When talking about the characteristics of a voltaic arc, it is worth mentioning that it has a lower voltage than a glow discharge and relies on thermionic radiation of electrons from the electrodes that support the arc. In English-speaking countries, this term is considered archaic and obsolete.
Arc suppression techniques can be used to reduce arc length or arc likelihood.
In the late 1800s, the voltaic arc was widely used for public lighting. Some low pressure electric arcs are used in many applications. For example, fluorescent lamps, mercury, sodium and metal halide lamps are used for lighting. Xenon arc lamps were used for movie projectors.
Opening the voltaic arc
This phenomenon is believed to have been first described by Sir Humphry Davy in an 1801 article published in William Nicholson's Journal of Natural Philosophy, Chemistry and Arts. However, the phenomenon described by Davy was not an electric arc, but only a spark. Later explorerswrote: “This is obviously a description not of an arc, but of a spark. The essence of the first is that it must be continuous, and its poles must not touch after it has arisen. The spark created by Sir Humphry Davy was clearly not continuous, and although it remained charged for some time after contact with carbon atoms, there was most likely no connection of the arc, which is necessary for its classification as a voltaic one.
The same year, Davy publicly demonstrated the effect before the Royal Society by passing an electric current through two touching carbon rods and then pulling them a short distance apart. The demonstration showed a "weak" arc, hardly distinguishable from a steady spark, between points of charcoal. The scientific community provided him with a more powerful battery of 1000 plates, and in 1808 he demonstrated the occurrence of a voltaic arc on a large scale. He is also credited with its name in English (electric arc). He called it an arc because it takes the form of an upward bow when the distance between the electrodes becomes close. This is due to the conductive properties of hot gas.
How did the voltaic arc appear? The first continuous arc was independently recorded in 1802 and described in 1803 as "a special liquid with electrical properties" by the Russian scientist Vasily Petrov, who was experimenting with a 4,200-disc copper-zinc battery.
Further study
At the end of the nineteenth century, the voltaic arc was widelyused for public lighting. The tendency of electric arcs to flicker and hiss was a major problem. In 1895, Hertha Marx Ayrton wrote a series of papers on electricity, explaining that the voltaic arc was the result of oxygen coming into contact with the carbon rods used to create the arc.
In 1899, she was the first woman ever to give her own paper before the Institute of Electrical Engineers (IEE). Her report was en titled "The Mechanism of the Electric Arc". Shortly thereafter, Ayrton was elected as the first female member of the Institute of Electrical Engineers. The next woman was admitted to the institute already in 1958. Ayrton petitioned to read a paper before the Royal Society, but was not allowed to do so because of her gender, and The Mechanism of the Electric Arc was read by John Perry in her stead in 1901.
Description
An electric arc is a type of electric discharge with the highest current density. The maximum current drawn through the arc is only limited by the environment, not by the arc itself.
The arc between two electrodes can be initiated by ionization and glow discharge when the current through the electrodes is increased. The breakdown voltage of the electrode gap is a combined function of pressure, distance between the electrodes, and the type of gas surrounding the electrodes. When an arc starts, its terminal voltage is much less than that of a glow discharge, and the current is higher. An arc in gases near atmospheric pressure is characterized by visible light,high current density and high temperature. It differs from a glow discharge in that the effective temperatures of both electrons and positive ions are approximately the same, and in a glow discharge, ions have much lower thermal energy than electrons.
When welding
An extended arc can be initiated by two electrodes initially in contact and separated during the experiment. This action can initiate an arc without a high voltage glow discharge. This is the way the welder starts to weld the joint by instantly touching the welding electrode to the workpiece.
Another example is the separation of electrical contacts on switches, relays or circuit breakers. High energy circuits may require arc suppression to prevent contact damage.
Voltaic arc: characteristics
Electrical resistance along a continuous arc creates heat that ionizes more gas molecules (where the degree of ionization is determined by temperature), and in accordance with this sequence, the gas gradually turns into a thermal plasma that is in thermal equilibrium as the temperature is relatively uniformly distributed for all atoms, molecules, ions and electrons. The energy transferred by the electrons quickly disperses with the heavier particles through elastic collisions due to their high mobility and large numbers.
The current in the arc is supported by thermionic and field emission of electrons at the cathode. Currentcan be concentrated in a very small hot spot on the cathode - on the order of a million amperes per square centimeter. In contrast to the glow discharge, the arc has a little discernible structure, since the positive column is quite bright and extends almost to the electrodes at both ends. The cathode drop and the anode drop of a few volts occur within a fraction of a millimeter of each electrode. The positive column has a lower voltage gradient and may be absent in very short arcs.
Low frequency arc
Low frequency (less than 100 Hz) AC arc resembles DC arc. On each cycle, the arc is initiated by a breakdown, and the electrodes change roles when the current changes direction. As the current frequency increases, there is not enough time for ionization at divergence in each half cycle, and breakdown is no longer needed to maintain the arc - the voltage and current characteristic becomes more ohmic.
A place among other physical phenomena
Different arc shapes are emerging properties of non-linear current and electric field patterns. The arc occurs in a gas-filled space between two conductive electrodes (often tungsten or carbon), resulting in very high temperatures capable of melting or vaporizing most materials. An electric arc is a continuous discharge, while a similar electric spark discharge is instantaneous. A voltaic arc can occur either in DC circuits or in AC circuits. In the latter case, she maystrike each half-cycle of the current. An electric arc differs from a glow discharge in that the current density is rather high and the voltage drop within the arc is low. At the cathode, the current density can reach one megaampere per square centimeter.
Destructive Potential
The electric arc has a non-linear relationship between current and voltage. Once the arc has been created (either by progressing from a glow discharge or by momentarily touching the electrodes and then separating them), the increase in current results in a lower voltage between the arc terminals. This negative resistance effect requires some form of positive impedance (like electrical ballast) to be placed in the circuit to maintain a stable arc. This property is what causes uncontrolled electrical arcs in a machine to be so destructive, as once the arc occurs it will draw more and more current from the DC voltage source until the machine is destroyed.
Practical application
On an industrial scale, electric arcs are used for welding, plasma cutting, electrical discharge machining, as an arc lamp in movie projectors and in lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is obtained in this way, since to achieve an endothermic reaction (at temperatures of 2500 ° C) a large amount ofenergy.
Carbon arc lights were the first electric lights. They were used for street lamps in the 19th century and for specialized devices such as searchlights until World War II. Today low pressure electric arcs are used in many areas. For example, fluorescent, mercury, sodium and metal halide lamps are used for lighting, while xenon arc lamps are used for movie projectors.
The formation of an intense electric arc, like a small-scale arc flash, is the basis of explosive detonators. When scientists learned what a voltaic arc is and how it can be used, effective explosives have replenished the variety of world weapons.
The main remaining application is high voltage switchgear for transmission networks. Modern devices also use high pressure sulfur hexafluoride.
Conclusion
Despite the frequency of voltaic arc burns, it is considered a very useful physical phenomenon, still widely used in industry, manufacturing and decorative items. She has her own aesthetic and is often featured in sci-fi films. The defeat of the voltaic arc is not fatal.
