ELECTROMAGNETISM. The art r process of magnetizing by means of an electric current, as distinguished from so-called natural magnetism as in the lodestone, or transient magnetism, as established in a wrought-iron bar. A magnet not only attracts steel and iron in a less degree, but cobalt, nickel, manganese, cerium and chromium. Hans Christian Oer sted (q.v.) is credited with being the first to recognize the identity of magnetic and electric phenomena. In 1820 he observed that wires connecting the poles of a voltaic pile affected the magnetic needle. In 1821 he wrote a long paper on electromagnetism. Arago, Ampere. Davy and Faraday all studied and contributed to knowledge of the phenomena. In 1825 Wil liam Sturgeon of Woolwich, England, began to experiment. His two first electromagnets were made in the shape of a horseshoe and a straight bar. The former was made of ,a bent rod of iron one foot in length and half an inch thick, around which a bare copper wire was wound 18 times, the iron having been previously covered with varnish to insulate the wire from the iron. The current was supplied by one large primary cell. This magnet was able to sustain a weight of nine pounds, though weigh ing itself only seven ounces. Subsequently Sturgeon constructed a horseshore electro magnet 18 inches in length, 23/4 inches thick and wound with 980 feet of copper wire one twelfth of an inch in diameter, which upheld 1,386 pounds. In 1831 Joseph Henry made electromagnets for both Yale and Princeton universities that lifted 3,000 pounds. Lag' electromagnets are now used in foundries and machine shops of a capacity of 12 or more tons.
The phenomena of electro-magnetism may be briefly described as follows: It is known that when iron filings are strewn over a cardboard or glass, if a bar or horseshoe magnet be placed under the cardboard the filings will tend to ar range themselves symmetrically when the card board is tapped. This is due to magnetic lines of force which are assumed to flow from the north to the south pole of the magnet, and the iron filings, becoming temporarily magnet ized by these magnetic lines of force, tend to set themselves parallel thereto. Similarly, when an electric current flows in a wire (elec tro) magnetic lines of force surround the wire in circles or hoops which increase in density with the strength of the current It is lcnown that iron is a much better conductor of mag netic lines of force (or magnetism) than air, in the ratio of 1 to 100 or 150, depending on the quality or °permeability° of the iron.
Hence when the wire is made in the form of a coil into which is inserted a soft iron bar, the magnetic lines of force, so to speak, use the iron as a circuit and the latter becomes .a mag net having north and south poles. A simple form of electromagnet consists of a U-shaped bar of soft iron, around which a copper wire is coiled in spirals, beginning at one extremity and extending to the other. If the iron core is placed in this position n, and winding be gins at the lower left side, turning the wire around clocicwise, the south pole of the mag netic will be at the starting point on the left.
The space between the poles of a magnet or wherever its magnetic lines of foice extend, or in the space around a wire conveying a current of electricity, is termed a magnetic °field.° The substances through which the lines of force pass, including the iron of the magnet, consti tute the magnetic circuit. The expression num ber of lines of force per square centimeter in the material, is at present used as a measure of magnetic density. The total number of lines of force in a magnetic circuit is termed the magnetic flux, and is obtained by multiplying the total cross-sectional area of the field in square centimeters by the density of a square centimeters of the circuit. The magneto motive force (that is, the force that, as it were, drives the lines of force through the circuit) is equal to the product of the strength of current in amperes in the coil by the number of convolu tions of the coil. This is also termed the am pere turns. The magnetic flux may be increased by increasing the magneto motive force or by decreasing the resistance (termed the reluc tance) of the magnetic circuit. Hence the re lation of the foregoing terms to one another is analogous to that between electromotive force, resistance and current, in an electrical circuit, and may be expressed by the equation: Magnetomotive force Magnetic flux= Reluctance.
It is to be noted, however, that the reluctance o'f a magnetic circuit containing iron is not a constant, but increases in other words, its per meability to magnetization decreases, after a certain degree of magnetization, which is termed saturation, has been reached.
Electro-magnets are extensively used in electric bells, in telegraph and telephone appa ratus, in dynamo machines, electric motors (q.v), and for many other purposes. See ELEC