adjacent lands and rights of way and for other reasons, is not very close to the falls. The Cataract Construction Company has established itself about a mile and a half above the American Fall, and has dug a canal of considerable width, of a depth of twelve feet, and length fifteen hundred feet. Along its edge for a distance of at present one hundred and forty feet is dug a great trench or slot one hundred and sixty feet down, with, arrangements in the form of gates in the masonry wall separating it from the canal, by which water may be admitted to penstocks placed vertically in the slot and supplying the turbine wheels. A penstock, as many of our readers are aware, is a great tube, usually, in these days, of boiler plate, of a diameter running up, it may be, to thirteen feet, conveying water under head into the wheel case in which the turbine revolves.
In the present instance the penstocks, which are seven and a half feet in diameter, seem very small, considering that they each, supply a pair of wheels of five thousand horse power, but that is on account of the enormous pressure under which the wheels work, giving a greater power for a given volume of water than with the smaller heads more commonly used.
The turbines discharge their waste water into the tunnel above referred to, which is no less than six thousand seven hundred feet long, and which discharges into the chasm below the falls just past the Suspension Bridge.
The details of this tunnel, which was excavated through three shafts, one in the face of the cliff and two vertical ones, are as follows: Length, six thousand seven hundred feet, and sectional area three hundred and eighty-six square feet throughout, the average height and width being about twenty-one and nineteen feet respectively. The cross-section somewhat resembles a horseshoe. The excavation was much larger than the finished inside dimensions, on account of the subsequent lining with four courses of brick. The mouth of the tunnel has, besides, a lining on the top and sides of iron. The work has been done most substantially and is built to stay. The tunneling was done through strata of limestone and shale, and harder material was met with than had been expected in the beginning, so that the three million cubic feet of excavation has cut a very important figure in the total cost of the power plant. The tunnel has a grade of 0·7 per cent (seven feet fall per thousand length), and runs directly under the city of Niagara Falls to the lower river level.
The work of excavation was carried on on three benches, dividing the total height of twenty-six feet about into three equal portions.
The whole undertaking has been so entirely novel in many ways that the engineers in charge have had their resources taxed to the utmost in overcoming the various difficulties that presented themselves during the design and construction of the power house, electrical and hydraulic apparatus, and tunnel. The power-house building is as yet of comparatively small proportions, but is intended to be enlarged as the number of dynamos and turbines is increased. It might be thought, and was thought at first by some of the projectors of the scheme, that the great amount of power that was to be developed would admit of considerable subdivision, not only of the units of power production (each unit consisting of a turbine and generator), but also of the ways in which the electrical power would best be sent out to consumers.
As already mentioned, a number of manufacturing establishments are locating themselves on the property owned by the Cataract Construction Company, and to these it would at first sight seem natural and best to deliver electrical power straight from the power-house generators to their motors, seeing that this could easily be done without much loss of voltage on the carrying line; and, on the other hand, for distant work, as at Buffalo and Rochester,
to use a high potential on the line with transformers at the consuming end or at both ends. It has, however, been decided not to thus take advantage of the mechanical subdivision of the plant to use different types of generators for different kinds of work, but to adopt as a standard one good form of machine and use it throughout, at least until the plant is increased.
