The unit of work shall be the joule, which is equal to 10,000,000 units of work in the centimeter-gram-second system, and which is practically equivalent to the energy expended in one second by an international ampere in an international ohm.

The unit of power shall be the watt, which is equal to 10,000,000 units of power in the centimeter-gram-second system, and which is practically 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 electromotive force induced in this circuit is one international volt while the inducing current varies at the rate of one ampere per second. See HENRY.

ELECTRIC ARC. The effect produced when an electric current is maintained between two electrodes or terminals at a gap or opening in the circuit. This phenomenon involves the production of light and the generation of heat, and consequently the arc is employed for purposes 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, whereas in the case of the arc 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 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 1843 carbon conductors formed from gas-coke instead of charcoal were made use of by Foucault, and later various 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 current of sufficient tension to force its way across the gap or opening where the arc is to be pro

minute spark is produced, and a part of the carbon or other material is volatilized and 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 terminals. 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 arc 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 intense heat, being at a temperature of about 3500° C. (Violle). The negative carbon takes a rapidly as the positive. Both carbons are incanpointed shape, but is consumed only one-half as siderable light is emitted, though about 85 per descent at their tops, and from these sources concent. of the total amount comes from the crater about 5 per cent., while the remaining 10 per just mentioned. The arc itself furnishes only affected by the magnetic influences, and the bowcent. comes from the negative carbon. The arc is shaped arc is produced with vertical carbons by the action of the earth's magnetism. It was from Many interesting effects take place in the arc, one this curved appearance that the arc took its name. 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 are varies with the material of the electrodes, and even with different qualities of carbon. However, 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 incandescent at the crater. Surrounding this is a nonluminous portion where a dark flame indicates bined with the carbon and carbon monoxide prothat the oxygen of the external air is being comduced. Outside of this is a luminous flame where the carbon monoxide is further oxidized and carbon dioxide formed. The arc plays an important part in electric lighting, and the dynamos and lamps used for that purpose will be found described in the articles on that subject. (See ELECTRIC LIGHTING; DYNAMO - ELECTRIC MACHINERY.) It also is the underlying principle of the electric furnace, where the intense heat generated is employed to melt the most refractory substances and perform important metallurgical operations. For a description of the electric arc in reference to practical conditions in electric 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, 1901). A popular treatise on electricity, in which some attention is paid to the are and its phenomena, is Thompson, Elementary Lessons in Electricity and Magnetism (New York, 1901). The studies of Mrs. Ayrton in The Electrician (London, 1900) and the Elec trical Engineer (London, 1899) are important contributions.

ELECTRIC BELL. See BELL.

ELECTRIC COLUMN. See DRY PILE. ELECTRIC DISCHARGE. See ELECTRICITY. ELECTRIC FISH. A fish which possesses the power of discharging electricity at will. Electric organs are well developed in three sorts of fishes: (1) Most strongly in the Brazilian electric eel; (2) in a genus (Malapterurus) of African catfishes; and, (3) most weakly, in the torpedo family of rays. Electric organs are present, but developed to a much slighter degree, in some other fishes, which are spoken of as pseudo-electric. The organs are modified muscular tracts and nerve-connections, by means of which the fish are able to give an electric shock. They lie, in the case of the torpedo, in the head and gill region; in the South American eel they are on the ventral surface of the long tail; and in Malapterurus they practically incase the entire body, or occur in lateral lines under the skin on each side of the tail. The electro-motive force seems to be under the influence of the central nervous system, and the nerves supplying the organs are enormously developed. In the torpedo the 'electric plates' are arranged vertically, and the current passes in the same direction from the lower to the upper side. In the other two they are arranged longitudinally, and the current in Gymnotus passes toward the head and in Malapterurus toward the tail. The discharge, when the animal is at its best, may stun an animal as large as the horse. This power is of use, doubtless, in defense, and probably in capturing smaller prey.

ELECTRIC EEL. This fish (Gymnotus, or Electrophorus, electricus) inhabits the rivers of the basin of the Amazon and Orinoco, wherever they are warm and sluggish. It is in shape like a thick, stout, blackish, scaleless eel, and may grow to be six feet long; but it differs so much in structure from ordinary eels that it has been set apart as a separate family or other group by Cope, Gill, and other ichthyologists. It is abundant, and seems to have the general food and habits of an eel, but little is known in regard to its generation or the use of its battery, except that it habitually kills more fish than it can consume. The flesh is filled with bones, but is said to be palatable, and is not only eaten, but is regarded by the native South Americans as having medicinal value.

