Electric Arc

Electric Arc

Electrical conduction in gases

Properties of the arc

Uses of electric arcs

An electric arc is an electrical discharge between electrodes in the presence of gases. In an electric arc, electrons are emitted from a heated cathode. Arcs can be formed in high, atmospheric, or low pressures and in various gases. They are used for highly luminous lamps, furnaces, cutting and welding, and as tools for spectrochemical analysis.

Electrical conduction in gases

Gases consist of neutral molecules, and are, therefore, good insulators; they do not supply free electrons that can move and so constitute an electrical current. Yet under certain conditions, a breakdown of this insulating property occurs, and current can pass through the gas. Several phenomena are associated with the electric discharge in gases; among them are the spark, dark (Townsend) discharge, glow, corona, and arc. In air under ordinary conditions, an electric field of intensity of about 30,000 volts per centimeter will separate electrons from air molecules and allow a current to flow—a spark or arc.

In order to conduct electricity, two conditions are required. First, the normally neutral gas must create charges or accept them from external sources, or both. Second, an electric field should exist to produce the directional motion of the charges. A charged atom or molecule, or ion, can be positive or negative; electrons are negative charges. In electrical devices, an electric field is produced between two electrodes, called anode and cathode, made of conducting materials. The process of changing a neutral atom or molecule into an ion is called ionization. Ionized gas is called plasma. Conduction in gases is distinguished from conduction in solids and liquids in that the gases play an active role in the process. The gas not only permits free charges to pass though, but itself may produce charges. Cumulative ionization occurs when the original electron and its offspring gain enough energy, so each can produce another electron. When the process is repeated over and over, the resulting process is called an avalanche.

For any gas at a given pressure and temperature there is a certain voltage value, called breakdown potential, that will produce ionization. Application of a voltage above the critical value would initially cause the current to increase due to cumulative ionization, and the voltage is then decreased. If the pressure is not too low, conduction is concentrated into a narrow, illuminated, “spark” channel. By receiving energy from the current, the channel becomes hot and may produce shock waves. Natural phenomena are the lightning and the associated thunder, that consist of high voltages and currents that cannot be artificially achieved.

An arc can be produced in high pressure following a spark. This occurs when steady conditions are achieved, and the voltage is low but sufficient to maintain the required current. In low pressures, the transient stage of the spark leads to the glow discharge, and an arc can later be formed when the current is further increased. In arcs, the thermionic effect is responsible for the production of free electrons that are emitted from the hot cathode. A strong electric field at the metallic surface lowers the barrier for electron emission, and provides a field emission. Because of the high temperature and the high current involved, however, some of the mechanisms of arcs cannot be easily studied.

Properties of the arc

The electric arc was first detected in 1808 by British chemist Humphry Davy. He saw a brilliant luminous flame when two carbon rods conducting a current were separated, and the convection current of hot gas deflected it in the shape of an arc. Typical characteristics of an arc include a relatively low potential gradient between the electrodes (less than a few tens of volts), and a high current density (from 0.1 amperes to thousands amperes or higher). High gas temperatures (several thousands or tens of thousands degrees Kelvin) exist in the conducting channel, especially in high gas pressures. Vaporization of the electrodes is also common, and the gas contains molecules of the electrodes material. In some cases, a hissing sound may be heard, making the arc “sing.” The potential gradient between the electrodes is not uniform. In most cases, one can distinguish between three different regions: the area close to the positive electrode, termed cathode fall ; the area close to the negative electrode, or anode rise ; and the main arc body. Within the arc body there is a uniform voltage gradient. This region is electrically neutral, where the cumulative ionization results in the number of positive ions equals the number of electrons or negative ions. The ionization occurs mainly due to excitation of the molecules and the gain of high temperature.

The cathode fall region is about 0.01 mm with a potential difference of less than about 10 volts. Often thermionic emission would be achieved at the cathode. The electrodes in this case are made of refractive materials like tungsten and carbon, and the region contains an excess of positive ions and a large electric current. At the cathode, transition is made from a metallic conductor in which current is carried by electrons, to a gas in which conduction is done by both electrons, or negative ions and positive ions. The gaseous positive ions may reach the cathode freely and form a potential barrier. Electrons emitted from the cathode must overcome this barrier in order to enter the gas.

At the anode, transition is made from a gas, in which both electrons and positive ions conduct current, to the metallic conductor, in which current is carried only by electrons. With a few exceptions, positive ions do not enter the gas from the metal. Electrons are accelerated towards the anode and provide, through ionization, a supply of ions for the column. The electron current may raise the anode to a high temperature, making it a thermionic emitter, but the emitted electrons are returned to the anode, contributing to the large negative space charge around it. The melting of the electrodes and the introduction of their vapor to the gas adds to the pressure in their vicinities.

Uses of electric arcs

There are many types of arc devices. Some operate at atmospheric pressure and may be open, and others operate at low pressure and are therefore closed in a container, like glass. The property of high current in the arc is used in the mercury arc rectifiers, like the

Key Terms

Artificial (hot) arc— An electric arc whose cathode is heated by an external source to provide thermionic emission, and not by the discharge itself.

Cold cathode arc— An electric arc that operates on low boiling-point materials.

Thermionic arc— An electric arc in which the electron current from the cathode is provided predominantly by thermionic emission.

thyratron. An alternate potential difference is applied, and the arc transfers the current in one direction only. The cathode is heated by a filament.

The high temperature created by an electric arc in the gas is used in furnaces. Arc welders are used for welding, where a metal is fused and added in a joint. The arc can supply the heat only, or one of its electrodes can serve as the consumable parent metal. Plasma torches are used for cutting, spraying, and gas heating. Cutting may be done by means of an arc formed between the metal and the electrode.

Arc lamps provide high luminous efficiency and great brightness. The light comes from the highly incandescence (about 7,000°F [3,871°C]) electrodes, as in carbon arcs, or from the heated, ionized gases surrounded the arc, as in flame arcs. The carbon arc, where two carbon rods serve as electrodes, was the first practical commercial electric lighting device, and it is still one of the brightest sources of light. It is used in theater motion-picture projectors, large searchlights, and lighthouses. Flame arcs are used in color photography and in photochemical processes because they closely approximate natural sunshine. The carbon is impregnated with volatile chemicals, which become luminous when evaporated and driven into the arc. The color of the arc depends on the material; the material could be calcium, barium, titanium, or strontium. In some, the wavelength of the radiation is out of the visible spectrum. Mercury arcs produce ultraviolet radiation at high pressure. They can also produce visible light in a low pressure tube, if the internal walls are coated with fluorescence material such as phosphor; the phosphor emits light when illuminated by the ultraviolet radiation from the mercury.

Other uses of arcs include valves (used in the early days of the radio), and as a source of ions in nuclear accelerators and thermonuclear devices. The excitation of electrons in the arc, in particular the direct electron bombardment, leads to narrow spectral lines. The arc, therefore, can provide information on the composition of the electrodes. The spectra of metal alloys are widely studied using arcs; the metals are incorporated with the electrodes material, and when vaporized, they produce distinct spectra.

See also Electronics.

Ilana Steinhorn