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 treated in Volume II. The subjects therein discussed include the principles, methods, and apparatus employed in direct- as well as in alternating-current distribution, underground and overhead conductors, recording-meters, house-wiring, arc and incandescent lamps. It will be noted in both volumes that the order in which the various subjects are treated follows the usual sequence of generating, transmitting, and utilizing the electrical energy for lighting or other purposes. This being the natural order is easier to follow than any arbitrary arrangement.

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.

In

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. 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, such as the entrance to 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 enclosing the lamp in a thick glass globe. If a lamp is in contact with or enclosed in cloth, wood or other combustible material, the heat may accumulate sufficiently to char or set fire to the latter.

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 practicable 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.

The incandescent light seems to be steadier than any other kind of light; but the author has heard of, or actually observed, cases where sensitive eyes were disagreeably affected by it. The slight fluctuations in speed and current due to the strokes of the engine often produce a perceptible flickering in the lights, which can best be detected by observing a piece of white paper held near the lamp. This may be overcome or reduced by higher speed or heavier fly-wheels, and certainly should be brought down until it is imperceptible. Two or more engines or dynamos working on the same circuit might shift the load from one to the other, or otherwise act inharmoniously. This would be more likely to occur with alternating currents which might surge back and forth, due to lack of perfect synchronism or equality of action in the generators. The sudden throwing on or off of motors or a considerable number of lamps, the intermittent slipping of a belt or inductive action between two or more alternating currents differing slightly in phase, are also causes of variation in lamps which should be guarded against.

3. The incandescent light is sometimes more expensive than gas light; but in isolated plants in hotels, factories, etc., where boilers, engines, and engineers are required in any case, the extra expense due to the electric light is small, and it costs less than the equivalent gas. When the exhaust steam is used for heating, the introduction of electrical generating machinery adds little to the coal consumption. With the arc, Nernst and Hewitt lamps, which require less watts per candle-power than the ordinary incandescent type, the cost of electric lighting is still lower.

As a matter of fact, however, the real importance and utility of the electric light is dependent upon its radical advantages over any other form of artificial light; and whether it costs a little more, a little less than, or exactly the same as, gas light, is not so very important. For example, gas light costs more than lighting by kerosene lamps; but the greater convenience and general superiority of gas are sufficient to practically eliminate the use of kerosene wherever gas is available. The advantages of the electric light over gas are similar in character to, and fully as great in degree as, the advantages of gas over oil; and this applies to the Welsbach burner and acetylene gas as well as to ordinary illuminating-gas. The advantages of the incandescent lamp

stated above, particularly the facts that it does not vitiate the atmosphere or produce as much heat, and can be lighted without matches, make it a superior kind of light in practically every respect; and it is probably a fact in nearly every case where electric light is introduced instead of gas, that this is the reason, and not because it is expected to be cheaper than gas. This, however, is only true when the cost of electric light is approximately equal to that of gas. If the cost were very much greater, it would prevent its use in many cases. If, on the other hand, electric light is actually cheaper than gas, in addition to its other decided advantages, then there would appear to be no reason why it should not be used almost universally wherever it can be obtained.

The standard incandescent lamp (carbon filament), giving 16 candle-power, consumes about 50 watts, and the ordinary charge for current to supply it is about three-quarters of a cent per hour. This applies to small quantities obtained from electriclight companies. For larger quantities a discount is generally made, and when a customer uses several hundred thousand kilowatt-hours per year the price may be one-half cent or less per lamp-hour. In most cases electric power for running motors is sold at a lower rate than for lighting because the former service usually extends over longer periods of time. The machinery in a factory, for example, runs about ten hours a day, whereas the lamps may be in use only an hour or two in winter and not at all in summer. On account of this difference a common charge for electric power is about five cents per K.W.-hour even for moderate quantities and corresponds to one-quarter of a cent for a 50-watt lamp. Large water-power plants under favorable circumstances are able to sell electrical energy at very low rates. At Niagara Falls the charge is about .015 cent per lamp-hour (50) watts), and about $20 per horse-power-year for twenty-four hours a day, which is less than one-third of a cent per K.W.-hour.

In isolated plants (steam), for the reasons stated on page 6, the cost of electric light or power is often less than the charges of electric-light companies. Fairly large installations properly designed and operated produce electrical energy at about 4 cents per K.W.-hour