AS the term “central-station” associates itself with some pretentious building, such as a railway terminus, it may be advisable to remark that the similarity is only in the words, and that central-station is an abbreviation of central generating station, or building designed to contain the plant for the public supply of electricity. In the early days of electric lighting the transmission of electricity to a distance was considered an impossibility; we find the late Sir William Siemens, in his Presidential Address at the Society of Arts on the occasion of the opening of the session in 1882, stating “that a quarter of a mile in every direction from the lighting station was the area which would be as much as could be economically worked;” and, in order to tap the most paying district, it was proposed to establish a station in the most central spot. Sir William Siemens suggested the utilisation of the public squares, which could be excavated to a depth of twenty-five feet, and then arched over to the existing ground level, and in this covered space the engines, boilers, and dynamos were to be fixed; the only erection above the surface was the chimney, which was to be of ornamental design and combined with the ventilating arrangements of the subterranean chamber. The great inventor, who so ably filled the presidential chair at the meeting where these words were spoken, would be astonished to find that in London one electric lighting company has already erected seventy miles of overhead wire, and that customers are supplied miles away from the so-called central-station. The changed position of electricity is due to the introduction of the transformer by Goulard, who showed, at the Turin Exhibition of 1884, that a high-tension current could be transformed into a low-tension working current of safe potential, fifty miles away from the generator, in a successful and economical manner, and that the generating station might, therefore, be located outside the area to be lighted. In large cities this is a great advantage, the value of land often precluding the erection of a big station in the working area; for this reason small stations are often arranged in basements, under a large building, which are, as a rule, specially designed. This plan is somewhat similar to that adopted in the United States, where it is not unusual to find a successful installation in a basement and sub-basement, the general arrangement being of similar character to the engine-room of a steamship.

A station is being erected in Philadelphia on a ground space of 72 feet × 100 feet, which is to supply 60,000 lights; the building is six-storied, the dynamos are on the first floor, the boilers on third, the coal stores on fourth, and the offices on the fifth.

The term “block station” is also used in the United States and in Germany, and is applied to an installation which lights a group of buildings or block without crossing any streets, and consequently without having any wayleave or permission from local authorities.

An American electrical engineer graphically sums up this question in the following manner:—“There are two ways of starting a central-station for electric lighting—the investment or the speculative plan, or the fair means or the foul. The first has its legitimate end, but the latter is the border ruffian or money-or-your-life policy, which enters a territory already sufficiently covered, not for fair competition, but to make money by being bought out.” Happily, here, we have at present only to deal with the first plan, and the question naturally arises, “Is electric lighting a paying investment?” It certainly will not be if the object in view is only to compete with gas in a limited district where perhaps it is being sold at 2s. 6d. per 1000 feet, for, as long as there is a ready market for the coke and other by-products, gas will remain in possession of the field. The heat from gas, which is found so undesirable by the wealthier classes, is advantageous to those who perhaps cannot afford a fire; in fact, gas has been truly called the “poor man’s friend,” and, until electricity can be supplied at a nominal price, it will be useless to expect any revenue from the poorer districts of large cities. Quite an opposite result may be looked for when the electric mains are laid at the doors of the wealthy householder, or through the business neighbourhoods. Shop-owners especially are found to immediately take up electric light, from the fact that no fumes are given off to destroy goods or tarnish silver or gilding, and because it can be so easily applied in a shop window so as to efficiently light the contents without producing shadows. The great object to be aimed at in selecting a district to be served from a central-station is a “constant demand,” and for this reason it is advantageous to include a business neighbourhood with shops, public-houses, and restaurants, which require the light for a definite period every day, and probably will each take more than double the amount of an ordinary dwelling-house; in fashionable neighbourhoods especially, it is not unusual to find a large number of houses vacated at the close of the season, the interest on that portion of the electric system which is unemployed will have to be set against the profits of other periods of the year.

