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supply of 2 X 220 volts as before, in spite of the contrary opinion expressed above.
In three-phase installations the four-wire system has gradually found more and more favor in preference to the three-wire system. It has important advantages, so that higher pressure is of less consequence. The author therefore recommends 3 X 120 volts star connections for three-phase installations, especially because there is then no difficulty in making an alteration in new areas if necessary. Arc Lamps
The revolution in lighting with small units which will accompany the general introduction of metal-filament lamps is sure also to affect the use of more powerful lamps.
For certain purposes for which arc lamps of a particular type have recently gained importance, metal-filament lamps are now entering into competition. In recent years a series of designs of arc lamps for small currents was brought out. The A. E. G. was the first to supply an arc lamp of this kind; it was called the "Rignon” lamp, after its inventor. Other firms followed suit, giving their lamps names referring to their size, such as Liliput, Mignon, Baby lamp, and so forth. Most of these lamps were intended for a current of two amperes, but they required a working pressure of about 110 volts per lamp. The first lamps were made for continuous current, those for alternating current being made at a later date. The principle embodied in all designs was a limited access of air, which produced a higher efficiency and a steadier arc, although the length of burning was not equal to that of the original long-burning lamp. Most of the new small arc iamps will burn from twelve to twenty hours. They found favor very quickly and were extensively used for lighting in private houses and workshops, in shop windows, for lighting in railway stations, and even for street lighting in small towns.
Let us consider the economy of these lamps. A continuouscurrent arc lamp in a 110-volt circuit consumes two amperes and gives a light of about 200 candle-power, thus requiring 1.1 watts per candle-power. A lamp for two amperes is therefore not sufficiently economical; even a 3.5-ampere lamp will give an average hemispherical light of only 100 to 120 candle-power. The specific consumption is therefore three to four watts per candle-power. It is evident from the above that the small continuous-current arc lamp is not so economical as the metal-filament lamp, and that the alternating-current lamp is worse still.
It should be borne in mind that metal-filament lamps can easily be made for units of 100 candle-power, so they are well able to satisfy the same requirements as to power of light as the smallest arc lamps. On the other hand, the latter require to be trimmed; they burn carbons and have a sensitive mechanism, a weak point with all these designs. In view of these facts, it is not difficult to foresee that progress in the manufacture of metalfilament lamps will cause these small arc lamps to vanish altogether.
Arc lamps for 500 candle-power are now considered to be the smallest lamps that it is worth while to make. They are represented by the four-ampere lamp now being manufactured, which has a high economy and will burn on continuous-current circuits. Any source of light of less power than 500 candle-power will be a metal-filament lamp.
It is an experience of the lighting industry that consumers seldom make use of a pecuniary benefit offered when a certain source of iight has to give way to a better and more economical one, and the ultimate result is always the use of more light for the same money, due to improvements in lighting. At present there seems to be no limit in this direction. The metal-filament lamp has no doubt created a demand for a better light for indoor lighting, and the eye will soon be accustomed to the improved light indoors and one will want equal improvements in streets and open places.
The arc-lamp industry is already prepared to meet the future demand for more light out-of-doors. Constant improvements in the design of flame arc lamps have now produced lamps which leave nothing to be desired in this direction. The smallest type of these lamps consumes six amperes and gives a light of 1500 candle-power. The 20-ampere lamps, however, will give up to 5000 candle-power. An equal power of light from a single arc lamp was quite unknown up to quite recently, and the 10ampere differential lamp for 1000 candle-power was considered quite sufficient for public lighting in large towns. Units of light giving 5000 candle-power are by no means beyond practical requirements in roads and places in large towns where traffic is congested, as is proved by the lighting of the Potsdamer Platz in Berlin with eight flame arc lamps, each for 20 amperes, giving a total of 40,000 candle-power. The lamps are mounted on two high posts so that the burning point is fixed at a height of 18 metres above the level of the road. By this means the source of light itself is not disagreeable to the eye and the lighting of the square is as uniform as possible.