Perhaps the most remarkable consequence of this step will be that the Pittsburg Reduction Company, which manufactures metallic aluminum by the action of electricity upon certain compounds of that metal in a state of fusion, and which expects to use some thousands of electrical horse power when established at the falls, will receive it in the form of an alternating current. which will be passed into an alternating-current motor driving a direct-current, low-voltage generator furnishing at last the desired electrolyzing current. It has seemed best to submit to this complication of apparatus in order to gain the advantage of entire uniformity and interchangeability of power units in the generating plant. Of course, if the power company were to put in a direct-current dynamo for the benefit of the Reduction Company, all that would be necessary would be to send the current over a wire straight to its work; and it seems remarkable, in view of the thousands of horse power required, that the extra expense of a motor and dynamo to transform this quantity appears preferable. The electrical power unit which has been decided on after the most exhaustive, and presumably competent, expert examination of the requirements of the situation, will be of a capacity for continuous work of five thousand electrical horse power (or three thousand seven hundred kilowatts), and will be directly connected with a pair of turbines of similar power. All the generators will be mechanically identical in construction and have parts interchangeable with each other. The advantage of this, besides the obvious one of having a single set of spare parts suffice against the breakdown of any machine in the station, is that, from a point of view of the electrical aspect of the case, of the machines being able all to be put in parallel, as it is called. The expression may not be a familiar one to some of our readers, and the following hydraulic analogy may be of service in leading to an understanding of what is meant by it. Let us assume that we have several pumping engines of equal power, and that we are using them all to pump water from one reservoir into another at a higher level. Obviously the total amount of water pumped will be what a single machine handles multiplied by the number of them. Had, say, one of the pumps been weaker than the others—had it, that is, not been strong enough to force water up to the height that the others did—the result would be that, instead of doing any work when put, as we may say, in parallel with the others, it would have been unable to withstand the head, and water would have forced itself back through it into the lower reservoir. The same way with dynamos, or generators as they are usually called when referring to the machinery in a power as distinct from a lighting station. The advantage of working in parallel is, that if we have, say, six machines all "pumping" current into the same mains and one breaks down, we may take it out of circuit, and, by temporarily overloading the other five, which can always be done for a short time with good machines, keep on supplying full current to consumers. Should the power company have decided to put in a special machine for aluminum, and other special ones for other local work, and still more for distant work. each would have its own circuit, and, if it broke down, the whole dependent system would be idle until repairs were completed. One of the great aims of the company appears to be to insure the permanence and continuousness of their power service—which is, of course, of the utmost importance to manufacturers.
A remarkable method of construction—not, however, unique—is employed in the generators to secure means for direct coupling to the turbine shafts. These latter are vertical, and come up over one hundred and forty feet out of the wheel pits from the rotating water wheels, which make two hundred and fifty revolutions per minute. In order to obtain direct driving—that is, without the intervention of toothed or friction gearing, or belt or rope driving—the revolving portions of the generator are arranged to rotate in a horizontal instead of, as is usual, a vertical plane.
A dynamo of any type whatever consists, as is well known, essentially of two portions, one of which possesses motion with respect to the other, viz., the armature and the field magnets. Since the field magnets are almost invariably much heavier and much less compact than the armature, the latter is usually chosen as the moving part. In the case under discussion the contrary has been decided on, the armature being fixed and the field magnets rotating. This gives certain advantages in the matter of less complicated electrical connections and of dispensing with the armature's rubbing collectors altogether; it also gives the advantage much more important in this case than with smaller machines—that, since the revolving magnets are arranged on a ring and point inward, the attraction between them and the armature core tends toward neutralization of the strains of centrifugal force. The greatest advantage, however, attained by this method, and again one which is of far greater value in the present case than in ordinary practice, is the high degree of insulation possible with fixed armature coils and connections. The requirements that had to be met in the way of limiting the centrifugal strains were that the product of the sum of the weights of the revolving parts in pounds and the square of their velocities in feet per second should not exceed eleven hundred million. The weight of the moving parts of each dynamo was also limited to eighty thousand pounds, while the weight of the turbine and its shaft amounts to seventy-two thousand pounds.
This whole weight of seventy-six tons acts in one vertical line—i. e., that of the turbine shaft—and revolves two hundred and fifty times per minute. It would have been very difficult to construct thrust bearings to take up the whole of this strain, and a hydraulic balancing piston has been resorted to for supporting it. This device is simply a circular piston fast on the vertical turbine shaft, set in a vertical cylinder. The supporting force consists of
hydraulic pressure admitted to the under side of the piston. This pressure is derived simply from the water in the penstock supplied to the turbine, and when the latter is working under full gate—that is, is taking water to its full capacity—the pressure in the penstock is decidedly less, just as the pressure in a water pipe is partly relieved by the opening of a faucet. This causes the supporting force on the under side of the piston to materially decrease, and a thrust bearing—that is, a bearing adapted to withstand either pressure or pull, so as to hold the shaft against the tendency to end play—has to be resorted to in order to take up the difference. As a matter of fact, the difference between the supporting force when the flow is a minimum and that when the gate is wide open is about two tons in the seventy-six. The way this is handled is to arrange the area of the piston and the depth below^ the upper water level so that at minimum flow the supporting pressure will be about one ton more than the total weight, and at full gate about the same amount less. At the normal rate of working there is very little to be taken up by the thrust bearings.
An idea of the magnitude of the proportions of the generators may be gathered from the fact that the designers were limited in the size of base plates that they could use by the inability of the railways to transport, even by especially large and powerful cars, pieces of proportions originally designed from the factories to he falls.
