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Electric Arc Lamps

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Electric Arc Lamps 4 UNIVERSITY OF ILLINOIS LIBRARY Class Book Volume m .. ... ...... : mm J0 _^.^:.^2^ ^i^^^'^P' .^^^^-^^;^^ I ... ... ;-W W.-.,.' ... Digitized by the Internet Archive in 2013 http://archive.org/details/electricarclampsOOjone ELECTRIC ARC LAMPS BY EDWARD WALTER JONES THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE IN ELECTRICAL ENGINEERING COLLEGE OF ENGINEERING UNIVERSITY OF ILLINOIS 19 11 , UNIVERSITY OF ILLINOIS May 29 19011 THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Sdward Ualte r Jones ENTITLED Slectric Arc Lsrops IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF "Bachelor of S o lence in Elect rical Engineering y'y^-^ Instructor in Charge APPROVED: HEAD OF DEPARTMENT OF Electrical Engineering TABLE OF CONTENTS page History of the Electric Arc 1 General Theory of the Direct Current Open Arc 3 The Direct Current Inclosed Arc 10 The Alternating Current Open Arc 14 The Inclosed Alternating Current Arc Lamp 17 The Flaming Arc Lamp 19 The Regenerative Flame Lamp 23 The Metallic Flame or Magnetite Lamp 25 The Titanium Carbide Arc Lamp 28 The Present Status of the Arc Lamp 30 ELECTRIC ARC LAMPS HISTORY OF THE ELECTRIC ARC In the year 1801 Sir Humphrey Davy, then a young man of 22, told in a lecture before the Royal Institution of a spark of "vivid whiteness" passing between two pieces of well burned car- bon. The spark occured when the carbon rods were touched to- gether and then separated while connected to the opposite poles of a galvanic battery. His source of energy was a battery of 850 pairs of copper and zinc plates placed in an electrolyte con- sisting of a solution of alum with a few drops of nitric acid added. Hence it is evident that simply a spark and not a continuous dis- charge resulted. In fact it was not until seven years later, in 1808 that the first true arc was produced. A huge galvanic bat- tery of 8000 cells had been placed in the laboratory of the Royal Institution. With this source of power at his disposal, Davy pro- duced the first continuous electrical discharge through air, the gap being about three inches long. Davy placed his carbon pen- cils, or rods, in a horizontal position and the upward current of heated air, together with the tendency to enlarge, caused the dis- charge to take the form of an arch, and because of this fact, in the year 1820 he definitely named the electric flame the arc. Until 1844 this phenomenon remained simply a brilliant laboratory experiment. In that year, however, Leon Foucault re- placed the soft charcoal electrodes with sticks of hard gas re- tort carbon and, using the newly invented Bunsen battery, pro- duced a steady and continuous light which was publicly exhibited in Paris. It attracted considerable attention and was utilized, in a small way, for the lighting of public squares and theaters. From 1844 on, inventors turned their attention to perfecting the lamp mechanism and it was soon developed to a rather high degree of efficiency. Because of its great cost, the inconvenience and labor involved in its care and maintenance, the weakening of the current and the evolution of obnoxious gases, the galvanic cell did not prove to be a commercial success as a source of energy. It was for this reason that no very extended use was made of the arc lamp until the discovery of magnetic induction and the inven- tion of the electro-magnetic generator. This invention stimu- lated practical as well as scientific investigations and in 1878 the present series arc lamp with its shunt coil was devised. From this date improvements and inventions followed one another in rapid succession until the present high degree of development of the arc lamp has been attained. 3 THE GENERAL THEORY OF THE DIRECT CURRENT OPEN ARC When two carbon rods are oonnected to the terminals of a battery or generator so that a constant difference of potential of forty volts or more is maintained and these rods are first brought together and then separated a short distance, a contin- uous electrical discharge takes place across the gap. This dis- charge is accompanied by a brilliant white light and the evolu- tion of intense heat. The circuit is maintained through the gap due to the presence of volatilized carbon and carbon mist. This conducting strip of mist and vapor, called the arc stream, which bridges the gap, flows from the negative electrode toward the positive electrode. Therefore, the character of the arc stream is determined very largely by the composition of the negative electrode. It also follows that the temperature of the