The number of gas lights which are actually used at one time in a house is found to average only two-thirds of the total number fixed, and with electric light this number is reduced to one-half; economy is at once the rule with electric light, partly because of the novelty of the illuminant, and also on account of the facility of lighting and extinguishing by simply turning a tap or switch. The number of hours artificial light is wanted in a residential district may be taken at about 1000 hours per annum, that is to say, the light is required for about four hours a day in winter and two hours in summer; this amount is very much exceeded in clubs, shops, and even in large houses, but 1000 hours is a safe figure, and, if the supply is taken by meter, an annual payment equivalent to 1000 hours’ supply should be a fixed amount to be paid for, whether used or not. Mr. Crompton estimates that a Londoner, who is a tenant or owner of a house having three reception rooms, ten bedrooms, and usual offices, spends about £25 a year for his lighting, which is made up as follows:—gas bill, £15; lamp, oil, candles, matches, about £10. There would be about fifty burners fixed; and, supposing fifty electric lights to be substituted, he could be supplied with electricity for £25 a year, at a fair profit to the supply company if they charged 8d. per Board of Trade Unit, as practice has shown that the total number of lamp-hours with fifty electric lamps is not more than sixty-two, so that two Units,[1] or 1s. 4d. per day, would be sufficient for the lights he would require.

The diagram, Fig. 1, taken from a London residential district, shows how the number of lamps on at one time vary; the district is supposed to be wired for 10,000 lamps, and the plant as equal to the supply of 600 kilowatts, or 600,000 watts; the number of lamps is small until about 3 o’clock, when it gets dusk on a winter afternoon; it then increases steadily until about 6.30 o’clock, when the curve goes up with a rush; about this time a great number of people are preparing for dinner, and probably the lights are on both in the dining-room and bedrooms. The curve falls, and at about 8 it begins to sink gradually until 10 o’clock; a great many people appear to go to bed about this time, but a few sit up to 1 o’clock; until 6 the next morning hardly any supply is taken, when the servants get up and prepare the rooms for the day. The diagram, Fig. 2, is taken from the Edison Company’s central-station at Cincinnati, and agrees fairly with the London demand for light. Another interesting fact has been ascertained from the observations taken at the Mauer Strasse station in Berlin, namely, that the output varies with the temperature, it goes up or down with the thermometer. The reason is easily explained; gas is laid on side by side with the incandescent lamps, and the burners are first lighted when it is cold to warm the apartments; in warm weather the electric light alone is used. From these and other diagrams the very important fact has been obtained, that the average daily output of a station throughout the year is less than one-third of the total capacity of the generating machinery, so that, although the station from which the diagram in Fig. 1 was taken could maintain 10,000 16 candle-power lamps simultaneously alight, the average daily output of electricity would only equal 3500 constantly lighted; and, as the first cost of the station is dependent on the size of the plant, the saleable output is the important factor which governs the profits.

The well-known expression “per 1000 Cubic Feet” is not applicable to electric light, and, instead, the Board of Trade Unit is employed. By this term Unit is meant the quantity of energy contained in a current of 1000 AmpÈres flowing under an Electro-motive Force of One Volt during One Hour. In the early days of electric lighting the term Volt-ampÈre was used, and has for convenience sake been shortened to Watt; that is, the Volt or Unit of Electro-motive Force (or pressure) is multiplied by the AmpÈre or Unit of current.

The Board of Trade Unit is, therefore, a Thousand Volt-ampÈres or Watts per hour.

For example: 16 candle-power Swan lamps are assumed to take 60 Watts, which, if the electrical pressure is 100 Volts, would mean a consumption of 0·6 AmpÈre; and, as an Electrical Horse-power equals 746 Watts, 12·4 lamps should theoretically be obtained per Horse-power, which is, however, reduced in actual practice to 10 at the most, often less.

The charge per Unit supplied by meter varies in England from 1s. to 7d.

Comparison of Cost of Gas and Electricity.

