ELECTRIC LIGHTING.

CHAPTER I.

INTRODUCTION.

ELECTRIC LIGHTING is the art of producing artificial illumination by means of electrical energy.

Generally speaking, an electric lighting system comprises three essential elements, viz. :

1. Apparatus for generating the electrical energy, for which purpose dynamo-electric machines driven by steam or gas engines or water-wheels are almost universally employed.

2. Means for transmitting and distributing the electrical energy, which consist largely of copper conductors.

3. Devices for converting the electrical energy into light, which are practically always either arc or incandescent lamps.

In addition to these three essential elements, certain auxiliary devices are commonly employed, such as transformers, secondary batteries, switching, regulating, and measuring apparatus, etc.

The words system, installation, and plant are all used to designate the collection of apparatus and other elements employed for electric lighting in any given case. The first term is used too freely; as, for example, when some trifling device is called "a new system of electric lighting." Nevertheless, these terms have their legitimate use in discussing electric lighting. Their significance in this connection is substantially identical with their ordinary meaning.

The dynamo-electric machines used in electric lighting are the various well-known forms of mechanical generators of electricity. They may be defined as machines for converting mechanical energy into electrical energy; or, in other words, they generate electric currents when driven by mechanical power.

The term dynamo-electric machine is so long that it is usually and almost unavoidably shortened into "dynamo," which

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has exactly the same meaning. The name "electric generator,' or simply "generator," is often applied to the dynamo, especially when it is used to produce current for electric railway or other motors; but this distinction is merely for convenience. An alternating current dynamo is commonly called an "alternator."

Two essentially different kinds of electric currents are in use, direct and alternating; and the differences between them give rise to very important variations in the construction and operation of electric-lighting plants.

A direct or continuous current flows in one direction only; whereas an alternating current reverses its direction of flow, and usually the reversals occur very rapidly, that is, 50 to 266 times. per second, the "frequency" or number of complete periods being between 25 and 133 per second in all systems in general use.

The steam or gas engines and water-wheels employed in electric lighting are practically the same as those used for other purposes, except that it is especially important that they should be very constant in speed.

Steam and gas engines and water-wheels being practically the only prime movers or sources of mechanical power used in electric lighting, are quite fully treated in Chapters VII. to XIV. inclusive.

The mechanical connection between the engine and dynamo is a matter of much consequence; in fact, it has been the cause of considerable trouble and discussion in electric-light engineering, and it therefore receives particular attention in Chapters XV. and XVI.

The dynamo being by far the most essential element in electrical engineering is treated in considerable detail. Chapter XVII. is devoted to the principles and construction of dynamos, Chapter XVIII. to typical forms, and Chapter XIX. to the practical management of these machines. The last subject is certainly of fundamental importance in electric lighting; and no fact concerning it, however small, is unworthy of consideration. Indeed, to nearly all electric-light engineers a knowledge of the construction of the dynamo is chiefly useful because it enables them to manage these machines more intelligently, and not because they are called upon to design or build them.

Accumulators are often used in connection with the generating

plant. An accumulator, also called a secondary or storage battery, consists of a number of voltaic cells containing plates or electrodes and a conducting liquid or electrolyte. Such a battery is inert in itself; but, on passing a current through it, certain chemical changes are produced, which render it capable of afterwards reproducing a large fraction of the electrical energy put into it.

In Europe accumulators have been more extensively and successfully employed than in the United States, but they are now being quite rapidly introduced into central stations and isolated plants in this country.

The principles, construction, and action of accumulators are discussed in Chapter XX.; and their use in electric lighting is considered in Chapter XXI.

Switchboards, including measuring instruments, switches, circuit-breakers, fuses, automatic cut-outs, rheostats, ground detectors, and other similar apparatus, are described in Chapters XXII. and XXIII.

Lightning arresters, which involve almost the only very uncertain questions in electric lighting, are carefully considered in Chapter XXIV.