arc is probably that of the boiling of the material composing the neg- ative electrode. In 1881 Sir Wm.Abney announced this deduction and said further that the temperature was constant. The tempera- lure was given by M. Viole as 3500oC and by Rosetti as 3900oC. The temperature of the negative carbon tip is probably about 3000**C. An exploration of the arc stream will show that it is by no means honogeneous. There is a thin layer of true carbon vapor covering the tip of the positive electrode; between this layer of vapor and the negative electrode the carbon is in the form of mist. The whole stream is surrounded by a greenish flame produced by the carbon mist combining with the oxygen of the sur- rounding air. This flame is almost a perfect insulator and hence plays no part in the conduction of current through the gap. The vapor near the positive electrode has a very much higher specific resistance than has the mist, and hence a large part of the energy consumed by the arc is absorbed in this layer of vapor and appears in the form of heat. It is this heat, in all probability, which causes the volatilization of the positive electrode. The volatil- ization of the positive electrode furnishes the carbon which is consumed or burned by oxidizing, and for this reason the positive electrode wastes away more rapidly than does the negative elec- trode, the ratio being about two to one in the ordinary open D. C. arc. This fact has given rise to the erroneous idea that the positive electrode feeds the arc stream. It does, however, furnish some of the carbon of the arc stream through heat evapor- ation. That the positive electrode does not feed the arc stream and its vaporization is not essential may be proved by making the positive electrode so large that the cooling effect prevents vol- atilization, when it will be found that the change in no way af- fects the operation of the arc. Near the periphery of the electrode the vapor layer, covering the positive carbon tip, is cooled by the surrounding air into mist. Now since volatilization can occur only where « the vapor is in actual contact with the carbon, it is evident that the inner part of the positive electrode tip is the only part of that electrode consumed by volatilization. This fact causes a hollow or crater-, as it is called, to be formed in the positive carbon, leaving a rim at the periphery. This rim is itself consumed, or rather burned, by combining with the oxygen of the surrounding air, with varying degrees of rapidity depending on length of arc and value of current flowing. For a short arc 5 the consumption is slow and hence the crater will be deep. The rate of consumption increases with an increase of arc length. The depth of the crater varies in general with the current except for long arcs. Thus it would seem that an increase of arc length would decrease the crater depth and, in fact, both crater and rim practically disappear for arc lengths greater than diameter of positive electrode. The tip of the negative electrode is covered with carbon mist. Hence the negative electrode is not volatilised, but is heated partly by the carbon mist and partly by radiation from the positive electrode. The tip of the negative electrode being covered by mist is consumed solely by burning and through supplying the arc stream carbon. The sides of the electrode being heated to a rather high degree and being unprotected from the air, burn away more rapidly than does the protected tip. These sides are heated by the crater of the positive carbon and hence the negative tip will be longer and more slender the larger the crater and the shorter the arc. In general, the length of the tapering parts of both carbons is increased by increasing the cur- rent and decreasing the arc length. If the current is sufficiently increased and arc length decreased, a mushroom tip is formed on the negative carbon. This mushroom tip is caused by carbon being deposited from the crater It interferes with the formation of the proper arc and obstructs the light from the crater. It is also undesirable in that it causes an unpleasant hissing noise. Hissing is an unstable con- dition of the arc and it may occur with any length of arc, pro- vided the current is sufficiently large. The real distinction between the hissing and silent arcs is that in the silent arc the crater occupies only the end of the positive carbon. The his- sing seems to be due to the crater becoming too large to occupy the end of the positive carbon, and hence extends up the side of the carbon. The air then rushes in radially, burns the carbon very rapidly and produces tho hissing sound.
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