These prices, of course, include the manufacturer’s profit as well as the loss in transmission through the mains and expenses of connecting up to the consumer. The actual manufacturing cost of a station maintaining 10,000 lights should not be more than 3d. per Unit, or equivalent to gas at 1s. 8½d. per 1000 cubic feet.[2]

As petroleum lamps are used for the street lighting of many foreign and colonial towns, the question arises, “Will it pay to substitute the electric light?” Comparing the light given by a kerosene or petroleum lamp with that from the incandescent electric lamp, the cost is greatly in favour of oil; and, in fact, where the price of kerosene is under 1s. per gallon, electricity cannot compete if labour is cheap. On the other hand, the trimming, lighting, and keeping in order of a number of lamps scattered over a large area greatly augments the working cost, to which must be added the breakages of chimneys, expense of wicks, also the danger of fire. It is the safety of electricity which has caused it to supplant oil both for public and private lighting in American cities; even where the price of kerosene is not more than 6d. per gallon there is a demand for the electric light, which is by far the dearer illuminant, after making a liberal allowance for labour in cleaning, filling, and lighting the oil lamp, also for depreciation of the burners.

Arc and Incandescent or Glow Lighting.

Electric lighting can be obtained by means of arc or incandescent lamps. The arc light is now well understood to be caused by the extremely high temperature of the end of one or both the carbon electrodes. The voltaic arc, Fig. 3, is formed by the minute particles of carbon in a high state of combustion which the current appears to break off and carry from one electrode to the other, the light, however, being mainly due to the incandescence of the crater shown in Fig. 3 on the upper carbon. In the incandescent or glow lamp light is produced by the passage of a current of electricity through a continuous fine thread or filament of carbon which becomes white-hot, the destruction of the filament being prevented through its enclosure in a glass bulb from which the air is exhausted. Figs. 4 and 5.

The first method is suitable for the lighting of streets where a high-class illumination is required; also will be wanted for the external lighting of shops, public-houses, and places of amusement, so that arrangements must be made for arc lighting. The usual plan is to charge at the same rate per Unit by meter as the incandescent lamps, but to make an additional charge of 5s. to 7s. 6d. per lamp per quarter for rent, and a further charge of 3s. per week for cleaning and trimming.

The principal types are the Edison and the Swan, Fig. 4 and Fig. 5.

Incandescent lamps can be obtained to order from 1½ candle-power upwards, but the 16 candle-power (nominal 20) or the 8 candle-power (nominal 10) lamps are almost invariably employed. The latter give the best effect, and can be worked to 10 candle-power without much risk, they take about 30 watts as against 60 watts for the 16 candles; and are not uneconomical, for nearly double the number can be worked with the same energy. A new type of glow lamp, called the “Sunbeam,” has been recently introduced, which contains a thick filament, and gives a light of from 200 to 1500 candle-power, and can be employed instead of an arc lamp with the same economy as the ordinary 16 candle-power type.

Life of Incandescent Lamps.

In estimating the annual cost of lighting, the renewals of lamps must be taken into account; and although some lamps have worked 3000 or even 4000 hours, a life of 1000 working hours is the highest average it is safe to assume in practical work under even the best conditions, that is, using secondary batteries and never over running. The average life of 130 lamps on H.M.S. troopship Malabar was 3799 hours each, the shortest life being 638½ hours for 18 yard-arm lamps of 32 candle-power. If the current is allowed to fluctuate, the average life would be very much less; it is an unsettled question whether long-lived lamps are really economical, by reason of the blackening of the globes, which takes place after the lamp has been worked some time, and is probably due to small particles of carbon thrown off from the filament being deposited on the glass. It has been suggested that attrition of the filament is going on all the time the lamp is at work, and that the heated atoms striking against the filament may account for the blackening, in that the mean free path of the atoms would be greater in a perfect vacuum than in the air, consequently they would abrade the filament with considerable force. If lamps were sold at 1s. each instead of 3s. 6d., which is now the price for not less than a thousand, it would be more economical to change them at the first signs of blackening, even if the life did not exceed 500 hours.

The diagram, Fig. 6, has been so arranged that the amount of light required in a given district can be ascertained for any period of the day or night; it has been calculated from the observations taken daily at one of the Berlin central-stations by the engineer to the company.

Six hundred and forty watts are assumed, for the purposes of the diagram, to be the equivalent of a horse-power, instead of 736, as the German electrical horse-power is 736 watts instead of 746 watts.

