These combine several lamps in series, and these series groups in multiple, or several lamps in multiple and these multiple groups in series, respectively.

They have but a limited application.

## Multiple Or Parallel Systems Of Distribution

By far thelargest number of lamps in service are connected to parallel systems of distribution. In this system, the units are connected across the lines leading to the bus bars at the station, or to the secondaries of constand potential transformers. Fig. 50 shows a diagram of ten lamps connected in parallel. The current delivered by the machine depends directly on the number of lamps connected in service, the voltage of the system being kept constant.

Fig. 48. Wiring Diagram Showing Introduction of the Current Regulator.

Inasmuch as the flow of current in a conductor is always accompanied by a fall of potential equal to the product of the current flowing into the resistance of the conductor, the lamps at the end of the system shown will not have as high a voltage impressed upon them as those nearer the machine. This drop in potential is the most serious obstacle that we have to overcome in multiple systems, and various schemes have been adopted to aid in this regulation. The systems may be classified as:

1. Cylindrical conductors, parallel feeding.

2. Conical " " "

3. Cylindrical " anti-parallel feeding.

4. Conical " " "

In the cylindrical conductor, parallel-feeding system, the conductors, A, B, C, D, Fig. 50, are of the same size throughout and are fed at the same end by the generator. The voltage is a minimum at the lamps E and a maximum at the lamps F; the value of the voltage at any lamp being readily calculated.

By a conical or tapering conductor is meant a conductor whose diameter is so proportioned throughout its length that the current, divided by the cross-section, or the current density, is a constant quantity. Such a conductor is approximated in practice by using smaller sizes of wire as the current in the lines becomes less.

Fig. 49. Wiring Diagram for A. C. System Showing Introduction of Mercury Arc Rectifier.

In an anti-parallel system, the current is fed to the lamps from opposite ends of the system, as shown in Fig. 51.

## Multiple-Wire Systems

In order to take advantage of a higher voltage for distribution of power to the lighting circuits, three- and five-wire systems have been introduced, the three-wire system being used to a very large extent. In this system, three conductors are used, the voltage from each outside conductor to the middle neutral conductor being the same as for a simple parallel system. Fig.

52 gives a diagram of this. By this system the amount of copper required for a given number of lamps is from five-sixteenths to three-eighths of the amount required for a two-wire distribution, depending on the size of the neutral conductor. The saving of copper together with the disadvantages of the system is more fully treated in the paper on "Power Transmission."