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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

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