It is stated that, had it not been for the tariff restrictions imposed on the importation of electrical machinery, the generators would probably have been purchased abroad. As it was, they, as well as the motors which will operate on their circuits, are the work of a great Pittsburg company. In the case of the turbines the design was by a Geneva firm, and the construction mainly done in Philadelphia. Certain of the fittings are French, and the governors Swiss.
One of the details in the power house is a traveling crane capable of handling pieces weighing up to fifty tons, which commands every portion of the floor of the building. The presence of this piece of apparatus is of the greatest importance in the case of anything going wrong with one of the generators or turbines. With its assistance any portion of either of these ponderous pieces of mechanism which may need repair can be moved with the greatest expedition, and a spare interchangeable part put in its place. Frequently in an installation of heavy machinery, although perhaps much less ponderous than these in question, a break occurs which may cause a shut-down of many hours, when, if sufficiently powerful means of moving heavy parts were at hand, the damaged piece could be replaced in a comparatively short time. A traveling crane of this description, as most of our readers are aware, consists of a long carriage having a pair of rails on which runs the crane truck carrying the lifting machinery. The long carriage, which is supported a suitable height above the floor, stretches across the width of space to be commanded, and itself has a sideway movement on several supporting rails which run the length of the space to be operated over. Thus by a combination of the two movements the crane truck commands the whole floor.
During the work of assembling the penstocks, wheel cases, turbines, etc., at the wheel pit, a view of this great slot with its contents was wonderfully impressive in giving an idea of the vastness of the whole enterprise. The great depth of this long, narrow pit, which made it impossible to see to the bottom except with the assistance of lamps in the lower part, the mysterious-looking pipes (the penstocks) rising vertically, new sections being constantly added much in the same way that a stovepipe is put together, except for the permanence given by the heavy riveted seams, and the enormous power and flexibility of operation of the immense traveling crane which rapidly conveyed in every direction great masses of iron and steel obedient to the turn of a switch, made a combination of impressive effects not quickly forgotten.
To obtain an idea of just what the relation to each other of the various parts in the installation is, the reader is referred to the sketches numbered 6, 7, and 8.
It may be mentioned that, to withstand the very considerable hydraulic pressure at the lower part of the penstocks, these tubes are built of thicker and thicker plates from the top downward.
There has been very little criticism of the mechanical details of construction so far referred to; on the contrary, very little can be said except in praise of the fertility of resource and high general competence of the engineers who have had this work in hand. With regard, however, to the particular design of the generators from an electrical rather than a mechanical standpoint much and lavish criticism, if not condemnation, has appeared in various quarters. Whether the grounds for this criticism are well founded or not it would be presumptuous at this time to attempt to declare, but we may say that where, as in this case, one man has had practically the entire control of the design of the electrical apparatus, we may usually look for, rather than be surprised at, a great amount of setting up of individual opinion against the views which he may embody in practice, often a good deal irrespective of the probably cogent reasons which may have induced him to adopt the course in question.
Without attempting to decide between the various views which are plentifully to hand in criticism of certain electrical
details in the design and proposed method of utilizing the current of the generators, we may glance at what has been decided on, and review the more important points raised in connection therewith.
In the first place, the use of an alternating as opposed to a direct current was decided on, as was to have been expected. The development within the last year or two of alternating-current motors has rendered possible the distribution of electricity for power (as opposed to lighting) purposes over distances before almost out of the question. It has been for a number of years past possible to transmit large quantities of electrical energy for lighting which was not suitable for running the then known motors. The method of electrical distribution for lighting purposes that is used in cities is available also for transmission to considerable distances. It consists, as is well known, of a dynamo supplying current at a high voltage to the street lines, and a system of transformers each taking a portion of this current at high voltage and giving in return a current of greater amperage or volume and of lower voltage for house consumption, the object being simply to avoid loss of voltage or pressure by transmitting a heavy current over a light wire. As this may not be quite clear to every reader, it may be as well to say a little more about it.
The energy of any current is determined by and is equal to the product of two of its properties, its volume or ampèrage and its pressure or voltage. Letting C represent the ampères and V the voltage, we have that the energy C V. In passing any current over any wire there is a loss of voltage determined by and equal to the product of two things—i. e., the ampèrage of the current and the resistance of the wire; so we have loss of voltage C R. Now, if we have two currents—one, say, of ten ampères and one volt, and the other of one ampère and ten volts—the energy will be the same, or ten watts as it is called. If we pass both through a given resistance, R, we shall have a loss of voltage ( CR) ten times greater in the first than in the second case. But a given loss of voltage amounts to only one tenth as much energy (C V) in the second case with C one ampère as it does in the first with C ten ampères, so that with only one tenth the given loss of voltage the energy lost will be only one one-hundredth that lost in the first case. What it amounts to is that the loss in passing a given amount of electrical energy through a given resistance is proportional to the square of the current, or amperage, and consequently inversely proportional to the square of the pressure, or voltage.
