Electrical Machine

Collier's New Encyclopedia (1921)

a machine for converting mechanical work, either by friction or by induction, into electric energy. The earliest known form was constructed about the middle of the 17th century by Otto von Guericke. It consisted of a ball of sulphur mounted on an axle which was rotated as rapidly as possible while the hand of the operator was pressed against the ball. The friction of the hand upon the sulphur generated electricity. Sir Isaac Newton replaced the sulphur ball with a glass globe, and later workers substituted woolen cloth or pieces of leather for the hand. In 1768, Jesse Ramsden constructed the first plate electrical machine, consisting of a glass disc, which was rotated rapidly between two leather rubbers. The glass became positively charged and the charge was collected by means of insulated metallic forks, while the negative electricity produced on the leather escaped to the ground. A modification of this machine was that of Edward Nairne, who used a glass cylinder, with insulated conductors on opposite sides. One conductor carried a leather cushion, and the other a row of metal points, while a silk apron covered the surface of the cylinder from the leather almost to the points. Positive electricity was formed on the conductor carrying the points and negative electricity on that carrying the leather cushion, while the silk served to prevent the loss of the electric charge while the cylinder was passing from the rubber to the collecting points. All these types of frictional machines have been rendered obsolete, however, by the introduction of the so-called "influence machines," which operate by induction. Suppose a conducting body ELECTRIC ENGINE is brought near to an electrified substance having a positive charge. Then negative electricity is attracted by the charged body, while positive electricity is repelled, and if the conductor be connected with the earth, the positive electricity will escape, leaving the conductor charged with negative electricity. This is the underlying principle of the influence machine. In the Topler machine, for instance, two glass discs are used, one of which is fixed, while the other will revolve. On the latter are fixed a number of strips of tinfoil and, on the other, two so-called "field-plates," also made of tin-foil. A charge is given to one of the field plates, and then the movable plate is revolved. Let us suppose that the charge on the field plate is positive. Then as a tin-foil strip moves into place opposite the field plate, negative electricity is attracted and positive electricity repelled. A brass wire brush makes contact with the metallic strip and the positive electricity escapes, leaving the strip charged with nega charged negative ele electricity. Another strip moves into place, and the same process is repeated. In the meantime, the first strip has moved on toward metal collecting combs, and communicates its charge to them. The electricity from combs passes to Leyden jars, where it is stored. The more modern Wimshurst machine the resembles Topler, but consists of two glass plates, each fitted with tin-foil discs, and each made to revolve in the opposite direction to the other. ELECTRICAL UNIT. A coulomb. The quantity of electricity that liberates .000162 grain of hydrogen from water. ELECTRIC BATTERY. AGE BATTERY. ELECTRIC DENSITY, or ELECTRIC THICKNESS, the quantity of electricity found on a given surface. ELECTRIC FURNACE. Electric furnaces may be broadly classified into (1) Resistance f nce furnaces, in which heat is produced by the passage of an electric current through a poor conductor, (2) Arc furnaces, in which the heat of the electric arc is used, (3) Induction furnaces, which are a modified form of the resistance furnace, and in which the heat is generated by an induced current. In this type of furnace the resistance element is frequently formed by the furnace charge, which also acts as the secondary of a transformer; current at high voltage and low amperage is supplied to the primary, thus inducing high amperage at low voltage in the secondary. Electric furnaces can only compete, economically, with the ordinary directfired furnace where electric power is generated cheaply by water-power, as, for instance, at Niagara. They also have the advantage that much higher temperatures can be produced in them, the maximum temperature obtainable in non-electric furnaces being about 2,000° C, whereas, in electric furnaces, a temperature of 3,500° C is easily maintained. There are some processes in ELECTRIC DYNAMO Direct current, lighting and power generator. See STOR- which these extremely high temperatures are necessary, as, for instance, in the manufacture of carborundum. In this process, a core of granular carbon, or coke, is placed between electrodes, and around the core is packed a mixture of coke and sand. The electric current flows through the core and raises the temperature of the sand and coke mixture to such a point that combination takes place with the formation of carborundum. The induction type of furnace is largely used in the steel industry, one of the best known examples being that designed by F. A. Kjellin. In this furnace, current at 3,000 volts is supplied to the primary, inducing a current of 30,000 amperes in the metallic charge, which forms the secondary. Electric furnaces are also used in the manufacture of aluminum, calcium carbide, phosphorus, various iron alloys, and in the production of artificial graphite. In the laboratory, small electric furnaces are in common use. In these a current is passed through a high resistance 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 insulating material to prevent loss of heat by radiation. (1) A device ELECTRIC FUSE. 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 induced by a voltaic or magneto-electric battery. (2) A safety device used to protect electric circuits against too great a volume 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. In electric ELECTRIC HEATER. heaters a coil of metal of more or less resistance is wound around a frame and is made the channel of a current sur- 1 rounded by insulating material, the heat closed in various forms of receptacle which radiate it. The wire or strip rip of ip metal may be surrounded by air or by a non-inflammable substance that serves Porcelain, asbestos, as a conductor. 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 materials such as hyposulphite of sodium and crystallized acetate of sodium. There are great varieties in the methods used. In the Parville system heat is engendered by means of rods of metallic powder mixed with fusible clay, compressed by a force of 2,000 kilograms per square centimeter and baked at a temperature of 1,350° C. The Prometheus 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 manufacture and production as in oil fields, where electric heaters are used to stimulate the heavy petroleum and cause it to flow more easily.