Part 4

The Edison meters are based on the electrolytic action of a small fraction of the current which passes through the meter. They are cells, with rectangular zinc plates immersed in a solution of sulphate of zinc of 1·054 density, the distance between the plates being a little over ¼ inch. The proportion of the current which passes through the meter to that which passes directly into the consumer’s house is 1 to 973. The resistance of the shunt circuit is 9·75 ohms, made up as follows: cell, 1·75 ohm; metallic portion, 8 ohms. The resistance of the metallic portion rises with the temperature, whereas that of the cells falls with a rising temperature; and in this manner the small variations of resistance which might take place in the cell are counter-balanced by the equally small variations in the resistance of the metallic portion. A complete meter consists of two similar-sized cells of the same resistance, placed in series. The object of employing two cells is, that when little current is passing, as in the summer months, one cell alone is used, and when the consumption is sufficiently large both cells are employed, and the mean between the two indications is taken as the basis for calculation in number of ampère-hours. The quantity of electricity passed through the cell is calculated by the loss of weight which has taken place in the positive plate. An employé of the society visits every meter monthly, taking away the old cells and substituting others freshly constructed. A book is kept in which the weights of the new plates and those of the returned plates are entered, and on the basis of these entries the accounts are made up. The largest plates are those in the 100-light meter, and are intended for a maximum current of 75 ampères in the main circuit; they are 6 inches long by 2 inches wide. In cases where a larger amount of current is taken, the capacity of the 100-light meter is increased by joining two or more copper strips across the terminals of the cells. The weak point of the system is the removal of the cells, which leaves the adjustment of the account to be paid entirely in the hands of the Electric-Light Company; in spite of this drawback, it is stated that there has not been a single complaint from consumers during the four years in which the meter system has been in use.

_Discovery of Faults._

It is evident that in so extensive a system of lighting a short circuit now and then between the lamp wires and the earth cannot altogether be avoided. Many of the lamps have been fitted to existing gas fittings, and are beyond the daily supervision of the company’s officers; the faulty place is often not easily accessible, so the first step taken is to discover on which of the two circuits the trouble has occurred. This is done at the station by joining two 16-candle lamps in series across the main conductors and the point of junction between the two lamps is connected to earth by a stout wire. As long as both circuits (positive and negative) are perfectly insulated from earth no current flows through this middle wire, and both lamps remain hardly incandescent; but, if one of the circuits should be in connection with the earth, the lamp which is joined on the other circuit will brighten up, because the potential of the middle wire and that of the faulty circuit are both zero, and consequently the lamp between the middle wire and the sound circuit receives the full pressure of 110 volts. To localise the fault, contact is made between the earth and the sound circuit by means of a fusible plug of known melting point, say for a thirty-lamp supply. If the fault is on a portion of the external circuit, supplying less than thirty lamps, its fusible plug will melt as soon as the sound main is put to earth. If, however, the fault is on a portion supplying more than thirty lamps, the fusible plug which has been inserted at the station between the sound main and the earth will melt instead. A series of fusible plugs are thus tried, increasing in melting capacity until one is found that does not go: in this case, the other plug on the faulty portion has melted, and the consumer’s lamps on that branch are extinguished; the position of the fault is thus localised, and the company proceed to remedy the defect without interfering in the slightest degree with the rest of their system.

THE ELECTRIC LIGHTING OF BERLIN.

The Edison system is also employed at Berlin, in fact the Deutscher Edison Gesellschaft have at the present time a monopoly of the supply of the city from three large central-stations, each of which serves the area in their immediate neighbourhood. The mains differ from those used at Milan in that stranded highly insulated cables, protected with steel wire on the outside, are laid under the pavement in every street throughout the district. With the exception of the Leipziger strasse and Unter den Linden, which are lit with arc lamps suspended from chains running between cast-iron poles 24 ft. high, about 100 to 250 ft. apart, gas is used for the street lighting, and electricity for the interior illumination of many public buildings and private houses; there are also a good many arc lights outside the shops and restaurants. The mains are on the Edison network system, the area of copper being such, that when all the lamps are on there is a loss of energy of 25 per cent.; but this does not occur on an average for more than half an hour a day. No sole concession is given to the company, who simply have the right to take up the pavement and cross streets, and for this permission they are bound to furnish any consumer in the district with a constant supply of electricity at the following charges:—

10-candle lamps 2·5 pf., about 0·29 _d._ per hour. 16- ” 4·0 ” 0·48 ” 32- ” 8·0 ” 0·96 ” 50- ” 12·5 ” 1·50 ” 100- ” 25 ” 3·00 ”

In addition to this an installation fee of 6_s._ per lamp is charged, which includes one lamp.

