ELECTRIC ARC. The effect produced when an electric current is maintained between two electrodes or termMals at a gap or Opening in the circuit. This phenomenon involves the pro duction of light and the generation of heat. and consequently the are is employed for purposes if illumination as well as for producing high temperatures. The arc is distinguished front a spark in that the latter is of extremely brief duration and has a disruptive character. whereas in the case of the are the vapor produced by the volatilization of the extremity of the electrodes is raised to a high temperature and forms a path across which the discharge takes place. Sir Humphry Davy in 1500 exhibited to the Royal Institution apparatus where a continuous spark was produced in a gap between two pointed pieces of charcoal, whether they were in air. or in water, or sonic other liquid. In 1505 Davy. by using a battery of 2000 elements. produced an arc nearly four inches in length. In 1543 carbon conductors formed from gas-coke instead of char coal were made use of by Foucault. and later vari ous substances were introduced into the carbons in order to increase the length of the arc and make it more steady.
The first essential of an arc is an electric cur rent of sufficient tension to force its way across the gap or opening where the arc is to be pro duced. Unless there is a very large difference of potential on both sides of the gap, there must first is' contact between the two carbons or other electrodes while the current passes. and then rfter they have been separated the are will he produced. As the terminals are separated a minute spark is produced, and a part of the carbon or other material is volatilized awl made conducting. The heat thus produced is so intense that it is necessary to employ electrodes of a highly refractory material. in order to prevent their melting or too rapid vaporization. and it is for this reason that use is made of carbon ter minals. The are can be produced by either an alternating or direct current from a battery or dynamo. In the case of a direct or continuous current. a voltage of about 45 volts is used to maintain the arc. The current necessary for an are between two carbons varies from 5 to 15 amperes. being about 10 amperes in commercial practice. The carbons, when used for lighting, are generally placed vertically above each other. and the positive carbon is distinguished by the formation of a crater. which is the most brilliant source of light as well as the place of most in tense heat, being at a temperature of about 3500 C. (Violle). The negative carbon lakes it
pointed shape, but is consumed only one-half as rapidly as the positive. Both carbons are incan descent at their tops, and from these con siderable light is emitted, though about 53 per cent. of the total amount comes from the crater just mentioned. The arc itself furnishes only about 5 per cent.. while the remaining 10 per cent. comes from the negative carbon. The are is affected by the magnetic influences. and the bow shaped arc is produced with vertical carbons by the action of the earth's magnetism. It was from this curved appearance that the arc took its name. Many interesting effects take place in the arc. one of which is the change from carbon to graphite experienced in the electrodes of arc lamps. The composition of the furnished by the electric are varies with the material of the electrodes, and even with different qualities of carbon. How ever. the light in general resembles sunlight, but is richer in violet rays. An electric are formed between carbon electrodes will be found to con sist of a central portion of violet hue. which is doubtless the vapor of the carbon rendered incan descent at the crater. Surrounding this is a non luminous portion where a dark flame indicates that the oxygen of the external air is being com bined with the carbon and carbon monoxide pro duced. Outside of this is a luminous flame where the carbon monoxide is further oxidized and car bon dioxide formed. 'The are plays an important part in electric lighting_ and the dynamos and lamps used for that purpose will be found de scribed in the articles on that subject. (See ELECTRIC LIGHTING ELI:cra• MA CHINERY.) It also is the underlying prineiple of the electric furnace, where the intense heat gen erated is employed to melt the most refractory substances and perform important metalluroical operations. For a description of the electric arc in reference to practical cowl it ions in elec tric lighting. in which, however, the theory of the subject has not been neglected. reference should be made to Crocker. Electric Lighting, vol. ii. (New York. 1001). A popular treatise on electricity, in which some attention is paid to the arc and its phenomena. is Thompson, Elementary Lessons in •lertrieity and llagartisin (New York. 1001). The studies of Airs. \yrton in The Electrician- (London, 1900) and the Elec trical Engineer (London, 1599) are important cent ribution:.