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Electro-Magnetism

the science which treats of the development of magnetism by voltaic electricity, and the properties or actions of the currents thus evolved. Professor Oersted, of Copenhagen, led the way in the discoveries which established the science; Ampere, Faraday, Barlow, Ara Arago, Nobili and others followed in his track. The Copenhagen, led the way in the discovtional to the intensity of the currents. In the case of an iron bar it is proportional to the number of windings. In a magnet it is proportional also to the square root of the diameter of the magnet. In solid and in hollow cylinders of the same diameter it is equal in amount.

The attraction of an armature by an ELECTRO-METALLURGY square of the intensity of the current, as electro-magnet is proportionate to the long as the magnetic moment does not attain its maximum. Two unequally strong electro-magnets attract each other with a force proportional to the For powerful magnets the length of the square of the sums of both currents. out influence on the weight which it can branches of an electro-magnet is withsupport.

ELECTRO-METALLURGY.

Under this term are included the processes of extracting metals from their ores, purifying them, and dealing with them by welding, plating, etc. where very high temperatures are re- Where electric power is cheap, or tions of the manufacture of graphite, where very high temperatures are reores in the electric furnace, and under carborundum, and steel by this process. tions of the manufacture of graphite, tric furnace, a mixture of lime and coke dust being heated to fusion by passing the mixture an alternating curdust being heated to fusion by passing through the a resistance furnace of a somewhat difrent of 4,000 amperes at 110 volts. In In this case, a direct current its ores. In this case, a direct current is used, and the furnace, which consists its ores.

Metallic alumiby electrolytic action is used, and the furnace, which consists of a metal case lined with aluminum, is filled with molten ore. (v. ELECTRO-CHEMISTRY) and accumunum is separated b lates upon an iron or carbon plate at the bottom of the furnace, this plate sodium, and potassium are made in fursodium, and potassium are made in furnaces of a smilar type, while steel and one of three processes. The first de- Electric welding is carried out by The first dezinc are made in arc furnaces. one of three processes. tric arc between the retallic surfaces to pends upon the production of an elecbe 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 insulated tongs, 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 procpositive pole is ess, the metal is connected to the negative pole, while the pos ess, the metal is connected to the negafilled with dilute sulphuric_acid, into filled with dilute sulphuric acid, into action is set up, and the metal becomes offering powerful resistance to the passage of the current. The metal soon reaches the necessary temperature, when 4 ELECTRO-METALLURGY it can be removed from the bath and welded on the anvil. The third process is known as the incandescent process.

The two pieces of metal to be welded are connected to opposite poles of a generator, and then brought together. The resistance offered raises the surfaces to incandescence, when they can be welded.

Electro-plating depends upon the electrolysis of a solution of a metallic salt, and by its means a coating of the metal in solution is deposited upon another metal which forms the electrode of the cell in which the electrolysis takes place. For silver plating a solution of potassium silver cyanide is used; for nickel plating, nickel ammonium sulphate; and for copper plating, a solution of copper sulphate. In all cases the general procedure is the same. The metal to be plated forms the kathode of the cell, and must be perfectly clean and free from all traces of grease. The solution must also be kept moving, either by means of paddles or by moving electrodes. The current required is small, and seldom exceeds 12 amperes per square foot, being, in the case of nickel plating, as low as four amperes per square foot.

Electric refining is a similar process to electro-plating, but in this case the impure metal forms the anode of the cell, the pure metal being first brought into solution and then deposited on the kathode. For instance, in the refining of copper, a solution is made containing about 2 pounds of copper sulphate and 6 ounces of sulphuric acid per gallon of water. This solution forms the electrolyte. The crude copper to be purified forms the anode, while the kathode consists of a graphitized plate of pure copper. When the current is passed, copper is dissolved from the anode and deposited in very pure form upon the kathode.

Electro-typing is the process by which reproductions are made of such articles as medals, engraved plates, busts, and so on. A reversed cast is first obtained in the usual manner, wax or gutta-percha being used for the purpose.

An impression of the cast is then made in a mixture of bees-wax, Venice turpentine and plumbago, and the surface of this impression is covered with graphite, and carefully cleaned with alcohol and water. It is then submitted to a process very similar to that of electroplating, a deposit of metal being produced all over the surface of the wax, until a sufficient thickness is obtained.

The metal shell, thus formed, is removed from the wax and "backed" with metal -an alloy of lead, antimony, and tin ELEPHANT being commonly used. See also ELEC TRIC FURNACE, ELECTRO-CHEMISTRY.