Fig. 2, and above them is a coal-bunker having a capacity of 10,000 tons. It will be observed that the whole arrangement is designed to save ground space and yet not sacrifice effectiveness, economy, and convenience of operation. The steam-piping system of this plant is shown on page 117.

converted into direct current to feed the three-wire network which supplies lamps, motors, etc., as already described on page 39. Each generator is directly connected to a vertical, compound engine. The boilers, 56 in number, are placed on two floors, as shown in

In most of the electric-light plants built from the beginning in 1878 to about 1890, the dynamos were comparatively small-20 to 100 H.P.-and belted to the engines. One plan was to have each machine or pair of machines driven by a separate high-speed engine.

The other method was to employ one or more large, low-speed engines which drove line-shafts by means of heavy belts, the various dynamo being also connected to the line-shafts by belting. These arrangements are still commonly used in direct-current arc-lighting plants, because for the most part they supply street lamps, the distances being considerable, so that the high-potential series system is adopted.

Fig. 4.

Typical Direct-current Arc-lighting Station.

The constant-current dynamo (Chap. XVIII.) required in this case is limited to a capacity of about 100 or 150 lights, so that many machines are needed. A modern example of such a station is illustrated in Fig. 4, a pair of dynamos being belted to each high-speed, tandem-compound engine. The series alternating is usually preferable to the series direct-current system, because thousands of arc lamps may be operated by one generator if desired, and incandescent lamps as well as motors at the same time, as stated on page 37. In such a case the general type of station represented in Figs. 2 and 3 may be adopted.

For several years after the alternating current was introduced in 1887, its use was limited to incandescent lighting and the generators were comparatively small, a few hundred lights' capacity, and usually installed in arc-lighting stations so that incandescent lamps could also be supplied. The consequence was that they were driven by high-speed or low-speed engines with belting, like the arc machines in Fig. 4. But the great increase in size of alternating as well as direct-current generators has resulted in the general adoption of direct connection in place of belting. Hence the modern station for electric light or power, whether direct, single-phase, or polyphase current, usually employs multipolar generators of large diameter, each directly connected to a steam-engine or other prime mover.

The isolated plant has followed similar lines of development, and is in fact a small central station except that high-speed engines of the various types shown in Chapters XI., XV. and XVIII. are generally adopted.

CHAPTER VI.

BUILDINGS FOR ELECTRIC-LIGHT PLANTS.

THE building in which an electric-light plant is placed may be designed and built specially for it, or it may be a building already in existence, and built for some other purpose. In the case of a central station the plant usually occupies the entire building, or a large portion of it; whereas, an isolated plant occupies a comparatively small space in the cellar or basement. In any case, the problem of constructing or arranging the building or space comes under the head of architecture rather than of electrical engineering; but it is always very desirable that the electrical engineer should at least be consulted, and have the plan submitted to him. Frequently, however, this is totally disregarded, and the electrical engineer is given a certain place in which to put the machinery; and the result is likely to be very unsatisfactory to all concerned.

This unwise practice is particularly common in regard to isolated plants; and it is the rule, rather than the exception, for the architect to provide a certain room or space which he may arbitrarily think sufficient for the electric-lighting plant. Very often this space is too cramped, or of wrong shape, to allow the machinery to be properly put in. In most cases the difficulty could have been entirely avoided if the electrical engineer had been given an opportunity to make suggestions or modifications in regard to the original plans. The author has visited many plants in which this trouble was very apparent. In one instance the machinery was located in what was little better than a hole in the middle of the cellar, in which ventilation was practically impossible. The consequence was that in summer the temperature of this place often rose as high as 125° F., thus causing the dynamos to run very hot; in fact, their actual output was reduced to about one-half, because they could not generate any more current without being heated above 160° or 170° F., which is the maximum allowable temperature. The attendants were,