ELECTRIC CATFISH. These shock-giving catfish compose the subfamily Malapterurinæ, of which the best known is the raash (Malapterurus electricus) of the Nile. It grows to a length of four feet, with the ordinary rayed dorsal fin replaced by a fatty dorsal fin just in front of the rounded tail. It is said to give a shock like that from a Leyden jar, which may be communicated by touching the creature with a conductor. Two other species are known.

THE TORPEDO. The electric rays, or torpedoes, constitute a family closely allied to the true rays. They embrace about six genera and fifteen species of the warmer seas of the world, and two species which approach our southern shores. They have rounded, disk-like bodies, with powerful tails, and may weigh 80 pounds; swim close to the bottom, and are dark-colored above and light beneath, like other fishes of their class. The bestknown species is Torpedo marmoratus of south

ern Europe, upon which Dr. Gunther, ichthyologist of the British Museum, made the following observations: He found that the phenomena attending the exercise of this extraordinary faculty closely resembled muscular action. The power is exhausted after some time, and it needs repose and nourishment to restore it. If the electric nerves are cut and divided from the brain, the cerebral action is interrupted, and no irritant to the body has any effect to excite electric discharge; but if their ends be irritated the discharge takes place, just as a muscle is excited to contraction under similar circumstances.

And,

singularly enough, the application of strychnine causes simultaneously a tetanic state of the muscles and a rapid succession of the involuntary electric discharges. The strength of the discharges depends entirely on the size, health, and energy of the fish. It seems to be "essential and necessary to them for overpowering, stunning, or killing the creatures [fish of various kinds] on which they feed, while incidentally they use it as the means of defending themselves from their enemies."

ANATOMICAL FEATURES. The electric organs in all the above-named fishes are to be regarded, according to Wiedersheim, as metamorphosed muscular tracts, and the nerve-endings belonging to them as homologues of the motor end-plates which are ordinarily found on muscles. As regards the minute structure of the electric organs, the same essential arrangements are met with in all the forms. The framework is formed of fibrous tissue, which gives rise to numerous polygonal or more or less rounded chambers (see Plate). Numerous vessels and nerves ramify in the connective tissue lying between these compartments, the nerves being inclosed in very thick sheaths, and having a great variety of origin, according to the species. In torpedo they arise from the 'electric lobe' of the medulla oblongata, a single branch coming also from the trigeminal; in all pseudo-electric fishes, as well as in Gymnotus, in which over 200 nerves pass to the electric organ, they arise from the spinal cord. The electric nerves of Malapterurus arise on each side from a single enormous nerve-cell, which, lying in the neighborhood of the second spinal nerve, is continued into a very large primitive fibre, which passes toward the end of the tail, dividing as it goes.

The end-organs in which the nerves terminate are disks, called 'electric plates,' formed of muscle-substance over one or the other side of which, according to the species, the terminal filaments of the nerve spread out. The side of the electric plate on which the nerve branches out is negative at the moment of discharge, while the opposite side is positive, and thus the different arrangements of the parts in Gymnotus and in Malapterurus render it clear that the electric shock must pass in different directions in these fishes-thus, in Malapterurus it passes from the head to the tail, but in the contrary direction in Gymnotus. In torpedo the discharge passes from below upward. The mechanism whereby the electric plates become temporarily charged with electricity is not known.

BIBLIOGRAPHY. Although the peculiar powers of the torpedo and of the gymnotus were well known to the ancients, the first scientific determination of the electrical character of the shock of the torpedo was by Walsh, in 1772-Of the

Electric Properties of the Torpedo (Philadelphia, 1773). From that date the electric organs of fishes have been made the object of special study by some of the greatest anatomists and physiologists, among them Jobert de Lamballe, who published a special work entitled Des appareils électriques des poissons électriques (Paris, 1858), accompanied by a magnificent volume of plates. More recent works are: Boll, Ueber elektrische Fische (Berlin, 1874); Sachs, Untersuchungen am Zitteraal (Leipzig, 1881); Fritsch, Die elektrischen Fische (ib., 1887-90); Schönlein, Beobachtungen und Untersuchungen über den Schlag vom Torpedo (Munich, 1894); Gunther, Study of Fishes (London, 1880); Wiedersheim, Com parative Anatomy of Vertebrates (London, 1897). ELECTRIC FURNACE. See ELECTRO-CHEMISTRY, INDUSTRIAL APPLICATIONS OF.

ELECTRIC FUSE. See FUSE.

ELECTRIC GENERATOR. See DYNAMOELECTRIC MACHINERY.

