Electric 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.