Meters are charged as follows:—

_£ s. d._ 10- to 16-candle-power 0 16 0 per annum. 25- ” ” 1 0 0 ” 50- ” ” 1 10 0 ” 100- ” ” 2 0 0 ”

A discount is allowed off this meter charge, varying with the number of hours the light is used in the year.

The cost of gas is about 4_s._ 9_d._ per 1,000 cubic feet, so the electric light is slightly the dearer illuminant.

The Aron meter, Fig. 19, is usually employed as the recorder of the electricity consumed. It consists of two pendulums, controlling two distinct clockwork gears. One oscillates at a regular speed, but the other has a permanent magnet, instead of a weight, and is variable in speed. The entire current passes through the solenoid, which is underneath the pendulum, with the magnet; the difference in speed between the standard and variable clocks is given in direct ampère-hours by a counter-gearing similar to the index of a gas-meter. An electro-magnet starts each pendulum when the current begins to flow, and immediately it ceases, two detents come into operation and hold the pendulums stationary.

CLASS III.

THE SERIES SYSTEM OF DISTRIBUTION.

This method dates back to the introduction of the incandescent light, and, although it has been frequently demonstrated that a small current of high potential could be employed to work incandescent lamps, the series system has never been installed on a commercial scale, and is confined to arc lighting. In the United States the usual pressure for arc lighting is 2,000 volts, and it is not an uncommon occurrence to have forty arc lamps in series upon a line over 10 miles in length, carrying a current of 10 ampères. To economically use this high pressure for glow lamps in series, they must be of such design as to enable the whole of the current to be passed through them without injury. The filament of an ordinary high-resistance glow lamp would be immediately destroyed, so that low-resistance lamps, having a much larger sectional area, must be employed. The Bernstein or the Cruto lamp, which can be made to have a “hot” resistance of about 0·7 ohm, and requires a current of 9·75 ampères, could be used, and the current might be economically brought from a great distance. Mr. Bernstein calculates that it would be possible to operate 6,000 of these 7-volt lamps from twenty dynamos, each giving a current of 10 ampères at a potential of 2,000 volts, and still have a margin for loss of current in the leads. An economical feature of this scheme is the easy way in which power could be saved when only comparatively few lights were required; for instance, in the daytime all the circuits could be looped together and fed by one dynamo, and, as the number of lights increased, so other machines could be switched in by having an auxiliary bank of lamps as a resistance. From the central-station twenty pairs of carefully insulated copper wires, say of No. 6 B. W. G., would lead to the houses; and, as a good-sized ordinary house takes on an average twenty lights, the conductor would pass through fifteen houses before it returned to the station. It is in the house that the practical difficulty commences, as in this series system the circuit must never be opened, so that the switches and safety appliances must be such that, whatever happens, there must remain some path for the current, otherwise all the lights on that particular circuit would be extinguished. Mr. Bernstein gives the designation of “short closed” if the current goes through the switch-lever, and “long closed” if the current is led through the lamps or other electrical devices.

Fig. 20 is a diagram of the lamps in any building. The street main, M, enters at the main switch, S, and continues from switch to switch, S¹ S¹, and returns to S before it leaves. It is necessary, to guard against any possible extinction, to construct all the switches so that it would be impossible to move the lever without a lamp was lighted; and, should the lamp give out, an equivalent resistance must be automatically inserted. These details have been investigated by Mr. Alexander Bernstein, who has designed a complete system for “series” lighting, and claims for it special economical advantages. It is, however, very doubtful if this plan can be recommended for adoption in private houses; but in public lighting, or in large establishments where an electrician could be kept to look after the fittings and the insulation of the conductors, there should be no more danger, in introducing the high-tension continuous current of 2,000 volts, than there is at present with the 100-volt alternating current, and the relative saving in weight of conductors would be an important item.

Installations on this method have been erected at Messrs. Brunner and Mond’s alkali works, and in several large factories in the United States where lights had to be distributed over a considerable area; the system has not, however, come into favour for central-station work.

CLASS IV.

THE MULTIPLE SERIES SYSTEM.

