Electric arc

“ The electric arc may be produced between any conducting materials maintained at different potentials, provided that the source of electric supply is able to furnish a sufficiently large current; but for illuminating purposes pieces of hard graphitic carbon are most convenient. ”

“ The earliest instance of applying Electricity to the production of light was in 1810, by Sir Humphrey Davy, who found that when the points of two carbon rods whose other ends were connected by wires with a powerful primary battery were brought into contact, and then drawn a little way apart, the Electric current still continued to jump across the gap, forming what is now termed an Electric Arc. ”

“ If such an arc lamp is connected to a source of electric supply having an electromotive force preferably of 100 volts, and if some resistance is included in the circuit, say about 5 ohms, a steady and continuous arc is formed when the carbons are brought together and then slightly separated. ”

“ The Electric Arc.—If the terminals of an electric circuit, in which is an electromotive force of forty or more volts, be formed of carbon rods, a brilliant and permanent luminous are will appear between the ends of the rods if they be touched together and then withdrawn a short distance from each other. ”

“ Such a current may eat away the copper of the positive pole and in time cause the wire to break, creating an electric arc and perhaps causing a fire. It may also cause wet wood through which it flows to become charred and finally ignited.
The ground is the bane of the electrical worker. If a system can be kept free of grounds, the chances of trouble are vastly reduced. The cause of most grounds is moisture. Nearly all substances except metals are fairly good insulators if dry; and nearly all of them are fairly good conductors if sufficiently wet. ”

“ It has been stated that modern theory recognizes two classes of electricity, the Static and the Dynamic. The difference is, however, solely noticeable in operation. Of the dynamic class there can be no more common and striking example than the now almost universal electric light. Yet, with a sufficient expenditure of chemicals and electrodes, and a sufficient number of cells, electric lighting, either arc or incandescent, can be as effectively accomplished as with the current evolved by a powerful dynamo. ”

“ He reasoned out the subject, and concluded that the electric spark between metals, the electric arc between the carbons, and other luminous electric phenomena are secondary effects due to the heating and illumination of electric carriers; that the electric spark of the conductors of ordinary electrical machines is simply a transfer of incandescent particles of metal, which effect a kind of electric convection, known as the disruptive discharge ”

“ Zeits., 1906, 27, p. 1070) reduced these intervals very considerably by using the electric arc to excite the waves, and in this way produced electrical waves possessing great energy. In these methods the terminals between which the arc is passing are connected through coils with self-induction L to the plates of a condenser of capacity C. The arc is not steady, but is continually varying. This is especially the case when it passes through hydrogen. These variations excite vibrations with a period 2π√ (LC) in the circuit containing the capacity of the self-induction. ”

“ In effecting a complete severance of the conductor for a short distance, after the current has been established, a very great local resistance is occasioned, giving rise to the electric arc, the highest development of heat ever attained. Vibration is another form of lost energy in mechanism, but who would call it a loss if it proceeded from the violin of a Joachim or a Norman-Neruda?
Electricity is the form of energy best suited for transmitting an effect from one place to another ”

“ Its whiteness and luminosity varies according to the oxygen playing on the lime cylinder, rendering the relative intensities of the different parts of the spectrum so erratic as to make it unreliable. This leaves the [Pg 20] (electric) arc-light as the only one which is really available. Remember how the arc-light is produced. A current of electricity passes between the ends of two thick black carbon rods, or poles as they are called, through an air space of small interval, and the passage of the current renders the tips of these rods white-hot (Fig. 2) . ”

“ The reference is to the fact that electricity cannot be transmitted through a vacuum. An electric arc begins to spread out as the density of the air decreases, and presently it is extinguished. An induction spark that will jump two or three feet in air cannot be made to bridge the tenth of an inch in an ordinary vacuum. ”

“ After a short time the carbons burn away, the arc becomes longer and establishes itself and the resistance becoming greater in passing from carbon to carbon and a correspondingly less current flows through b and a greater one through c. This makes the electro-magnets j and k strong enough to draw D to them in spite of spring Q. When D is attracted by j and k, C (or H) falls and again the arc lengthens, always being kept about 3/32 inch long. This is frequently and continually repeated, the delicacy depending upon the strengh of the spring Q as compared to the electromagnets strength.
Fig. 24. ”

“ In the arc-lamp the electric circuit is broken at the point where light is required. In glow or incandescent lamps the current is only hindered by the interposition of a bad conductor of electricity, which must also be incombustible. Just as a current of water flows in less volume as the bore of a pipe is reduced, and requires that greater pressure shall be exerted to force a constant amount through the pipe, so is an electric current choked by its conductor being reduced in size or altered in nature. ”

“ Now when electromagnets are to be employed with rapidly alternating currents, such as are used for electric lighting, the frequency of the alternations being usually about 100 periods per second, slitting the cores is insufficient to guard against eddy currents; nothing short of completely laminating the cores is a satisfactory remedy. I have here, thanks to the Brush Electric Engineering Company, an electromagnet of the special form that is used in the Brush arc lamp when required for the purpose of working in an alternating current circuit. ”

“ Especially when large gratings are employed do we find that the electric arc alone seems sufficient to give vibrations of the requisite power. The metals may be introduced into the arc in various ways, and in some cases where they can be obtained in sufficient quantity the metallic electrodes may be used in the place of carbon poles. ”

“ When the current is turned on the circuit aa is almost instantly broken and most of the current goes through c as the distance between carbons being a quarter of an inch the arc has a very large resistance. The electromagnets j an k attract D which lets loose C, which falls to lower carbon, and the resistance being almost nothing, most of the current goes through b. ”