This completes the list of elements which form part of the generating-plant; and the remainder of the subject, comprising the distribution and utilization of the electrical energy, is to be included in a second volume, to follow the present one. It is impossible to consider the generating-plant as a whole in this volume, since a knowledge of the distributing-conductors, lamps, etc., is essential to an intelligent general view of the station. This extremely important matter will therefore be reserved for the second volume, which will also include underground and overhead conductors, transformers, recording-meters, house-wiring, arc and incandescent lamps, applications of electric lighting, general management, finance, etc

ADVANTAGES AND DISADVANTAGES OF THE ELECTRIC LIGHT.

Before entering upon the detailed study of electric lighting, certain general questions present themselves for consideration. In the first place, the relation of the electric light to other forms

of artificial light is a matter upon which its ultimate success or failure necessarily depends. In other words, if the electric light does not possess decided advantages over the gas light and other means of lighting already in existence, it is obvious that its introduction is of no utility, and the study of it is unnecessary. In short, the very existence of the electric light in practical use depends upon its exact value compared with that of its rivals ; and therefore it will be well to carefully consider its various advantages and disadvantages.

The marked advantages of electric light over gas light may be enumerated as follows:

1. It does not vitiate the atmosphere; that is, it neither consumes the oxygen upon which the life and health of human beings depend, nor produces carbonic acid or other gases which are deleterious.

2. It is much cooler; i.e., it produces less than one-tenth as much heat for the same amount of light.

3. It can be lighted without the aid of matches, which is not only a great convenience, but also largely reduces the danger of fire.

4. The incandescent light is much steadier than gas light, and does not flicker even in a strong current of air.

5. The incandescent lamp itself is practically free from the possibility of setting fire to anything, because the source of light is hermetically sealed in a glass globe; and even if the globe is broken in a barrel of gunpowder or kerosene, it will not ignite them.†

6. The lamps are capable of much more convenient and æsthetic arrangement; that is to say, lamps can be put close against a wall or ceiling, or they can be placed pointing upward or downward, or inclined at any angle, all of which arrangements are impossible in the case of gas or other kinds of lamps.

7. The lamps can be lighted and controlled from any desired point. For example, the switch may be placed at the entrance of a building or room.

This is strictly true only of the incandescent lamp, but it practically applies to the arc lamp also.

† A mixture of explosive gases, however, might be exploded in this way. But this danger is largely avoided by inclosing the lamp in a thick glass globe.

8. Incandescent lamps can be obtained of any power, from a small fraction of one candle-power up to several hundred, and one can be substituted for the other in a few seconds, which is not practical with any other means of illumination.

It should be remarked that the above advantages apply more particularly to the incandescent electric light than to the arc light; but the former is the one used almost entirely for interior illumination, the latter being used more for street lighting and other rougher uses, where fine points of advantage are not of so much consequence.

The only disadvantages of the electric light to offset the numerous and important advantages stated above are:

1. The electric light cannot be turned down like a gas or oil lamp.

This objection is often urged; but it amounts to very little, because it is rarely desirable to turn down a light, and ninetynine times out of a hundred when it is done it is to save the trouble of relighting. To avoid danger of fire, and for other reasons, it is ordinarily a positive advantage to turn out a light entirely when not required; and this can be done in the case of the electric light without involving any trouble in relighting it.

Furthermore, the incandescent light can be dimmed, if desired, in several ways. A resistance can be used for a direct current, and a choke coil for an alternating current; and either of these can be applied without much trouble or expense, and the only reason they are not often used is that they are not needed. For a sick-room, or other place where a dim light is required, a low-candle-power lamp can be employed, or the light can be shut off by a shade or screen.

2. It is often stated that the electric light has an injurious effect upon the eye. The intense glare and usual unsteadiness of an arc light are often unpleasant, and would probably be harmful to the eye if exposed to it for any length of time. But the arc lamp is generally used for lighting streets, halls, railway stations, and other places where sight is general, and not applied to small objects. For lighting small spaces, or for any case where reading, writing, or other fine work has to be done, the arc light should be shaded, or arranged so as not to throw its glare directly into the eye.