The table, Fig. 6, has two vertical scales, A and B, each giving the kilowatts[3] and corresponding horse-power. A is drawn to a scale ten times greater than B, with the object of noting the smaller amount of lights required for street illumination. The horizontal line is divided into hours, and represents a day’s lighting in the middle of December and the end of July, so as to show the maximum and minimum amount of current that will be required. In the lighting of a town there are two classes of illumination, the amount taken by the public, which is uncertain, and that employed for street lighting, which is a known quantity.

The curves, II and II A, represent the private lighting of houses, hotels, theatres, and shops of different kinds in December and in July, the curve, II A, being in dotted lines clearly shows what a vast difference there is in the amount of light, and consequently the amount of energy required in the generating station, as compared with curve II, which is taken when the days are longest.

The rectangles, I and I A, show the street illumination, and are drawn to suit scale A; half an hour after sunset all the lamps are turned on, and the work reaches its maximum suddenly, and continues the same until 12 o’clock, when, according to the municipal decrees, it either falls one or two gradations until half an hour before sunrise, when all the lamps are extinguished. The calculations are based on the assumption of 640 watts to the horse-power, instead of 736, which is the theoretical efficiency of a German horse-power.

If a number of diagrams are taken on this method for different periods of the year, the constant work can be ascertained. This knowledge is most valuable when calculating the most economical area for the mains, which is then easily accomplished by means of Forbes’ tables, which are based on Sir William Thomson’s well-known rule.

The lines, 2 and 2 a show the constant work at the same two periods of the year from which the diagrams are taken. The constant work at the end of December will be found to amount to 20 per cent. of the total work, and that at the middle of June to 15 per cent. By summing up the average work for all the days in the year we obtain the cost per annum, and adding to this the expense of management, interest, &c., and knowing the local conditions, we can fix what proportion of the day’s work is admissible as loss. With the Edison system at Berlin, 5 per cent. is taken as average loss; thus, at the end of December, it amounts, with the maximum number of lights, to 18·8 per cent., and with the minimum to 1·1 per cent.; in the middle of July the maximum is 15·8 per cent., and the minimum 0·5 per cent. The dynamos must, of course, be of sufficient power to be able to overcome this loss, which only shows itself periodically; therefore the generating plant may be constructed to give, nominally, 20 to 30 per cent. less than the maximum work, and be capable of being pushed to the full amount for a short time only.

Uncontrolled financial speculation, aided by the stringent clauses of the Electric Lighting Act of 1882, have been a great deterrent to the extension of old or the introduction of new schemes for the supply of electricity to the public in the same manner as gas. The President of the Board of Trade, replying to a question in the House of Commons, said that, “since the passing of the Electric Lighting Act of 1882, fifty-nine provisional orders and five licences had been granted to companies, and fifteen provisional orders and two licences to local authorities. He was not aware that, in any single case where these powers had been obtained, they had been exercised.” Up to the present time no company supplying electricity has been under the necessity of applying for compulsory powers, and has either obtained permission from the local authorities to take up the streets, or has carried the electric mains over the houses, and, regardless of the question of overhead wires, has depended on wayleaves granted by the too-confiding householder, who has no idea that his roof is supporting a cable weighing 1¾ tons to the mile.

An amendment of the Act of 1882 has passed both Houses without hindrance, and has received the Royal assent. It provides that in the case of Provisional Orders the period after which the undertaking may be bought up by the Local Authority shall be extended from twenty-one to forty-two years, and that portion of the previous Act which referred to the compulsory purchase of the undertaking by a local authority at the end of the term has been altered, and more favourable terms given to the electric companies.

On the Continent, and in the United States, where each city may be said to legislate for itself in matters relating to the general welfare of its citizens, the electric lighting industry is in a very different position, and central-stations are either established or about to be started in every important town. There were, in 1887, 121 Edison central-stations alone, supplying over 323,000 incandescent lamps, and paying dividends from 6 to 14 per cent. The Westinghouse Company, who use a transformer system which is a modification of the Goulard and Gibbs, have a hundred stations, maintaining 191,000 lamps, although the first Westinghouse plant was put down only three years ago. The progress in the United States is so rapid, and there are so many successful applications of central-station lighting, that the subject becomes too large to be even summarised, so that it is proposed to treat in the following pages with some of the principal installations on the Continent and at home.

