ELECTRIC FUSE 491 ELECTBICITY 80,000 amperes in the metallic charge, which forms the secondary. Electric furnaces are also used in the manufacture of aluminum, calcium car- bide, phosphorus, various iron alloys, and in the production of artificial gra- phite. In the laboratory, small electric fur- naces are in common use. In these a current is passed through a high resist- ance wire, made of nichrome or some similar alloy, which is wound around the furnace, the latter being made of fire clay. The whole is inclosed in in- sulating material to prevent loss of heat by radiation. ELECTRIC FUSE. (1) A device used in blasting to explode the charge. The fulminate or the charge itself is lighted by means of an electric spark or a resistance section of fine platinum wire, which is heated to redness by the passage of an electric current in- duced by a voltaic or magneto-electric battery. (2) A safety device used to protect electric circuits against too great a vol- ume of current. The regular or metal circuit is broken by the introduction of a wire of lead or soft alloy, formed to melt at a point beyond which a current would be harmful. The melting of the fuse will stop the current by breaking the circuit. ELECTRIC HEATER. In electric heaters a coil of metal of more or less resistance is wound around a frame and is made the channel of a current sur- rounded by insulating material, the heat closed in various forms of receptacle which radiate it. The wire or strip of metal may be surrounded by air or by a •non-inflammable substance that serves as a conductor. Porcelain, asbestos, enamel, or glass into which the coils or circuits are imbedded are largely in use for fireproof insulation, while some classes of heaters are imbedded in ma- terials such as hyposulphite of sodium and crystallized acetate of sodium. There are great varieties in the methods used. In the Farville system heat is engendered by means of rods of metallic powder mixed with fusible clay, com- pressed by a force of 2,000 kilograms per square centimeter and baked at a temperature of 1,350° C. The Prome- theus heater has a strip of selected metal fused to an enamel covering which receives the heat sent through the strip of metal. In certain types a metallic paint is fired upon mica strips, which are so grouped as to determine the size of the heater. Electric cars are usually heated by sets of conducting wires coiled round porcelain tubes and connected with the motor. Electric heaters are somewhat too costly for house-heating, but they are of great use in appliances such as flatirons, cooking utensils, and small radiators. The industries devoted to the production of small heaters of this class have grown steadily in recent years. The larger kind are taking their place in certain processes of manufac- ture and production as in oil fields, where electric heaters are used to stimu- late the heavy petroleum and cause it to flow more easily. ELECTRICITY, a powerful physical agent which makes its resistance mani- fest by attractions and repulsions, by producing light and heat, commotions, chemical decompositions and other phe- nomena. About 600 B. C. Thales dicovered that when amber was rubbed with silk it be- came capable of attracting light bodies. The ancients seem to have known no more than this regarding electricity; nor for the first 16 centuries of the Christian era was much addition made to the solitary known fact in electricity. In 1600, Gilbert, who was surgeon to Queen Elizabeth and to James I., pub- lished a book, "De Magnete," in which for the first time the word "electric" was used in connection with science. He died in 1603. He regarded magnetism and electricity as two emanations of one fundamental force. He showed that not merely amber, but sulphur, glass, etc., are electrics. Otto Guericke, of Magde- burg, discovered that there was a repul- sive as well as an attractive force in electricity, and about 1647 constructed the first electrical machine. Newton, in 1675, observed signs of electrical excitement in a rubbed plate of glass. Hawkesbee, who wrote in 1709, also observed similar phenomena ; and Dufay, in the "Memoirs of the French Academy," between 1733 and 1737, gen- eralized so far as to lay do-svn the prin- ciple that electrified bodies attract all those which are not so, and repel them as soon as they have become electric by the vicinity or contact of the electric body. ' Dufay also discovered that a body elec- trified by contact with a resinous sub- stance repelled another electrified in a similar way, and attracted one which had been electrified by contact with glass. He thence concluded that the electric- ity derived from those two sources was of different kinds, and applied the names vitreous and resinous to them. Franklin attributed this difference to an excess or deficiency of the electric fluid, the former condition existing in elec- trified glass and the latter in resins. Otto Guericke had discovered that his sulphur globe, when rubbed in a dark
