30 per cent of the load goes to power users; therefore the system had to be adapted to all classes of motor service, as well as for lighting. For this service in such a district, the Edison direct-current three-wire system is certainly the most satisfactory, and this system has been pretty generally adopted. It permits the use of storage batteries; requires less copper than does the alternating-current system, because in it there is no loss due to inductance; gives better regulation; and is far better for general all-around power service.

To consolidate several systems of this class was a simple matter. It required merely that the separate networks of mains be tied together, and a uniform pressure kept on the system by each station feeding into it. In the outlying districts, however, the problem was more

ondary pressure ranged from 104 to 125 volts, while some of the earlier systems still maintained a secondary pressure of 50 volts. A few of the later stations had 2,000-volt lines, with a frequency of 60 cycles; and there were also polyphase (generally two-phase) lines for serving a motor load. A view of some of these earlier alternating-current stations is given in Figs. 19 and 20.

High-Voltage Polyphase Systems

This conglomerate mass, then, had to be unified. A careful study of various systems showed the four-wire threephase system, with a frequency of 50 to 60 cycles, to be the best suited to the distribution of such a load; and this system is being freely adopted. Good examples of it are found in Chicago, Milwaukee,

and the single-phase lighting feeders are switched on to this 2,300-volt connection. The various feeders are connected each to one of the phases and the neutral, so that the three phases are aproximately balanced. For a power load, then, connection is made to all three phases; and the motor is usually of the polyphase induction type, of which one is shown in Fig. 21. Where the capacity of the motor is very small-say under 3 H. P.the single-phase type of induction motor, equipped with some special starting device, is often used. Stepdown of the voltage to the service pressure is, of course,

pole line carrying principally alternatingcurrent primaries. Transmission to substations, which in this case are merely distributing centers, is then done over the three phases at 4,000 volts, the neutral preferably being carried along also. At the substation, only feeder regulators are required beside the switching apparatus; and the feeders here also are connected between a phase wire and the neutral, 2,300 volts. A diagram of a four-wire three-phase distribution is shown in Fig. 24(p. 154); and in Fig. 25 (p. 155) is seen a switchboard installed for such a system.

FIG. 22. ELECTRIC LIGHT POLE, WITH 150-LIGHT
TRANSFORMER.

a neutral conductor is not used and all load is connected between phase wires). Brooklyn has a two-phase 2,300volt 60-cycle system in the residence section; Philadelphia also, though in the latter city the current is generated at 5,500 volts and stepped down to 2,300 volts for alternating-current distribution.

A 500-volt two-phase generator is shown in Fig. 26. For higher voltages the revolving-field type is used, thus avoiding collector rings and brushes for the high-voltage current. At the South Boston station of the Boston Electric Light Company (now part of the Boston Edison Illuminating Company), the generators are wound for 2,300-volt threephase 60-cycle currents.

FIG. 23. POLE LINE CARRYING ALTERNATING-CURRENT

means

PRIMARIES.

of transmission. To provide enough copper for satisfactory transmission and regulation, would bankrupt a company. To build and to operate a new generating station in each section of

tioned, showed that system to be the proper one to adopt; and the three-phase system was chosen as the means to transmit current to substations located at or near the electrical center of the load. Such a transmission system, from a large alternating-current station in preference to several direct-current generating sta

tions, is considered good engineering whenever the total energy generated is large as compared with that used at any one locality. The voltage determined upon depended to some extent on the local conditions, but more largely on the efficacy of the insulation of underground cables, since in the cities all lines must

insulating material used in these hightension cables consists of paper treated with a resinous compound, the thickness in the 30,000-volt cable being about onehalf inch. Over this is a lead sheath about 8-inch thick, to protect the cable against moisture and mechanical injury. These cables are drawn into ducts laid below the surface in the streets, as shown in Figs. 3 and 4.

One of the earliest instances of the use of a three-phase transmission to a substation, for conversion to direct current of an Edison system, was in Chicago,

Electrical Age, February, 1904, p. 68.

stepped down again; and, after passing through the rotary converters, the current was fed into the direct-current system at 115 volts, one rotary being connected to each side of the Edison system. Such was the humble beginning of the very extensive system of high-tension transmission lines and substations which, at double the initial voltage of 4,500, is now in operation in Chicago.

In the same year there was installed in Brooklyn, N. Y., a similar transmission system with a rotary converter substation. Here the current was generated at 6,600 volts, 25 cycles, three-phase, at the