ELECTRIC HEATER. A device for the conversion of electricity into heat for purposes of artificial heating. Electric heaters consist essentially of coils or circuits of some refractory metal through which the current is passed, these coils or circuits being surrounded by air or some insulating material, and the whole being placed in a metallic box or radiator, which throws off or radiates the heat produced. In the simplest form of electric heater exposed coils of wire or strips of metal are wound around insulating material or left surrounded by air. Another common form consists of wire or strips of metal imbedded in asbestos, either in the form of coils or in flat layers. A third class of heater, to which belong the Leonard, Carpenter, Crompton, and other heaters, is made by imbedding the resistance wire in some fireproof insulation, such as enamel or glass. The Tommasi heater consists of a coil of wire imbedded in a material having great latent heat of fusion, such as crystallized acetate of sodium and hyposulphite of sodium. In these heaters the current is turned on until the

desired temperature has been reached, and is then turned off and the latent heat allowed to dissipate itself. It is claimed that the heater remains active about four hours after the current

is shut off. The Prometheus system, extensively used in Germany, consists of fusing a broad strip

of rare metal on to an enamel which forms the outside of the vessel, and passing the current through the metal strip or film. Tests have shown the efficiency of this apparatus to be between 84 and 87 per cent. The Le Roy system consists of inclosing sticks of crystallized carbon in glass tubes. In the Parville heater there are rods of metallic powder mixed with fusible clay, compressed under a pressure of 2000 kilograms per square centimeter and baked at a temperature of 1350° C. The above constructions are used

in electric cooking and heating apparatus. Electric heaters have their chief field of usefulness in supplying heat for cooking and for laundry irons and for warming electric cars. Unless electricity is produced at a very low cost, it is not commercially practicable for heating residences or large buildings. Nevertheless it is generally considered that the electric heater has a field of application in heating small offices, bathrooms, cold corners of rooms, street railway waiting. rooms, the summer villa on cool evenings, etc. It

has the peculiar advantage of being instantly available and portable, and it does not vitiate the atmosphere or make dirt. For heating electric cars the electric heater commends itself for reasons that are plainly obvious to all. For heating laundry irons it is commonly figured that eleetrically heated and gas-heated irons are on a par in economy when gas costs $1.25 per 1000 cubic feet and electricity costs 1 cent per horse-power per hour. Numerous tests and estimates of the efficiency of electric heaters for cooking purposes have been made, and the reader interested will find them adequately summarized in H. A. Fos ter's Electrical Engineer's Pocket-Book (New York, 1901). Generally speaking, it may be concluded that the efficiency of electric cooking ap paratus varies from 60 per cent. to 90 per cent. (for ovens), depending upon a number of variable conditions, such as time, size, quantity to be heated, and temperature rise. The efficiency of an ordinary cooking-stove using solid fuel is only about 2 per cent., 12 per cent. being wasted in obtaining a glowing fire, 70 per cent. going in the chimney, and 16 per cent. being radiated into

the room.

In a gas-stove, considering that the number of heat units obtainable at a certain price is but small compared with solid fuel, the ventilating current required for the operation alone consumes at least 80 per cent. of the heat units obtained by burning the gas. In the case of an electrical oven more than 90 per cent. of the heat energy can be utilized; and thus, although possibly 5 to 6 per cent. only of the heat energy of the fuel is present in the electrical energy, 90 per cent. of this, or 4% per cent. of the whole energy, actually goes into the food, and thus the electrical oven is practically twice as economical as any other oven, whether heated by solid fuel or by gas. See WELDING; ELECTRO CHEMISTRY.

ELECTRICITY. Few sciences can claim as great an antiquity as that of electricity. It is believed that Thales of Miletus (c.640-546 B.C.) knew that amber, after being rubbed, acquired the property of attracting light bodies; and Theophrastus (c.372-287 B.C.), in his treatise On Gems, mentions the fact that this power is not peculiar to amber. No definite scientific information was acquired, however, until the close of the sixteenth century. William Gilbert (q.v.) published in 1600 his great work De Magnete. In

this book he used for the first time the terms

electric force' and 'electric attraction,' and distinguished between ‘electrics' and 'non-electrics'— the former name being given to bodies which act as amber does when rubbed, the latter to bodies. such as metals, which, when held in the hand and rubbed, do not acquire the power of attraction. electric action, as Cardan (1501-76) had also He clearly distinguished between magnetic and done before, but in an imperfect manner. Robert Boyle, Sir Isaac Newton, and others made many interesting observations on electrical phenomena. the former showing that electric attraction takes place through a vacuum. Otto von Guericke, the inventor of a rude form of electrical machine (q.v.), also discovered electric induction, the phenomena of which were studied with special care by Canton nearly a century later. Hawksbee made several important advances, being the first to show how to electrify metals by rubbing. and also the first to observe that electric charges