If, therefore, current is used in a house at fifty volts and transmitted to the house at one thousand volts, the loss will be only one four-hundredth as much over a given wire as it would be if transmitted at fifty volts.
The advantage that alternating currents have over direct for long-distance transmission is that they may easily be transformed up or down—that is, their voltage at the generating end may be increased (at the expense, of course, of their amperage) and reduced at the consuming end. In point of fact, it is frequently and usually unnecessary to employ such devices at the generating end, for the reason that the generators themselves can work perfectly well at the high voltage requisite to transmit. The objection to using the same high voltage on the consuming machinery is simply that there is more danger of accident with numerous small motors scattered in various places and in the hands of unskilled persons than in a power station containing only two or three highly guarded machines attended by trained operatives.
With this fact of the possibility of generating currents of a voltage suitable for immediate transmission, it at first sight appears strange that direct-current transmission is not a more common thing than it is. The method of the so-called "motor transformer," "rotary transformer," or "dynamotor," might be adopted. A transmission plant working on this method would operate as follows: The power station would contain preferably several highly insulated direct-current generators, all of similar construction, for very high potential (four thousand volts would be easily obtained); these would run in series that is, each would add its voltage to that of the others, and there would preferably be a spare machine to substitute for any one of the others which might become injured. If four machines were in series, the resultant current would be put to line at, say, sixteen thousand volts, would be received at the other end by a number of motors, also in series, which in their turn would drive low potential dynamos supplying current for local use.
There are two objections to this as compared with alternating-current transmission: One is the fact that there has grown up a very tangible, we may almost call it, superstition against the use of high-voltage direct-current machines of large size among very many electricians. The reasons for this are not difficult to trace; prominent among them being the simple fact that no commercial application has ever yet required such machines. The only high-potential direct-current dynamos are those used for arc lighting, and on account of the great subdivision of arc-lighting circuits the units of generation are invariably small, at least by comparison with the ponderous machinery used in the Niagara Falls power plant.
There is no reason why they could not be made large (in point of fact, arc-lighting requirements are continually making demands for the construction of larger and larger machines, and the requirements are just as steadily being met without difficulty), and yet this very tangible dislike of their use for power transmission undoubtedly exists. The result is that, without undertaking considerable work on new ground in the way of patterns, designs, etc, no company could obtain such machines; and since the alternating current has had practically the exclusive attention of the laborers in the field of electrical power transmission there is no
method, tried on the large scale, for the other. The second disadvantage referred to is the greater cost of motor transformers over the simple stationary ones for alternating work. In view, however, of the fact of the proposed installation of these very motor transformers in adapting the alternating current to the arc lighting of Buffalo, and to the aluminum smelting works at Niagara, it would seem that this objection could not count for very much.
In connection with the Niagara Falls work there is the further advantage which the alternating current has over the direct, and that is what may be termed the "flexibility," commercially, of the former. The alternating-current machines operated in parallel at, say, two thousand volts, may have a portion of their current taken from them at that voltage for use in the immediate neighborhood and the rest transformed up for distant transmission.
The advantages of the direct-current system would be two: First, the simpler methods of motor operation by its means, and the availability of the current for electrochemical work and storage-battery operation direct. Second, the smaller weight of copper necessary on the transmitting wire, for the three reasons of the evenness of flow, the absence of self-induction on the line, and the absence of skin resistance in direct-current transmission. The effects of the two latter phenomena will be discussed later.
Inventive effort has, singularly, stayed in the rut of work on alternating-current transmission, and in attempting anything on the scale of the Niagara Falls undertaking it would be perilous, even had it been considered for other reasons advisable, to depart more than necessary from usual practice.
Lately, and particularly owing to the brilliant work of a young man, a native of Smiljan Lika, a border country of Austria-Hungary, by name Nikola Tesla, there have been devised forms of apparatus, generating as well as consuming, by means of which alternating currents may be economically used for operating motors. To express it very roughly, his method amounts to arranging an armature within a magnetic ring and causing opposite magnetic poles to revolve around the ring so as to cause rotation of the armature.
