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by some form of cut-out, which short-circuits the lamp when the filament is broken. One type of this device is called a "film cutout," and consists of a thin sheet, F, of paper or other material interposed between the points P and P connected to the conductors A and B which enter and leave the lamp, as represented in Fig. 8. This film obliges the current to pass through the lamp so long as the filament is intact; but when the latter breaks, the difference of potential rises from its ordinary value of 10 or 20 volts to the full E.M.F. of the circuit, which is usually several thousand volts. This high pressure is sufficient to puncture the film, allowing the current to pass directly across between the points P and P, thus short-circuiting the lamp and reestablishing the continuity of the circuit. In some cases a small automatic switch is employed, which is caused to close and short-circuit the lamp by means of a magnet connected across as a shunt between the leads of the lamp. The coils of this magnet are of high resistance, and carry little current until the filament is broken, when the full current is thrown. through them, causing the switch to close.

A

Fig. 8.

B

Film Cut-Out for Series Incandescent Lamp.

"Municipal" Series Incandescent Lighting Systems. These are similar to the preceding; but instead of operating with a standard arc-lighting current of 10 amperes, they are usually designed for about 3 or 3.5 amperes. This gives a filament of sufficient length and cross-section to be durable, and yet does not require excessively large leading-in wires. The lamps are made of various sizes, requiring about one volt per candle-power. The practice with this system is to feed the circuit with a constant potential, usually from 500 to 1,000 volts, several of such circuits being ordinarily operated in parallel by the same dynamo, D, as represented in Fig. 9. This arrangement is therefore a parallel-series system. When the filament of a lamp breaks, and it is automatically cut out of the circuit, the current increases in strength, since the total resistance is reduced, the potential remaining constant. This increase of current is indicated by an ampere meter, or current indicator A placed in each circuit, and is corrected and brought back

to its normal value by switching in an extra or "relief" lamp L1, at the station. This is usually done by an attendant who is kept on duty to watch the various circuits. The system is rather a crude one, and is rarely used except for street-lighting in place of arc lights where the more powerful light of the latter is not required. Either the direct or alternating current is applicable to this method of distribution, and both have been used. The current capacity of the dynamo must be sufficient to supply the various circuits in parallel. In the case shown (Fig. 9), the current required would be 15 amperes, since there are 5 rows of lamps, each taking 3 amperes. With 10 lamps of 50-candle-power and 50 volts in series, the dynamo should operate at a constant potential of 500 volts. A shunt or compound wound direct current machine, or a separately excited or composite alternator, would be suitable for the purpose.

A

D

Fig. 9.

Parallel-Series System of Distribution.

In the case of the compound or composite machines they should simply give an absolutely constant potential, since the number of lamps, and therefore the drop, on each circuit is constant.

Series-Parallel Incandescent Lighting Systems may be arranged in the manner indicated in Fig. 10. Several lamps are arranged in parallel to form a group, and a number of such sets are connected in series, as shown. It is not necessary for the groups to be identical, provided they are all adapted to take the same current in amperes, which should be kept constant, and provided the lamps of each set agree in voltage. For example, on the ordinary 10-ampere arc circuit, one group might consist of 5 lamps, each requiring 50 volts and 2 amperes; the next might be composed of 10 lamps, each taking 100 volts and 1 ampere, and so on.

Such groups have been used directly on the ordinary series arclighting circuits (constant current), like the series incandescent

lamps described on page 24. The former arrangement is even less practical than the latter, and is also inferior to the "municipal" system, since a lamp which breaks or burns out cannot be either short-circuited or compensated for by adding a lamp in the station. To provide for this contingency, which is likely to be of frequent occurrence, a local device is required for each group, which will either connect a new lamp whenever any one of the

Fig. 10. Series-Parallel System of Distribution.

lamps fails, or short-circuit the entire group. This is such a complicated and unreliable arrangement that the system is not a very practical one.

Alternating Current Series Systems. Each of the direct current series systems that have been described has, or at least might have, a counterpart alternating current system. The general arrangement and method of operation would remain substantially the same; but as the phenomena of alternating currents differ in some respects from those of direct currents, the discussion of such systems will be given in the chapters on Alternating Current Distribution.

CHAPTER III.

PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION.

IN contradistinction to the series connection of lamps or other devices to be supplied with electrical energy, the other common method of distribution is the parallel or multiple arc arrangement represented in Fig. 11. Assuming that four lamps, each taking one ampere, are to be fed, the current generated by the dynamo D should be 4 amperes, which divides at the point where the first lamp is connected, and 1 ampere flows through it. The remaining 3 amperes pass on to the next lamp, and so on. The current sup

plied by the source should be equal to the sum of the amperes

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required by all of the lamps or other devices that are connected at any given time. The voltage should be as nearly constant as possible; hence the system is designated as constant potential, but this is only approximately true. In the case illustrated, the dynamo generates 112 volts, which is slightly reduced by the resistance of the wires until it falls to 110 volts at the last lamp.

Parallel systems are far more important in electrical distribution than series systems; since practically all incandescent lamps, a large proportion of arc lamps, and nearly all electric motors, are supplied by them. Constant potential circuits are usually more complicated than the simple series systems, there being only a

single path for the current in the latter case, while with parallel connections there are a number of branching paths. Furthermore, the maintenance of a uniform voltage over a large district is exceedingly difficult. The "drop The "drop" or loss of voltage, due to the resistance of conductors, which has already been discussed on page 9, is particularly objectionable in incandescent lighting, since the slightest decrease of potential produces a very considerable diminution of light. For example, the candle-power of an ordinary lamp is reduced from 16 to 15, which is more than 6 per cent, when the pressure falls from 110 to 109 volts, or less than 1 per cent. Such a very small variation in pressure would hardly be appreciable in any other practical work, such as steam or gas distribution.

The drop in pressure produces three different effects in the lamps or other devices supplied by parallel circuits:—

(1) All of the lamps receive a lower voltage than that generated by the source of electrical energy.

(2) Some lamps may be supplied with a lower pressure than others.

(3) The potential at some lamps may vary when others are thrown on or off the same circuit.

The least harmful of these effects is the first, which merely requires the generator to be run at a little higher voltage, and does not necessarily involve any difference between the candle-power of the lamps, since the drop may be made substantially the same for all of them by some of the methods described later.

On the other hand, variations in the candle-power of lamps, due to either of the last two effects, are extremely objectionable and difficult to overcome. In order to study these problems let us take a specific case, and assume that 100 incandescent lamps are to be supplied with electric current. They are supposed to be divided into five groups of 20 lamps each; each lamp requires a current of 110 volts and one-half ampere, and gives 16 candle-power; therefore one group takes 10 amperes, the total current being 50 amperes. The members of each group of lamps are connected in parallel in the usual manner, but will be indicated by a single line in the following diagrams in order to avoid confusion. These groups are assumed to be 200 feet apart in a straight line, making a total distance of 800 feet between the extreme groups, as shown

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