ELECTRICAL UNITS. 749 The unit of work shall he the joule, which is equal to 10.000,000 units of work in the centi- meter{;raiu-secoiul system, ami whidi is practi- cally equivalent to the encrjjy expended in one second hy an international ampere in an inter- national olim. The unit of power shall be the watt, which is equal to 10,000.000 units of power in the ccnti- nieter-firam-second system, and which is prac- tically equivalent to the work done at the rate of one joule per second. One thousand watts is called a kilowatt. The unit of induction shall be the henry, which is the induction in a circuit when the electro- motive force induced in this circuit is one inter- national volt while the inducing current varies at the rate of one ampere per second. See Henry. ELECTEIC ARC. The eflfect produced when an eleilriu current is maintained between two electrodes or terminals at a gap or opening in the circuit. This phenomenon involves the pro- duction of light and the generation of heat, and consequently the arc is employed for jiurposes of illumination as well as for producing high temperatures. The arc is distinguished from a spark in that the latter is of extremely brief duration and has a disruptive character, wliereas in the case of the arc the vapor produced liy 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 Jlimiphry Davy in 1800 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 some other liquid. In 1808 Davy, by using a battery of 2000 elements, produced an arc nearly four inches in length. In 1S43 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 suiBcient tension to force its way across the gap or opening wlure the arc is to be pro- ELECTRIC BELL. ELECTRIC ARC. duced. Unless there is a very large difTerence of potential on both sides of the gap, there nnist first 1k> contact between the two carbons or other electrodes while the current passes, and then cfter they have been separated the arc will be produced. As the terminals are separated a minute spark is produced, and a part of the carbon or other material is volatilized and made conducting. The heat tlius produccti is so intense that it is necessary to employ electrodes of a liighly refractory material, in order to prevent their melting or too rapid vaporization, and it is for this reason that use is made ol carbon ter- minals. The arc 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 arc between two carbons varies from 5 to 15 amperes, being about 10 amperes in commercial l>ractice. 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 3.500° C. (Viollef. The negative carbon takes a pointed shape, but is consumed only one-h.ilf as rapidly as the positive. Both carbons are incan- descent at their tops, and from these sources con- siderable light is emitted, though about 8.5 per cent, of the total amount comes from the crater just mentioned. The arc itself furnishes only about per cent., while the remaining 10 per cent, comes from the negative carbon. The arc is affected bj- 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 are took its name, ilany interesting effects take place in the are, one of which is the change from carbon to graphite experienced in the electrodes of arc lamps. The composition of the light furnished by the electric arc 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 tiame 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 llame where the carbon monoxide is further oxidized and car- bon dioxide formed. The arc 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 : Dyxamo - Eucctric Ma- chinery.) It also is the underlying principle of the electric furnace, where the intense heat gen- erated is em))loycd to melt the most refractory substances and perform important metallurgical operations. For a description of the electric arc in reference to practical conditions in elec- tric lighting, in which, however, the theory of the subject has not been neglected, reference should be made to Crocker, Electric Lif/htiiifi. vol. ii. (Xew York. 1901). A popular treatise on electricity, in which some attention is paid to the arc and its phenomena, is Thonipson. Elemenlary Lessons in Electricit;/ and .Un/;»c/i.«m (Xew York. 1901). The studies of Ifrs.Ayrton in The Electrician (London. 1900) and the' Elec- trical Enqineer (London, 1899) are important contributions. ELECTRIC BELL. See Bell.