The operation of these devices is preferably by means of a polyphase alternating current—that is, a flow of electricity having more than one pulsating current.
Before finally deciding on what system of transmission to use, the Cataract Construction Company asked for plans for a system for the purpose from a number of electrical engineering establishments. Twenty-four distinct ones were submitted, more than one of the tendering companies having sent several different plans to be chosen from. No individual one was, however, accepted in toto, but instead a design was adopted embodying such points of value as could be assembled in one suitable type of machine, and the Westinghouse Company received the contract for it. The system on which the generators work is the Tesla two-phase, and is notably peculiar on account of the low periodicity of alternation.
The number of pulsations of commercial alternating currents is usually over one hundred per second and is frequently double that amount. The reasons for this high frequency are mainly two: The first, that with any given alternating-current dynamo the number of alternations depends directly on the speed, and, as this must usually be high in order to get as much work as possible out of the machine, the periodicity is also high. The second reason is that in lighting work it is, of course, highly undesirable to employ a current of which the pulsations are so slow as to leave the incandescent filament or the arc visibly dimmer between separate beats, as we may call them, than during the passage of the full current strength. In the case in hand one is impressed with the effort that has been made to steer a middle course in the design of the generators so as to obtain a portion of
the advantage of the direct current for motor work and of the alternating for transformation. The periodicity for the first portion at least of the electrical equipment is to be as low as twenty-five per second, and this at once limits the scope of the use of the current in the matter of electric lighting. Prof. Forbes states that lighting by the current direct is a comparatively small portion of the work in contemplation, and that the plant is rather to be regarded as essentially for power distribution. The expression, "lighting by the current direct," is used because a very important branch of the power work will be the lighting of the city of Buffalo. This is at present done by the ordinary direct-current arc machines operated by engines of some three thousand horse power. In changing over to the Niagara Falls power the whole electrical system will be untouched, but the engines will be replaced by motors operated by current from the falls station.
As has been justified by the importance of the subject, there have been some quite exhaustive experiments undertaken by various scientists to determine the frequency of alternation at which unsteadiness of the light from both incandescent and arc lamps is observable or at least objectionable. Several independent experimenters have arrived at results sufficiently satisfactory to themselves, but which unfortunately can not be used as reliable data, for the reason that they are highly discrepant with each other. One reason for this is the evident one of the difference between different makes of lamps, but the discrepancies are of a character not altogether to be explained on that ground. With the ordinary fifty-volt filament, however, it would seem that we may place the working rate of alternation at about thirty or over; with arc lamps, at about fifty or over.
As above mentioned, the arc lighting will be done by making use of the motor transformer (a motor operated by the power current driving a dynamo generating, if we may call it so, the secondary current), but it is expected that by means of a special form of incandescent lamp—the Bernstein, which has, indeed, been on the market for several years—the twenty-five-period current will be available for direct use for illumination by means of incandescent lamps. It is evident that the thicker the filament the longer will its incandescence take to die out (as well as to start up), and a current of twenty-five pulsations, which may not be available for the high-resistance (thin) filament, may be quite sufficiently so for a low-resistance one, which the Bernstein lamp above mentioned is.
The voltage at which the first installation of generators is to operate is somewhat over two thousand. Considering the perfection to which European practice has been carried in the construction of alternating-current machines for much higher electrical pressures than the above, it seems strange that this voltage should have been decided on in a situation where one would expect the very highest degree of perfection to be attained. It is stated, however, that it was largely on account of the comparatively backward condition of that branch of electrical engineering construction in America that the voltage had to be placed so low.
In a case like the present one, where the power station will be under the supervision of skilled engineers, and not merely of men whose chief qualifications are those of sobriety and an ability to stay awake at night, there appears no sufficient reason why the generators should not be operated at five times the voltage named. The fact of the armatures in these machines being fixed gives, moreover, additional security against danger consequent on such high voltage on account of the very much more perfect insulation possible.
The advantage, of course, of using a very high electrical pressure lies in the principle stated above of the loss in sending a given amount of energy over a given wire being inversely proportional to the square of the voltage.
By the use of step-up transformers it will, of course, be possible to transmit at any voltage that the insulation of the line can withstand; but if this high voltage could be reached by the machines directly, the loss (we may liken it to a friction loss in machinery) of efficiency in the transformers, and, even more important, the great cost of that part of the equipment, would both be avoided.
