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could also be used as a compensator to subdivide the total voltage in the three-wire system indicated in Fig. 56. Indeed, it is customary to use motor-dynamos for these purposes.

The Oxford System. — One of the most prominent examples of transmission and distribution by means of high-tension direct currents is the plant that has been in operation at Oxford, Eng-. land, for several years. Similar systems are also used in London (Chelsea), Shoreditch, and other places in Great Britain, the name "Oxford System " being applied generally to this class of installations. They may be regarded as extensions of the simple arrangement shown in Fig. 72.

In the diagram, Fig. 74, which represents such a system, D D are the main generators supplying direct currents at 1,000 or 2,000

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volts to the high-tension 'bus bars H H. Their field coils F F are fed from the low-tension 'bus bars EE, that receive current at 100 or 200 volts from the secondary circuit of the motor-dynamo K, the< primary L of which is supplied from the high-tension 'bus bars HHhy the wires IV. The storage battery B is also connected to the low-tension 'bus bars E, being charged by the machine K, in order to give current for lighting the station when all the generators (D D) are stopped, and also for exciting their field magnets in starting them. The current is carried from the high-tension 'bus bars in the generating plant over transmission conductors T to the two 'bus bars at the distributing station, that may be placed at a considerable distance without involving large expenditure for the conductors, since the energy is transmitted at high voltage. From these 'bus bars, the high-tension current is conveyed by the pairs of feeders F F to the primary circuits of the motor-dynamos Q (located at sub-stations), the secondary circuits of which connect with the three-wire mains MMM, supplying the outside wires at about 200 volts.

The lamps JJJ are fed from these three-wire mains in the usual manner. The motor-dynamos Q at the various sub-stations may either be controlled by attendants at the sub-stations, or they may be started and stepped from the distributing station by means of the starting rheostat 5 placed in each feeder circuit. In the latter case, the motor-dynamos are provided with series field coils Z in order to give magnetization for starting up, after which excitation is produced by a shunt winding (not shown) supplied from the secondary circuit. The latter is connected to or disconnected from the mains M\>y the switch C, that may be operated from the station by means of the magnet X and wires P. The latter also serve as pressure wires, the voltage on the mains being indicated in the station by a voltmeter V.

Compensators U are connected to the three-wire mains M at various points to equalize the voltage on the two sides of the system. These machines may be simple, like M N in Fig. 56, or they may be provided with series winding, as in Fig. 58, in order to raise the pressure on the more heavily loaded side when the system becomes unbalanced. The storage battery G is connected to the mains M for the purpose of supplying current during the day, or when the load is light, thus enabling all of the main generators D D, and motor-dynamos Q Q, to be stopped a considerable portion of the time. This battery is charged when the generating plant is running, the increased voltage required being produced by the booster T, or a differential booster may be used for the purpose.

If desired, the storage battery G may be employed to supply current for the "peak " of the load-curve (i.e., the short period of maximum load), thereby relieving the generating plant, the feeders FF, and the motor-dynamos Q Q, at the time of heaviest load. The battery may be charged when the load is lighter, so that this plan of working would tend to secure a uniform load on the machinery while running, and would also allow it to be stopped when the load is very light. Storage batteries may be installed in the generating plant (as at B), in the distributing station, in sub-stations on the mains (as at G), or in all three places; the nearer they are to the lamps, the more of the apparatus and conductors they may relieve at times of maximum load. In fact, one of the advantages of this or other direct-current system is the ability to use storage batteries in connection with it. In some cases the distributing station may be omitted, the feeders FF being run directly from the bus bars HH in the generating station to the motor-dynamos Q in the sub-stations. The Electrical World (N. Y.) of March 12, 1898, Contains a description of this system as used on a large scale at Chelsea, England, also the variable ratio direct-current transformers that are employed there. A description and illustrations of a more recent installation of this character at Bromley, England, are given in the Electrical World and Engineer (N. Y.) of Feb. 17, and in the London Electrician of January, 1900.

An important method of electrical distribution consists in transmitting the electrical energy by means of alternating currents, usually two- or three-phase, from the generating plant to stations at which it is transformed into direct currents by means of rotary converters, and distributed for lighting and other purposes. Such systems will be described after the principles of alternating currents have been considered.

CHAPTER VI.

NETWORKS OF ELECTRICAL CONDUCTORS.

The most complete system of parallel distribution is that in which the conductors are interconnected to form a network. This arrangement was developed from the "feeder and main" method of Edison,* and is also due to him. It is used in most of the large systems throughout the world for low-tension, direct-current distribution, and is often employed for the secondary circuits of alternating-current transformers, especially where the system is a large or important one. The enormous networks of mains constructed by the Edison Electric Illuminating Companies in New York, Chicago, Philadelphia, Brooklyn, Boston, and other large cities, may be cited as very prominent examples. Networks are sometimes adopted in the interior wiring of buildings; but they are usually quite simple in such cases, being seldom developed much beyond the ring mains represented in Figs. 19 and 20, which may be regarded as the simplest form of network.

A two-wire network of conductors is indicated in Fig. 75, A B CD being composed of two sets of positive mains at right angles to each other, and connected at the points where they intersect; E FG H being a similar network of negative mains represented by dotted lines. The mains are supplied with current from the generating station .S by feeders which are not shown in Fig. 75, because they would confuse the diagram. At any desired point a lamp will be fed with current if connected between the + and — networks. In fact, the case may be considered as equivalent to that in which two parallel sheets of copper are respectively connected to the terminals of a source of electrical energy, lamps being connected across from one sheet to the other.

Distribution of Curfent and Drop in Voltage in Networks. — In order to study the flow of current, let us consider by itself onef * See page 30.

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quarter of the positive network, and suppose it to be supplied at the point J by a feeder from the station S, as represented in Fig. 76. Assuming that the portions of the three horizontal and the three vertical mains included between the points A V X and Z are uniformly loaded, and not considering the effect of any load outside of this region, it follows that one-quarter of the current will flow out from the feeding-point J on each of the four mains leading therefrom. If ten lamps, each taking one ampere, are connected to each section of the mains, the initial current in the main J a will be 30 amperes, since three sections {Ja, a A, and a Y) must be supplied by it. When the current reaches the point a, it will have been reduced to 20 amperes, since 10 amperes are consumed in the sec

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tion J a. Hence the average current between J and a is 25 amperes; and if the resistance for each section be taken as .04 ohm, the drop will be 1 volt. The initial current from a to A is 10 amperes, and its final value is zero; hence it averages 5 amperes, and the drop is .2 volts for that portion. The same is true of the section a Y, provided that the load beyond Y be ignored, as already stated. Having thus determined the drop in voltage on the positive mains, it is evident that precisely the same drop will also occur on the negative conductors. A lamp at J will receive the full pressure supplied by the feeder, which may be assumed to be 112 volts; a lamp at a will have 112 — (1 + 1) = 110 volts; and a lamp at A will be fed with 112 - (1 + 1 + .2 + .2) = 109.6 volts. Similar statements apply to the lamps at the other points, WZ. etc.

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