Each substance is characterized by the type of relationship of molecules and atoms. There are three fundamental state of matter – solid, liquid and gaseous, they are different distances between the atoms or molecules. In the solid and liquid states, these distances are small, this explains the low compressibility of solid and liquid substances and their common name – "condensed state". In gases, the distance between molecules is much larger, so they can relatively easily compressed under external pressure, electrical conductivity of gases also significantly different from the electrical conductivity of substances in condensed state. In solids and liquids outer shell electrons can easily lose touch with the core and move freely through the material. Free electrons, called conduction electrons carry the current in the conductors of condensed gases, in which electrons bound to their nuclei in normal conditions the current is not carried out, but in an electric arc ionized and acquire the electrical conductivity. The electric arc is one of the types of electrical discharges in gases, at which the passage of electric current through a gas gap under the influence of an electric field.
Passage of electric current through a gas is possible only if it charged particles – electrons and ions. The appearance of charged particles in the arc gap is due to emission (emission) of electrons from the surface of the negative electrode (cathode) and ionization are in the range of gases and vapors. Electric arc used for welding metals, called the welding arc. In contrast to conventional arc welding arc is an electrical arc discharge in an ionized mixture of not only gas but also the metal vapor and components that make up the electrode coatings, fluxes, etc. If the ionized air gap is in an electric field, then moving gas ions come in motion mapping and create an electrical current. However, when ionization occurs dynamic equilibrium consisting in the fact that each unit of time recovering the same molecules from the ions (, recombination), but falls apart. Thus, as soon as terminates the action of ionizing factor disappears, and electrical current stops.