What will be done will be to use these step-up transformers and put current on the transmitting line at about twenty thousand volts; it is likely, however, that in any subsequent enlargements of the generating plant the three original machines will be used for local work only, and a radical change made in the direction of an enormously higher generated voltage.
Intimately associated with this question is the problem of how to convey current at this tremendous potential of twenty thousand volts to distances. An idea of what it means may be had from the facts that two thousand is relied on to be sufficient to instantly kill a human being, and that the energy of a current given up in passing through any given resistance varies as the square of the voltage.
The chief difficulty to be met in such line construction is that of efficiently insulating the wires. If one attempted to use a line insulated merely as an ordinary telegraph line is, there would be an enormous loss, amounting practically to the whole of the transmitted current, in moist weather, by leakage over the damp surfaces of the glass or other insulators. The remedy for this leakage would, however, be a comparatively simple matter by means of well-known oil-holding arrangements for the insulators were it not for the further fact that it is imperatively necessary not to have the two wires, the going and return ones, farther apart than can not be avoided on account of what are known as the effects of self-induction. The wires strung on telegraph poles would have to be so far apart in order to insure their never, by any possibility, coming in contact, that the self-induction losses would make that method impracticable.
The self-induction of a circuit has the effect of retarding both the starting up and the dying out of a current flowing in the circuit. The phenomenon gives a resemblance of the possession of a property analogous to mechanical inertia to the current. Since inertia, however, is a property dependent solely on the mass of a body, and is the same for all situations or conditions of the body, we shall see that self-induction has but a very faint likeness to it, for self-induction is a property of a conducting path or circuit, and not at all of the current. To dip lightly into the theory of the phenomenon, we may say that the inception or the stoppage of an electrical flow in any conductor involves the starting up or stoppage of a movement in the dielectric medium surrounding the conductor. The time requisite for this movement to start up or stop gives a perfect analogue to mechanical inertia. If, now, we have a circuit consisting of a wire returning on itself, the two halves being as close together as they may be without touching, we see that a flow starting up in this wire means a current in each half in opposite directions. For the present it suffices to say that the effect above referred to of the starting up of a movement in the surrounding medium is rendered less and less by the canceling effect of the opposite electrical flows the nearer the two halves of the circuit are brought together.
The evil effects of self-induction are directly proportional to the number of alternations of the current in a given time, and consequently the twenty-five-period current adopted for the Niagara Falls work is highly advantageous from this point of view.
The so-called "skin resistance of an alternating-current circuit is, in brief, due to the fact that an alternating current penetrates only a short distance into
The form decided on in which to construct the conveying lines is that of a conduit or subway of large proportions. One which has been already constructed for a length of half a mile is as follows: The walls are arched, and the width is greatest at about two thirds of the height. The conductors are carried on insulated brackets along the sides, spaced at intervals of thirty feet. The subway is lined with concrete, and manholes at intervals allow of access; besides, there are small pieces of pipe let in at the bottoms of the manhole ducts for the purpose of inserting such wires as may from time to time be required to tap the line conductors. The subway is five and a half feet high and three feet ten inches wide. A track runs along it, and the line inspectors will make their trips on an electrically propelled car; heavy wire screens the height of the subway, extending on both sides of the track, protecting the occupants from any possible discharge from the main conductors.
The Cataract Construction Company expect to be able to deliver power in Buffalo at a cost per horse power, for twenty-four hours a day yearly, greatly below the cost of steam power as now produced in Buffalo with coal at one dollar and a half per ton. The generators are expected to operate at five thousand horse power each, with an efficiency of ninety-eight per cent on the power delivered to them by the turbines, and there will be only three and a half per cent drop of pressure in transmitting at twenty thousand volts to the northern part of Buffalo. This last appears wonderful when we consider that it is less than the drop from the generators of an electric railway system to the motors of cars within as short a distance as half a mile, quite apart, moreover, from the extra losses in the latter case due to imperfect trolley contacts. It is hoped also to transmit power before long to the Erie Canal, on which at the close of last season there was an interesting development in the line of electrical canal-boat propulsion. What else may be in store for the closing years of the century in still further applications of transmitted electrical power, notably in the displacement of steam in railroad operation, can only be foreshadowed. Suffice it to say that the Niagara Falls Power Company will probably soon find their initial fifteen thousand horse-power equipment entirely insufficient to meet the demands upon it.