Electric arc

[2] In 1801, he described the phenomenon in a paper published in William Nicholson's Journal of Natural Philosophy, Chemistry and the Arts.

[4] In the same year Davy publicly demonstrated the effect, before the Royal Society, by transmitting an electric current through two carbon rods that touched, and then pulling them a short distance apart.

The demonstration produced a "feeble" arc, not readily distinguished from a sustained spark, between charcoal points.

The first continuous arc was discovered independently in 1802 and described in 1803[8] as a "special fluid with electrical properties", by Vasily V. Petrov, a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs.

In 1895, Hertha Marks Ayrton wrote a series of articles for the Electrician, explaining that these phenomena were the result of oxygen coming into contact with the carbon rods used to create the arc.

[10] She petitioned to present a paper before the Royal Society, but she was not allowed because of her gender, and "The Mechanism of the Electric Arc" was read by John Perry in her stead in 1901.

The energy given to electrons is dispersed rapidly to the heavier particles by elastic collisions, due to their great mobility and large numbers.

Unlike a glow discharge, an arc has little discernible structure, since the positive column is quite bright and extends nearly to the electrodes on both ends.

The arc occurs in the gas-filled space between two conductive electrodes (often made of tungsten or carbon) and it results in a very high temperature, capable of melting or vaporizing most materials.

This negative resistance effect requires that some positive form of impedance (as an electrical ballast) be placed in the circuit to maintain a stable arc.

Calcium carbide is made in this way as it requires a large amount of energy to promote an endothermic reaction (at temperatures of 2500 °C).

Once the voltage reaches the air-breakdown threshold, an arc ignites across the spark plug and short-circuits the terminals of the unit, thus protecting the latter from the overvoltage.

For example, a spark gap can be fitted with arcing horns − two wires, approximately vertical but gradually diverging from each other towards the top in a narrow V shape.

The SF6 technology mostly displaced the similar air-based one because many noisy air-blast units in series were required to prevent the arc inside the switch from re-igniting.

Similarly to the arcing horns, the spark gap is formed by two wires diverging from the base to the top.

When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc.

The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil or any other source of high voltage in the 10,000–30,000-volt range, such as a neon sign transformer (5–15 kV) or a television picture tube circuit (flyback transformer) (10–28 kV), and two coat hangers or rods built into a V shape.

The laser-guided arc technology could be useful in applications to deliver a spark of electricity to a precise spot.

Devices which may cause arcing include switches, circuit breakers, relay contacts, fuses and poor cable terminations.

When an inductive circuit is switched off, the current cannot instantaneously jump to zero: a transient arc will be formed across the separating contacts.

An arc flash describes an explosive electrical event that presents a hazard to people and equipment.

[20] Arcing over some types of printed circuit boards, possibly due to cracks of the traces or the failure of a solder joint, renders the affected insulating layer conductive as the dielectric is combusted due to the high temperatures involved.

In industrial, military and consumer electronic design, the latter method generally applies to devices such as electromechanical power switches, relays and contactors.

These chemicals can be produced by high-power contacts in relays and motor commutators, and they are corrosive to nearby metal surfaces.

An arc formed in air will ionize oxygen and nitrogen, which then can re-form into reactive molecules such as ozone and nitric oxide.

An arc that occurs outside is less of a hazard because the heated ionized gases will rise up into the air and dissipate into the atmosphere.