distribute itself as shown on account of the lesser resistance of the wire relatively to that of the lamps. At the first lamp, the 3 amperes will divide, ½ ampere flowing through the lamp and the remaining 2½ amperes passing on to the next lamp and so on through the entire circuit. The reduction of pressure from 112 volts across the brushes to 110 volts at the last lamp is due to the resistance of the conducting wires.

Ques. What three effects are due to this drop in pressure?

Ans. 1, All the lamps or motors in the circuit receive a lower voltage than that at the dynamo, 2, some lamps or motors may receive a lower voltage than the others, and 3, the voltage at some lamps or motors may vary when the others are turned on or off.

Fig. 784.—Parallel system of distribution. This is a constant voltage system and is used principally for incandescent lighting and electric motor circuits.

The first is the least harmful and may be counteracted by running the dynamo at a little higher voltage; but the second and third are very objectionable and difficult to overcome. They are counteracted successfully in practice, however, by various methods of regulation, the use of boosters, and the operation of dynamos in parallel.

Ques. What are the principal applications of parallel or constant pressure systems?

Ans. They are used on practically all incandescent lamp and electric motor circuits, and on some arc lamp circuits.

Ques. Why is it specially applicable to incandescent lamp circuits?

Ans. Incandescent lamps cannot be made to stand a pressure much over 220 volts, and therefore have to be operated on low voltage systems.

Ques. What is the principal disadvantage of a parallel system as compared with a series system?

Ans. The greater cost of the copper conductors.

Fig. 785.—Arrangement of feeder and mains in parallel system. By locating the feeder at the electrical center, less copper is required for the mains. The cut does not show the fuses which in practice are placed at the junction of feeder and main.

Ques. What is the usual arrangement of parallel systems?

Ans. Conductors known as a feeder run out from the station, and connected to these are other conductors known as a main to which in turn the lamps or other devices are connected as shown in fig. 785.

Ques. In what two ways may feeders be connected?

Ans. They may be connected at the same end of the mains, known as parallel feeding, or they may be connected at the opposite end of the main, called anti-parallel feeding.

The main may be of uniform cross section throughout, or it may change in size so as to keep the current density approximately constant. The above condition gives rise to four possible combinations:

1. Cylindrical conductors parallel feeding, fig. 786;
2. Tapering conductors, parallel feeding, fig. 787;
3. Cylindrical conductors, anti-parallel feeding, fig. 788;
4. Tapering conductors, anti-parallel feeding, fig. 789.

Figs. 786 to 789.—Various parallel systems. Fig. 786, cylindrical conductors parallel feeding; fig. 787, tapering conductors parallel feeding; fig. 788, cylindrical conductors anti-parallel feeding; fig. 789, tapering conductors anti-parallel feeding. The term "tapering" is here used to denote a conductor made up of lengths of wire, each length smaller than the preceding length, the object of such arrangement being to avoid a waste of copper by progressively diminishing the size of wire so that the relation between circular rails and amperes is kept approximately constant. In an anti-parallel system, the current is fed to the lamp from opposite ends of the system.

Series-Parallel System.—This is a combination of the series and parallel systems, and is arranged as indicated in fig. 790. 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 2 amperes at 50 volts; the next might be composed of 10 lamps, each taking 1 ampere at 100 volts, and so on.

Fig. 790.—Series-parallel system of distribution. It consists of groups of parallel connected receptive devices, the groups being arranged in the circuit in series.

Parallel-Series System.—In this method of connection, one or more groups of lamp are connected in series and the groups in parallel as shown in fig. 791.