This method of using a high-tension current has already been referred to in connection with house-to-house lighting at Brighton, it was first employed for the street lighting of Chesterfield by the Brush Company. The electric lighting of the town of Temesvar, in Hungary, is on a far larger scale, and has, from November 1884, successfully superseded a combination of gas for the more important streets, and petroleum for the outlying ones, the total cost of which was 26,480 florins per annum. A twenty-four years’ concession was given to the International Electric Company, the plant remaining their property at the expiration of the term, subject to purchase by the municipality at their own valuation. The public lighting is stipulated to be effected by means of 731 glow lamps of the intensity of 16 candle-power; but the option is given to the company of switching out a fixed proportion of these lamps at 11.30 P.M., or of leaving the whole number in operation with their light-intensity reduced from 16 to 8 candle-power from 11.30 P.M. till dawn. The total number of lighting hours per annum is 3,597½ for the lamps which are in operation from dusk until dawn, and 1,816 for those which are extinguished at 11.30 P.M. The price fixed in the concession for public lighting is 1·5 kreutzer per 16 candle-power lamp per hour, equal to 53 florins 95 kreutzers per lamp per annum of 3,597½ hours, or 27 florins 24 kreutzers per lamp per annum of 1,816 hours. The company has found it more convenient to exercise the option reserved to it, of keeping all the 731 lamps in operation from dusk till dawn, reducing their light-intensity to 8 candles after 11.30 P.M.; and the municipality has agreed to pay a round sum of 29,000 florins (£2,416 13_s._ 4_d._) per annum for this lighting, and 41·95 florins (£3 10_s._) per annum for each additional lamp worked in the same way. Comparing these figures with what precedes, it will be found that the electric lighting of the streets now in operation costs 2,520 florins more than it did on the former plan of combined lighting, partly by gas and partly by petroleum. On the other hand, the streets are lighted throughout with 16 candle-power lamps from dusk until 11.30 P.M., and with 8 candle-power lamps from 11.30 P.M. until dawn. For electric light supplied to private consumers the concession fixes the price at 1·81 kreutzers per 16 candle-power lamp per hour, or 0·1131 kreutzers per candle per hour, with the right to charge 15 per cent. more for lamps of less intensity than 16 candles. In all these prices the renewal by the company of lamps failing from legitimate wear is included.

One central generating station has been provided for the whole town, from which at present four distinct circuits have been laid, each fed by a separate dynamo. The street lamps are connected up in “multiple series,” that is to say, in groups placed in series on the circuit, the lamps in each group being connected up in parallel.

Fig. 21 shows the arrangement diagrammatically. Each group consists of eight lamps in parallel; at present three of the circuits have twenty-four groups in series, and the fourth circuit has twenty-three groups in series, giving a total of ninety-five groups, comprising 760 lamps, of which 731 are public lamps and 29 are used at the central station. To meet the risk of interruption in any circuit through the failure of individual lamps, an automatic switch is arranged so as to put in a reserve lamp, in the event of a whole group being interrupted. Another self-acting device will short circuit the whole group, so that the other groups in the circuit will be unaffected. The automatic lamp-switch is contained, together with the reserve lamp, in the lantern, and the automatic group cut-out consists simply of an electro-magnet with a coil of high-resistance connected up in parallel with the group of lamps it protects. These appliances have been found to work well. The main conductors are formed of insulated single copper wire, 4·6 millimetres in diameter; they are carried overhead on porcelain insulators, fixed to telegraph posts or to wooden arms let into the walls of houses; the resistance of this conductor is about 1·1 ohm per kilometre. The glow lamps are placed in reflectors at an angle of about 45° from the vertical, and are carried on brackets either fixed to the walls or on special cast-iron posts. Fig. 22 shows the details of street bracket and reflector with automatic lamp-switch and lamps in place. The brackets are for the most part fixed to the walls of houses or to painted wooden posts.

The under side of the reflector, which is made of enamelled iron disposed in the form of a flat inverted cone, reflects the upward rays from the lamp and causes the extreme ones to strike the ground at a distance of about 50 metres from the foot of the lamp-post. The increase of lighting effect in the streets due to those reflectors is very marked. The upper part of the reflector serves the purpose of a case and weather protector for the automatic lamp-switch which is inserted from the top, and the lower end of which is fitted with copper hooks to which the two lamps are fixed. The glow lamps are fitted with holders of a type designed by the engineer, which provide the lamp terminals with large and strong eyes affording considerable contact surface and adapted for hooking on direct to 2·5 mm. copper wire, the ends of which have merely to be bent into a suitable form for maintaining the lamp in any required position. These lamps are of an improved Lane Fox type, manufactured by the Electrical Company, at their works in Vienna. Although originally intended for 16 candle-power lamps they have so far been worked at 18 candle-power, taking 53·618 volts and about 1·183 ampères, which is equivalent to 3·522 watts per candle-power, or about 211 candles per horse-power. The current is maintained at 10 ampères, and the potential between independent groups of lamps is 53·6 volts. The aggregate energy lost, in overcoming the resistance of the main leads, switches and cut-outs, is 12·8 per cent, of the total electrical energy generated at the central-station—a very satisfactory result on a system of over 37 miles of streets. The electro-motive force in the conductors is about 1,400 volts, which is below the normal capacity of a Brush machine, thus allowing more lamps to be operated from the four machines. The machinery is driven by a 300 horse-power horizontal compound-condensing tandem steam-engine, running at the normal speed of 100 revolutions per minute. During the first 1,200 hours of lighting, only three lamps out of 760 failed, and one of these had been broken maliciously. The engineering arrangements are due to Mr. C. F. de Kierskowski Steuart, M. Inst. C.E., the various difficulties incidental to a novel work having been surmounted with experienced workmen. Although the system at Temesvar has more complicated arrangements than are now required if secondary generators are used, it has shown that it is quite practicable to light all the streets in a town by electricity; also it has enabled a comparison to be made between the useful effect obtainable from arc and from glow lamps. Each group of glow lamps was found to absorb practically the same energy as one arc lamp of from 800 to 1,000 candle-power, and ninety-one or ninety-two of these could have been run with the same expenditure of power as 731 glow lamps. The eight glow lamps forming one group are in many cases scattered in different streets, often quite out of sight of each other. Under such circumstances, the substitution of one light centre, however powerful, for every eight could only be done by leaving many spots in complete darkness. To give a usefully diffused light by means of arc lamps, their number would have to be considerably greater than ninety-two, or, in other words, the standard of street lighting would have to be raised, and for this the town was not prepared to pay.