In America there has always been a great tendency to suspend powerful sources of light in towns at a very great height, in order to obtain the above-mentioned advantages. It is, however, not possible to develop this system completely without having a source of light similar to the flame arc lamp for 20 amperes, which is now being manufactured.
Another great advantage accompanies the introduction of the flame arc lamps, and that is the fact of their suitability for alternating current as well as for continuous current. The ordinary alternating-current lamp for 15 amperes will not give a light of very much more than 500 candle-power, and it is not very well possible to increase the power of the light by raising the current, on account of the size of wires. This was therefore the limit of lighting with powerful units in alternating-current circuits. Flame arc lamps for alternating current are in every respect equal to those for continuous current, so that single units for 4000 candle-power can easily be manufactured for both cases.
The consumption of the alternating-current flame arc lamp is not higher than that for continuous current when taking the power-factor of the arc into consideration (85 per cent). The progress made in this direction is of decisive importance for future lighting with arc lamps, because alternating current is being used more and more for feeding circuits covering extensive areas.
Certain points in connection with flame arcs at first hindered their introduction; for instance, it only seemed possible to obtain a yellow light, which did not suit the public taste, especially in Germany. Conditions were more favorable in England and America, because one had become more accustomed to an unusual color of arc lamps, owing to the extensive use of long-burning lamps of a violet hue. It is, however, now possible to make highly efficient carbons, giving a very agreeable light that is almost white in color. The steadiness of the arc in flame lamps is not as perfect as it is in differential lamps burning carbons that are not saturated. It is impossible to prevent the arc from flaring up now and then, owing to slight irregularities in the composition of carbons. One has, however, become accustomed to this deficiency in street lamps. A comparison of fame arc lamps with long-burning lamps as used in America for street lighting is in favor of the former type, the arc being very much steadier than in the American lamps, which burn very thick carbons, allowing the arc to wander continually.
The cost of effect carbons is higher than that of ordinary carbons. To a certain extent this is due to a metal core which is necessary on account of the high resistance of the carbons, due to their great length. The expenses for trimming lamps, and so forth, are also higher than similar expenses for other types of lamps, because the mechanism is more complicated, and effect carbons leave ashes and other deposits which require to be removed very carefully
According to the experience of a large central station in Germany using lamps that burn 16 hours at a time, the costs for every hour of burning are split up as follows: Trimmers and supervision, about.
2 pfennigs Effect carbons, about.
4 to 5 Maintenance, repairs, spare globes, etc..
These figures are comparatively high, and may not always be fully covered by the saving in current. The superiority of flame arc lamps will, however, still remain unquestioned, since there is no other type of lamp that will give more than 100Q candle-power.
A new source of light, which is gradually gaining ground in Europe and is competing with other sources of light, is the mercury-vapor lamp. It is primarily suited for indoor lighting and is used in engine-rooms, boiler-houses and workshops. An attractive effect is obtained in shop windows and for lighting advertisements. The Berlin Electricity Works has already installed about 60 of these lamps in its boiler-houses and engine-rooms. The peculiar color of the mercury light is at present a hindrance to its use, but custom and improvements will no doubt overcome this defect in time.
When comparing the power of light, mercury lamps will be found to range between incandescent lamps (metal-filament lamps) and arc lamps. Mercury lamps will always retain a certain sphere of usefulness of their own, owing to the peculiar shade of their light.
In giving a general aspect of electric lighting in Germany it is impossible to overlook the progress made in gas lighting in recent years. The so-called Lukas light, an improvement upon the Auer light, was followed by powerful lamps burning compressed gas, such as the Millennium light and others. As yet, however, no type of gas lamp has given results approaching those obtained with ordinary arc lamps, and no single gas lamp will give a power of light equal to that of a single flame arc lamp.
The claim for the superiority of electric street lighting is fully substantiated by test results obtained in Berlin streets by Dr. Bloch. His careful measurements show a consumption of about 18 watts per lux and per 100 square metres with ordinary arc lighting, as against about 50 litres of gas per hour for the same effect produced with Millennium light.
Although improvements in gas lighting have not been able to question the superiority of arc lamps, they have certainly created a state of keen competition which electrical engineers are bound to meet by similar progress if they do not wish to lose ground.