Travellers abroad are accustomed to find electric lighting installed in the most out-of-the-way places, especially in Switzerland, where water power is abundant and is utilised to generate electricity, so that in small hamlets arc lighting is often employed, and the visitors to the local hotel will find it lit throughout by electricity. Electric light stations in England are, with one exception, small in comparison with those on the Continent. The most important is that at the Grosvenor Gallery, London, which has increased from small beginnings until it now supplies 20,000 glow lamps on sixteen circuits, the total length of which is seventy miles. The next largest, which have been in practical work for some time, are the Brighton and Eastbourne stations, from which small installations of glow and arc lights are maintained in various districts of the two towns. That the question of cost or trouble, and the annoyance of machinery when erected in a dwelling-house, do not altogether prevent the adoption of a superior light, is clearly proved by the increasing number of householders, who, after waiting in vain for electricity to be brought to their doors, have set up the plant necessary to produce it themselves, and find no practical difficulty in doing so. There are also many important public works where electric light has been exclusively adopted. For instance, at the Tilbury Docks there are 1350 glow and 80 arc lamps, distributed over an area of 300 acres, and including the lighting of an hotel, dock sheds, warehouse, signal-boxes, and offices. The London, Chatham, and Dover station at Victoria has also been electrically lighted for the past three years, the current being obtained from a central-station, which was erected for the purpose of supplying electricity to the Victoria district, and for which a provisional order was obtained. This, however, has since been abandoned, although £16,000 had been expended on plant and buildings by the promoters, who preferred to postpone the scheme rather than to submit to the onerous 27th clause of the Electric Lighting Act. Another still larger installation has been put down to supply electricity to the Paddington station and district of the Great Western Railway, as far as Westbourne Park. It embraces an area of sixty-seven acres, and is lighted by 4115 glow and 98 arc lamps. The system adopted is that designed by Mr. J. E. Gordon, and has now been successfully worked for some time; but the many accessories which are introduced, such as telephones, telegraphs, and indicators, make it complicated in comparison with gas, or even with the ordinary electric light systems. The current is generated by two dynamos, each weighing 45 tons, and having revolving magnet wheels 9 feet 8 inches in diameter, 22 tons in weight, a third machine being kept in reserve. These dynamos are separately excited, and produce alternating currents. The electricity is led to a large switch-board for distribution throughout the district by means of five sub-stations; and from this board there branches a double system of mains, which run everywhere side by side, one-half the mains being connected to the first machine and one-half to the second, so forming an excellent arrangement for the prevention of total extinction of the light. The mains running to the sub-stations are on the divided system, which is introduced for the purpose of saving copper, as in a solid cable the loss of pressure is greatest when the full number of lamps is on, and decreases as the lamps are extinguished. With the divided main system it is intended to follow out Sir William Thomson’s formula, which equates the value of the loss of head, and the interest on the saving on the copper. If for a certain main this formula shows that a fall of 20 per cent. is the most economical condition for working, then, since by the divided main the pressure can be kept within a variation of 2 per cent. at the distant end, it follows that a considerable saving can be effected over an ordinary solid main. Special arrangements are adopted at Paddington to keep the pressure constant, a fall of potential being allowed for; thus at the engine-house the pressure is 150 volts, in the passenger station it is 120 volts, and at Westbourne Park it is 100 volts. The arc lamps are fed by the same mains, and are arranged two in series.

A small installation at Kensington Court, erected two years ago, for the purpose of supplying the houses in the immediate neighbourhood, has rapidly developed, and underground mains have been led in many directions from the station, and a constant service of electricity is provided for by means of secondary batteries. As this is the first practical exposition of the secondary batteries’ system of distribution, it is proposed to describe the installation under that head. Central-stations are also at work in Liverpool, Leamington, Taunton, Exeter, and there are also five large installations nearly completed in London, besides the Kensington Court station, all of which will probably be in full swing before the end of the year.

Electric Lighting from Central-Stations is now practically carried out on five different methods.

• I. By Secondary Generators or Transformers, with alternating current.
• II. The Edison or Parallel System, with continuous current.
• III. The Series or Bernstein System.
• IV. Multiple Series.
• V. Distribution with Secondary Batteries or Accumulators.