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Electric Arc - Arc Lamp

Long before the incandescent (very bright) electric light bulb was invented, arc lamps gave birth to the science of electric lighting. When the first large batteries were being built in the early 1800s, researchers noticed that electric current would leap across a gap in a circuit, from one electrode to the other. The result was a brilliant light.

The arc lamp has found a home for the future, however, in support of certain medical procedures. Lasers and arc lamps often work together to help stop chronic (recurring or happening for a long time) nosebleeds and other non-healing wounds. In addition, arc lamps are used in laparoscopic surgery where small incisions are used and a small video camera guides the surgeon to provide light for the procedure.

The electric discharge in the form of an arc is allied to the gas discharge which takes place when electricity is passed through rarefied gases and which is the basic principle of fluorescent lamps. The arc discharge take place when two carbon electrodes are brought into contact with each other and are then moved apart a distance of about one eighth of an inch (minimum voltage should be 55 volts).

Just before the carbon rods separate and direct material contact between them is broken, such a high electric resistance is developed at their boundary that the tips of the carbons begin to glow. This is associated with the emission of electrons which because of the high emission temperatures (upto 4000oC) produces a high degree of ionization of the air. With direct current the electrons are emitted from the cathode, i.e., the negative electrode; with alternating current the emission occurs at both electrodes alternately.

As a result of this ionization, the air in the immediate vicinity of the carbon tips becomes conductive to electricity, so that the current will continue to flow when the electrodes are no longer actually touching each other. The bombardment of electrons to which it is exposed causes the positive electrode (anode), in particular, to become white hot, and a crater forms at its tip. In the actual arc itself, which merely gives off yellowish violet light, the gas molecules of the air dissociate. They lose some of their enveloping electrons and form a mixture of positive ions i.e. electrically charged atoms and electrons i.e. negatively charged, which is externally neutral and which, on account of its particular properties is called thermal plasma (Fig.1).

The temperature of this gaseous state can be determined by spectroscopic investigations of its dissociated condition. It is found to be between 20, 0000 and 500000 C in the arc. In the arc lamp the arc serves as a source of light, but most of the light comes: from the incandescent tips of the carbons (Fig.2) and especially from the positive crater if the arc lamp is fed with direct current (Fig.3). As the carbons burn away, they have to be fed forward so as to keep the gap between them fairly constant. If this gap becomes too large, the arc will be extinguished.

In modern arc lamps the electrode feed is performed automatically (Fig.4). The springs F1 and F2 keep the carbons in contact with each other when the lamp is not functioning. When the current is switched on, the electromagnets E1 and E2 draw the carbons apart and thereby strike the arc. If the rate of burning away is too low, the resistance of the arc will increase. As a result, the current will become weaker, the pull exerted by the electromagnets will diminish, and the springs will draw the carbons closer together. This kind of control mechanism is still sometimes used in arc lamps of cinema projectors, but high-pressure gas discharge lamps are now superseding the arc lamp for this purpose.

In electric furnaces the intense heat is developed by the arc discharge is utilized for the melting of metals such as steel. If the material to be melted is a poor conductor of electricity, the heat radiated by the arc formed between two carbon electrodes is used to melt it (Fig.5). On the other hand, if the material does conduct electricity, then the arc discharge may either be passed direct from the electrodes to the material (Fig.6) or the electrodes may be actually buried in the material (Fig.7). In both cases the considerable heat developed in the electrodes help the current to generate heat in the material and thus attain the melting temperature.

Arc lamps are very strong sources of ultraviolet, visible and infrared light. They are excellent approximations of very bright point sources due to their small arcs. This feature makes arc lamps a good choice when precise collimation is required. Arc lamp light sources are also good choices for fiber optic applications, which require high intensity light focused on a very small point.