“ The armature and the arc will both become electrified by this new accession of electric matter. We cannot therefore suppose one armature to be positively and the other negatively so; and for this reason no transmission of electricity, and no sensation of taste, can be expected. ”

“ All that I have said about the voltaic arc, the incandescence resulting from resistance to the current's flow, and so forth, in relation to electricity generated by galvanic batteries, applies to electricity generated by induction coils, or by magneto-electric and by dynamo-electric machines. Only it is to be noticed that in some of these machines the currents alternate in direction with each revolution of the swiftly turning coil, in others the currents are always in the same direction, and in yet others the currents may be made to alternate or not, as may be most convenient. ”

“ Suppose, for instance, that a piece of fine wire be fixed between the pole wires of the battery; it will be heated “white hot.” Or if two carbon points be approached in a glass of water, as in the illustration (fig. 227) , they will emit a brilliant light in the fluid from the voltaic arc which has given us the electric light. The current is the passage of electricity along the wire, and continues [Pg 224] until the working power or “potential” of one conductor is equal to that of the other. When they become equal of course the action ceases, as there is equilibrium. ”

“ It will be seen that the flame arc lamp has an enormous advantage over other types in the light yielded for a given electric power consumption.
The practical employment of the electric arc as a means of illumination is dependent upon mechanism for automatically keeping two suitable carbon rods in the proper position, and moving them so as to enable a steady arc to be Arc lamp mechanism. maintained. Means must be provided for holding the carbons in line, and when the lamp is not in operation they must fall together, or come together when the current is switched on, so as to start the arc. ”

“ Division of the Light.—Hitherto a separate pile, or a separate machine, has been necessary for the production of each electric light, and it has been found impossible to place two lamps in one circuit. This is readily understood when we consider the mechanism of the regulating apparatus. In electric lamps the approximation and the separating of tlie carbons are controlled by an electro-magnet, which itself follows the variations of resistance in the circuit produced by changes in the length of the voltaic arc. ”

“ The rays from the electric arc, as it is called, either pass directly through the glass lens, or are caught by the parabolic reflector and shot back through it in an almost parallel pencil of wonderful intensity, which illumines the darkness like a ray of sunshine slanting through a crack in the shutter of a room. The search-light draws its current from special dynamos, which absorb many horse-power in the case of the powerful apparatus used on warships. ”

“ An electric current of high intensity is made to pass into them; they may become heated, but they will not give out light unless they be separated by a little distance from each other. On separating them, by the hand or other-wise, the voltaic arc appears and gives out a very strong light. This light persists, provided the carbons are a few millimetres apart; but, as the carbons waste away, the distance between their tips becomes greater, the voltaic arc is lengthened, and soon the light goes out, unless the points are again brought near to each other. ”

“ Do you know that if that vault is taken by either an electric-arc or an oxy-acetylene blow-pipe, I will be suspected?”
“I didn’t think of that.”
“It is a fact! You see the police and the private agencies know a man by his work. How can we take that vault in a new way? How can I go through three layers of vanadium steel?” ”

“ The alternating current will operate incandescent lamps, arc lamps, electric heating apparatus, alternating current motors. It will flow through a straight wire with slightly increased retarding effect, but if the wire be wound on an iron bar its strength is greatly reduced. ”

“ Tonight I start work over the safe. We shall enter the vault by Sunday, when no one is in Pope’s office.”
“How are you going to cut that vanadium steel? Father told me once it was the hardest kind of metal.”
Fay glanced at the skylight which had been changed to a new position. “I told you,” he smiled, “that I had an idea. It’s so far out of my line that the police wont suspect me. I’ve been accused of using thermite, the oxy-acetylene blow-pipe, the electric-arc, with a water-rheostat, and other devices. This time I’ll go everybody one better. ”

“ Carbon in a comparatively pure state is difficult to ignite, owing to its great resistance to heat. At about 7,000 degrees it will fuse, and pass into a vapor which causes the intense illumination.
The earliest form of electric lighting was by means of the arc, in which the light is maintained so long as the electrodes were kept a certain distance apart.
To do this requires delicate mechanism, for the reason that when contact is made, and the current flows through the two electrodes, which are connectedp. 163 up directly with the coils of a magnet, the cores, or armatures, will be magnetized. ”

“ Some carbon rods have soft cores containing calcium salts. These vaporize in the arc producing the flaming arc light of a bright yellow color, and give more light than the ordinary lamp.
Important Topics
1. Heat effects of electric currents, uses and applications.
2. Computation of the heat developed in a circuit. Three formulas.
3. Electric lamps; incandescent and arc; construction, uses, efficiency.
[Pg 324]
Exercises
1. Sketch a circuit containing 10 incandescent lamps in parallel. If each lamp when hot has a resistance of 220 ohms, and the E.M.F. is 100 volts, what current will flow? ”

“ It seems to pile up at points and corners, and we say that it is denser at such places than at well-rounded parts of a charged conductor. All points and sharp places should be removed from a conductor, if it is desired to keep a charge for any length of time.
Electrification may escape from a point so rapidly that currents are produced in the surrounding air. As the particles of air become charged, they repel each other. The movement of the air particles may be so great that a lighted candle will be affected when placed near the point. This current of air is called electric wind. ”

“ There is still another way of generating electricity besides those that we have named; which are friction, chemical action, and the magneto-electric mode of generating a current. Electricity may be generated by heat. If we connect antimony and bismuth bars together and apply heat at the junction of the metals and then connect the free ends of the two bars to a galvanometer, it will indicate a current. These pairs can be multiplied, and in this way increase the voltage or pressure, and, of course, increase the current, if we assume that there is resistance in the circuit to be overcome. ”