The business has now passed into the hands of the Anglo-American Corporation of London, who are extending the installation by placing alternating current dynamos at the station to work transformers for the supply of houses so as to utilise the original plant for street lighting only, as, even with the advanced knowledge of the present day, it is doubtful whether for this purpose a more economical system could be employed.

CLASS V.

THE DISTRIBUTION WITH SECONDARY BATTERIES, OR THE BATTERY TRANSFORMER SYSTEM.

Mr. Lane Fox was the first to put forward a complete system of electrical supply on this plan, Fig. 23.

The system is discussed by him as follows:[4]—

“The chief points of the system is the use of a generator in a central position, from one pole of which insulated conductors or mains are led to the several points where the electric energy is to be utilised, being branched and sub-branched as much as required, and thence back to the other pole of the generator by an uninsulated conductor, such as the gas or water pipes. At certain points, storage or secondary batteries are set up in connection, on one hand, with the mains, sub-mains, and branches, as the exigencies of the case may require, and, on the other, with the return conductor.”

[4] Hedges on the Supply of Electricity by Local Authorities. Proceedings of Association of Municipal and Sanitary Engineers and Surveyors, vol. ix. (1882-83), p. 159.

“The combination of generators, circuit and storage batteries is such, that when the current from the generators falls below the demands made on it from the various outlets to the mains at which its energy is utilised, the deficiency is made up from the storage batteries, which act in unison to supply the requisite quantity of energy. On the other hand, when the current from the generator exceeds in point of quantity the demands upon it at the various outlets, the excess goes to charge the storage batteries and to create a reserve to be called upon in case of need.”

The objection to the system which prevented it being put in practical operation was the use of the earth as a return conductor. Besides the great danger of short circuit, the gas and the water pipes, which are so thickly laid in most cities, would conduct the current and interrupt telegraphic and telephonic communication. The experiment of using storage batteries as reservoirs, from which a constant supply of electricity could be drawn as required, was tried on a considerable scale at Colchester, where a large installation was started in 1884, secondary batteries being placed in favourable positions, and charged by a high-tension current. The plan adopted is shown by Fig. 24.

The dynamos were two of the Brush type, each dynamo giving a current of 9·5 ampères, with an electro-motive force of 1,800 volts, when rotated at a speed of 700 revolutions per minute. They were driven by a semi-portable engine indicating 90 horse-power. The dynamos were coupled in parallel circuit for quantity, and excited by a small machine giving 10 ampères. The current was led some distance by a seven-strand 19 B. W. G. cable to the batteries, which were charged in series, the 60-volt lamps being placed in parallel on separate mains connected to the batteries. The danger of introducing a high-tension current of 1,800 volts into the houses was obviated by a rocking-switch worked automatically, so as to throw the batteries out of the charging circuit. The operation was accomplished by means of a master cell M, C, Fig. 24, similar to the others, but fitted with an arrangement to collect the gas evolved, which extended a diaphragm attached to a make-and-break arrangement which worked the rocking-switch. The Colchester installation did not turn out commercially successful, and has been abandoned; but the experiment has been valuable, and there is little doubt that, with simplification of details, a high-tension charging current could be led from a dynamo fixed in any convenient site where power is available; also in very crowded districts the batteries could be placed in cellars and be drawn from as reservoirs, so as to furnish a constant supply of electricity.