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an air pressure was maintained in the subway it would prevent outside gas from entering. The fallacy of this hypothesis was soon evident, owing to the law that if two gases are separated by a porous diaphragm, even though greater pressure is maintained on one side

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than the other, the gases will still mix. The brick walls in the subway acted in this case as the diaphragm; and explosions still occurred after the introduction of this ventilating system, although of less frequency and violence. Tests showed, moreover, that the blower system produces practically no pressure except within a radius of a few blocks from the station.

It was then decided to employ gangs of men who would visit various manholes, remove the covers, and let the subway at these points become ventilated. Owing to the excessive cost of these methods they were finally abandoned. The ventilation of manhole covers was then considered. The chief objection to ventilating covers was that they would allow the subway to fill up with water and dirt, and thus cause trouble. This proved, however, not to be the case in practice; for late experiments show that the dirt and water accumulated in very small amounts, and that only a small force of men was needed to remove such accumulations. The amount of gas which collects in the conduits proves relatively much less than with the blower system, and the total saving is very considerable.

The escape of gas from the gas mains still constitutes a source of much annoyance and danger, and even at the present time the precautions above mentioned have not proved sufficient to entirely eliminate the dangers of explosions.

For further information regarding Underground Electrical Conductors reference may be made to the following works:

Electric Transmission of Energy by A. V. Abbott, Second Edition, N.Y., 1899.

Electric Distribution by Kilgour, Swan and Biggs, London, 1893. Electric Light Cables by S. A. Russell. London and N. Y., 1892. Localisation of Faults in Electric Light Mains by F. C. Raphael, London and N. Y., 1898.

Ligues et Transmissions Electriques par Wciller et Vivarez, Paris, 1892.

CHAPTER XIV.

THE ELECTRIC ARC.

Definition. — The electric arc is the phenomenon of light and heat occurring when an electric current persists in maintaining itself across an opening made in its circuit. When of short duration and of disruptive character it is known as a spark; the term arc being used to designate a continued discharge across a bridge of conducting vapor.

History. —The spark was first observed by Volta in 1800, in which year, too, Sir Humphry Davy discovered the particularly bright spark between charcoal points separated in air or under liquids, and exhibited it before the Royal Institution with the aid of a battery of 150 elements. It was not until 1808 that Davy, with a battery of 2000 elements, was able to exhibit the first true arc, an extended flame nearly four inches long, before the Royal Institution. This discharge was maintained between horizontal charcoal points, and owing to the current of heated air which is created, assumed a bow or arch shape; hence its name of "arc."

The intense brilliancy and whiteness of this light resulted in wide-spread efforts to utilize it practically; and numerous improvements followed, chief of which was Foucault's introduction in 1843 of gas-coke carbons to replace those of charcoal hitherto used. Another early step in advance was Grove's use of the salts of sodium and potassium to steady and increase the length of the arc. The application of the arc to practical purposes was often attempted, but the cost of electrical energy generated by a primary battery is so high that no commercial success was accomplished until the dynamo had been developed.*

General Features of Arc.— An arc may be maintained by either direct or alternating current. Under ordinary circumstances the

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two electrodes must be brought together before being separated to establish an arc, otherwise several thousand volts pressure would be required to strike across the air-gap in the first place. As soon as the separation of the terminals commences, the spark, which tends to form at any break in a circuit, vaporizes a portion of the material of the electrodes, thus establishing a bridge of conducting vapor through which the current flow is maintained. The concentration of energy in a small space produces an intense heat, which vaporizes the electrodes rapidly, so that a highly refractory terminal must be employed to avoid its rapid consumption. Moreover, the intensity of light given out by the arc depends on the temperature to which the electrodes can be heated without being vaporized, therefore a highly refractory substance like carbon best fulfills the requirements.

Appearance. When an arc is sprung between two carbon rods placed vertically one over the other, and kept about one-eighth of an inch apart, a constant current of about 5 to 15 amperes will produce a stationary condition after a few minutes burning. If observed through smoked glass, or, better still, if the image of the arc and carbons be projected upon a white screen by a lens, it will be seen that both carbons tend to become bluntly pointed, because oxidized away by the heat; but the positive carbon, which is usually the upper on account of its emitting more light, will have a hollow, or "crater," at the tip. This may be .04 inch deep and 2. inch across under average conditions. This is the hottest and most luminous portion of the carbons, attaining a temperature of approximately 3500° C. as Violle proved, breaking it off and dropping it into a water calorimeter. The intense heat thus generated can be realized when the melting point of platinum is considered, which is 1,775° C. The negative electrode exhibits no tendency to become hollowed, and remains pointed. In fact, the carbon particles burned from the tip of the positive carbon tend to deposit in the shape of a point or nib on the negative carbon, which is much cooler and less luminous than the positive. Both carbons appear luminous some distance away from the tips, this being especially noticeable on the positive. If the carbons contain impurities, these may generally be seen in beads near the tips, to which they often work their way to be instantly volatilized. Between the carbon points is the arc stream proper, which assumes a bow shape even when the carbons are vertical, owing to the magnetic action of the earth's lines of force on the current. The inner portion of the arc stream consists of a violet hub, probably of incandescent carbon vapor, surrounded by a thin non-luminous portion where the carbon combines with the oxygen of the atmosphere in dark flame to form carbon monoxide (CO). This is enveloped in turn by a layer of luminous flame in which the carbon monoxide burns to carbon dioxide (C02). The magnified image of an arc on a screen will show occasional carbon particles flying from the positive (which on the screen seems to be the lower carbon) to the negative, while other particles are thrown off into space by the action of the heated air.

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258. Appearance of Arc Light Carbons.

Noise. Under favorable conditions the arc is perfectly quiet, but emits a hissing sound like frying if the current exceeds the proper value for the length of arc employed.

Odor. A distinct odor is noticeable close to the arc, especially in damp weather, probably due to the presence in small quantities of hydrocyanic acid gas. Besides this, carbon monoxide and dioxide as well as nitric oxide are usually present; but none of these gases is given off in sufficient quantity to be injurious where the voltage of the arc does not rise above 55 in air.

After some minutes burning, it will be observed that both carbons waste away, the positive as a rule being consumed in the open direct current arc about twice as fast as the negative, the ratio depending on various conditions. For this reason the carbons must be fed together by hand or by some automatic device, otherwise the length of the gap would increase until its resistance exceeded the power of the generating apparatus to maintain a current through it.

Physics of the Arc. — Under commercial conditions direct current open arcs usually consume about 10 amperes at 45 volts or 450 watts. Thus nearly one-half k. w. of energy is concentrated in heating up the small extent of the crater and arc, resulting in the production of the very high temperature of 3500° C.

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