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LEFT ELECTROLYSIS 499 ELECTRO-METALLUr.GY, duction of ozone, peroxides and sodium, and in the preparation of various or- ganic compounds. . The fixation of atmospheric nitrogen can be carried out by passing an electric spark through the air. The nitrogen and oxygen of the atmosphere combine, producing oxides of nitrogen, which are dissolved in water, nitric acid being the final product. In the laboratory, the electric current is used in analysis for determining con- centrations, for titration, a galvanome- ter being used in place of a chemical in- dicator, and for determining metals quantitatively, by deposition on a plati- num electrode, and weighing. ELECTROLYSIS, the decomposition of chemical compounds by electricity. This word is one of the many that have come into common use since electricity has played so important a part in every-day affairs, and means the condition which causes decomposition of gas and water pipes buried near the wires of electric railroads. As long ago as 1833 it was discovered that the earth could be used as a part of a circuit to carry electric currents, and until the introduction of electric cars the earth was almost wholly depended upon for the return current required by telephone and telegraph ap- paratus. Now the best telephone cir- cuits have carefully insulated wires for the return current. The interference with the telegraph is much less than with the telephone from this cause. When electricity passes through moist earth it causes the decomposition of the water and the formation of oxygen and hydrogen gases. The oxygen, reaching metallic pipes, causes oxidation and ulti- mate destruction. The time required is, of course, wholly dependent upon the conditions, such as the volume of the current, the size of the conductor, and the amount of oxygen liberated. ELECTRO-MAGNETISM, the science which treats of the development of mag- netism by voltaic electricity, and the properties or actions of the currents thus evolved. Professor Oersted, of Copenhagen, led the way in the discov- eries which established the science; Am- pere, Faraday, Barlow, Arago, Nobili and others followed in his track. The temporary magnetic moment is propor- tional to the intensity of the currents. In the case of an iron bar it is propor- tional to the number of windings. In a mag:net it is proportional also to the square root of the diameter of the mag- net. In solid and in hollow cylinders of the same diameter it is equal in amount. The attraction of an armature by an electro-magnet is proportionate to the square of the intensity of the current, as long as the magnetic moment does not attain its maximum. Two unequally strong electro-magnets attract each other with a force proportional to the square of the sums of both currents. For powerful magnets the length of the branches of an electro-magnet is with- out influence on the weight which it can support. ELECTRO-METALLURGY. Under this term are included the processes of extracting metals from their ores, puri- fying them, and dealing with them by such special processes as annealing, welding, plating, etc. Where electric power is cheap, or where very high temperatures are re- quired, metals are extracted from their ores in the electric furnace, and under that heading will be found brief descrip- tions of the manufacture of graphite, carborundum, and steel by this process. Calcium carbide is also made in the elec- tric furnace, a mixture of lime and coke dust being heated to fusion by passing through the mixture an alternating cur- rent of 4,000 amperes at 110 volts. In a resistance furnace of a somewhat dif- ferent type, aluminum, is extracted from its ores. In this case, a direct current is used, and the furnace, which consists of a metal case lined with aluminum, is filled with molten ore. Metallic alumi- num is separated by electrolytic action (v. Electro-chemistry) and accumu- lates upon an iron or carbon plate at the bottom of the furnace, this plate forming the negative pole. Magnesium, sodium, and potassium are made in fur- naces of a smilar type, while steel and zinc are made in arc furnaces. Electric welding is carried out by one of three processes. The first de- pends upon the production of an elec- tric arc between the metallic surfaces to be welded and a rod of carbon. The metal is connected to the positive pole of a generator, and the carbon to the negative pole. The carbon, held in in^ sulated tongfs, is brought into contact with the metal and then drawn back a few inches, an arc being thus produced, the heat from which melts the metallic surfaces together. In the second proc- ess, the metal is connected to the nega- tive pole, while the positive pole is formed by a lead-lined vat. This vat is filled with dilute sulphuric acid, into which the metal is plunged. Electrolytic action is set up, and the metal becomes covered with bubbles of hydrogen, thus offering powerful resistance to the pas- sage of the current. The metal soon reaches the necessary temperature, when