If we talk 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 emission of electrons from electrodes supporting the arc. In English-speaking countries, this term is considered archaic and outdated.
Arc suppression methods can be used to reduce its duration or the likelihood of formation.
In the late 1800s, the voltaic arc was widely used for public lighting. Some low pressure 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 film projectors.
Voltaic arc opening
It is believed that this phenomenon was first described by Sir Humphrey Davy in an 1801 article published in the Journal of Natural Philosophy, Chemistry and Arts by William Nicholson. However, the phenomenon described by Davy was not an electric arc, but only a spark. Later researchers wrote: “This is obviously a description of not an arc, but a spark. The essence of the first is that it must be continuous, and its poles should not touch after it has arisen. The spark created by Sir Humphrey Davy was clearly not continuous, and although for some time after contact with the carbon atoms remained charged, most likely there was no arc connection, which is necessary for its classification as voltaic. "
In the same year, Davy publicly demonstrated the effect in front of the Royal Society by transmitting electric current through two contacting coal rods, and then pulling them a short distance from each other. The demonstration showed a “weak” arc, hardly distinguishable from a stable spark, between the points of charcoal. The scientific community provided him with a more powerful battery of 1000 plates, and in 1808 he demonstrated the emergence of a voltaic arc on a large scale. He is also credited with her name in English (electric arc). He called it an arc because it takes the form of an ascending bow when the distance between the electrodes becomes close. This is due to the conductive properties of the hot gas.
How did the voltaic arc appear? The first continuous arc was recorded independently in 1802 and described in 1803 as a “special fluid with electrical properties” by the Russian scientist Vasily Petrov, experimenting with a copper-zinc battery consisting of 4200 disks.
Further study
At the end of the nineteenth century, the voltaic arc was widely used for public lighting. The tendency of electric arcs to flicker and hiss was a serious problem. In 1895, Hertha Marx Ayrton wrote a series of articles on electricity, explaining that the voltaic arc was the result of oxygen contact with the carbon rods used to create the arc.
In 1899, she was the first woman to ever read her own report to the Institute of Electrical Engineers (IEE). Her report was entitled “Electric Arc Mechanism”. Soon after, Ayrton was elected the first female member of the Institute of Electrical Engineers. The next woman was admitted to the institute already in 1958. Ayrton requested to read the report to the Royal Scientific Society, but she was not allowed to do this because of her gender, and the Electric Arc Mechanism was read by John Perry in her place in 1901.
Description
An electric arc is a type of electric discharge with the highest current density. The maximum current strength conducted along an arc is limited only by the external environment, and not by the arc itself.
The arc between two electrodes can be initiated by ionization and glow discharge when the current through the electrodes increases. The breakdown voltage of the electrode gap is a combined function of pressure, the distance between the electrodes and the type of gas surrounding the electrodes. When the arc begins, its voltage at the terminals is much less than that of a glow discharge, and the current is higher. The arc in gases near atmospheric pressure is characterized by visible light, high current density and high temperature. It differs from a glow discharge by approximately the same effective temperatures of both electrons and positive ions, and in a glow discharge, ions have much lower thermal energy than electrons.
When welding
An elongated arc can be initiated by two electrodes that are initially in contact and spaced apart during the experiment. This action can initiate an arc without a high voltage glow discharge. This is the way a welder begins to weld a joint by instantly touching a welding electrode to an object.
Another example is the separation of electrical contacts on switches, relays or circuit breakers. In high energy circuits, arc suppression may be required to prevent contact damage.
Volta arc: characteristics
The 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, which is in thermal equilibrium, since the temperature is relatively uniformly distributed across all atoms, molecules , ions and electrons. The energy transmitted by electrons is rapidly dispersed with heavier particles due to 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. The current can be concentrated in a very small hot spot on the cathode - about a million amperes per square centimeter. Unlike a glow discharge, the arc has a slightly distinguishable structure, since the positive column is quite bright and extends almost to the electrodes at both ends. A drop in the cathode and a drop in the anode of several volts occurs within a fraction of a millimeter of each electrode. The positive column has a lower voltage gradient and may not be present in very short arcs.
Low frequency arc
A low-frequency (less than 100 Hz) AC arc resembles a DC arc. On each cycle, the arc is initiated by breakdown, and the electrodes change roles when the current changes direction. As the current frequency increases, there is not enough time for ionization when there is a divergence at each half-cycle, and breakdown is no longer needed to maintain the arc - the voltage and current characteristics become more ohmic.
Place among other physical phenomena
Various forms of electric arcs are emerging properties of non-linear models of current and electric field. An arc occurs in a gas-filled space between two conductive electrodes (often made of tungsten or carbon), which leads to very high temperatures that can melt or vaporize 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, it can repeatedly hit every half-cycle of the current. An electric arc differs from a glow discharge in that the current density is quite high, and the voltage drop inside the arc is low. At the cathode, the current density can reach one megaampere per square centimeter.
Destructive potential
An electric arc has a nonlinear relationship between current and voltage. As soon as the arc is created (either by progressing from a glow discharge, or by instantly touching the electrodes, and then separating them), an increase in current leads to a lower voltage between the arc terminals. This negative resistance effect requires some form of positive impedance (like electrical ballast) to be placed in a circuit to maintain a stable arc. This property is the reason that uncontrolled electric arcs in the device become so destructive, because after its occurrence the arc will consume more and more current from a constant voltage source until the device is destroyed.
Practical use
On an industrial scale, electric arcs are used for welding, plasma cutting, machining by electric discharge, as an arc lamp in film projectors and in lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is obtained in this way, since a large amount of energy is required to achieve an endothermic reaction (at temperatures of 2500 ° C).
Carbon arc lights were the first electric lights. They were used for street lamps in the 19th century and to create specialized devices, such as spotlights, before World War II. Today, low pressure electric arcs are used in many areas. For example, fluorescent lamps, mercury, sodium and metal halide lamps are used for lighting, and xenon arc lamps are used for film projectors.
The formation of an intense electric arc, like a small-scale arc flash, is the basis of explosive detonators. When scientists learned what the Volta arc is and how it can be used, the variety of world weapons was replenished with effective explosives.
The main remaining application is a high voltage switchgear for transmission networks. Modern devices also use sulfur hexafluoride under high pressure.
Conclusion
Despite the frequency of burns with a voltaic arc, it is considered a very useful physical phenomenon, which is still widely used in industry, production and the creation of decorative objects. She has her own aesthetics, and her image often appears in science fiction films. A defeat by a voltaic arc is not fatal.