Practical Applications of Electricity

PRACTICAL APPLICATIONS OF ELECTRICITY This historical introduction to the subject will be concluded with a brief account of the history of the practical applications of electricity.

Telegraphy.—Immediately of ter Oersted's discovery of the action of a current on a compass needle Ampere, in 1820, sug gested that the effect might be used for transmitting messages between distant points. Practical systems of electric telegraphy were developed about 1836 by Weber and Gauss in Germany, Cooke and Wheatstone in England, and Henry and Morse in the United States. A submarine cable across the English channel was laid down successfully by T. R. Crompton about 1851. A sub marine cable across the Atlantic Ocean was laid successfully, after several earlier failures, in 1866. Lord Kelvin made the operation of long submarine cables possible by inventing his mir ror galvanometer and syphon recorder besides other instruments. (See TELEGRAPHY.) Dynamos and Motors.—Faraday's copper disk, rotating be tween the poles of a magnet and generating a steady current, was the forerunner of the modem dynamo or generator. Numerous inventors devised generators, notably Pixii (1832), Gramme, Sie mens, Wilde, Varley and Wheatstone. Commercial machines, largely due to Siemens, Wilde and Gramme, came into use about 1866. Siemens, J. Hopkinson, Crompton, Elihu Thomson, Eicke meyer, Parker, Edison, Brush, Kapp, S. P. Thompson, Steinmetz and many other scientists and inventors rapidly developed the earlier crude dynamo into the modern efficient and reliable machine.

Alternating current generators were introduced and developed by Jablochkov, Lucien Gaulard, Zipernowsky, Blathy and S. Z. de Ferranti about 1883. Polyphase alternators were introduced by Ferraris, Nikola Tesla, Dolivo-Dobrowolsky and C. E. L. Brown about 1891. The idea of using alternating currents and trans forming them from low to high potentials and vice versa, by means of transformers, was originally due to Jablochkov and Gaulard. In 1873, at Vienna, it was discovered that a Gramme ring type of dynamo can act as a motor when supplied with direct current. Transmission of power by electricity was then seen to be possible, and was soon made practicable and carried out by S. D. Field, Edison and many others. Electric trams and street cars came into use about 1882, and the use of electric motors for driving all kinds of machines rapidly became general in Europe and America. At the present time (1928) single generators of 50,000 kilowatts or nearly 70,00o horse power capacity are fairly common.

Electric Lighting.

Commercial electric lighting was intro duced by Jablochkov and by C. F. Brush about 1876. The electric arc, discovered by Sir Humphrey Davy many years before, was used as the source of light. Incandescent filament lamps were introduced by Edison, Lane Fox and Swann about 1878. The earlier .carbon filament lamps have been replaced by much more efficient tungsten filament lamps in recent years. By filling the bulbs with argon, evaporation of the filament is diminished, so making it possible to increase greatly the efficiency by running the tungsten filament at a higher temperature. Electric street and house lighting is now widespread.

Neon and argon lamps are discharge tubes in which a current is passed through one or other of these rare gases at a pressure of a few millimetres. These lamps are now used for advertising pur poses, and give a pleasing brilliant glow. Mercury arcs are dis charge tubes of glass or fused quartz in which a current is passed through mercury vapour. They give a bright greenish light and are sometimes used for factory illumination. The quartz mer cury arc gives strong ultra-violet light and is used for medical light treatments, for which purpose iron and carbon arcs are also much used. Enormously powerful arc lamps are now used in light-houses and search-lights.

The Telephone and Wireless Communication.

The tele phone was invented in 1876 by Graham Bell, and was improved by Edison, Elisha Gray, Hughes and many others. At the present time, by means of telephones almost anyone anywhere on the same continent, or, in many cases, on other continents, can be engaged in conversation.

Communication by means of electric waves was developed in 1896 by G. Marconi. Marconi's wireless telegraphy was rapidly adopted for the purpose of communicating with ships at sea and from one ship to another. It is now used also for communicating between distant countries, e.g., England and Australia.

Thernaionic Vacuum Tubes. The emission of electrons by hot bodies in a vacuum, investigated by 0. W. Richardson about 1900, has proved of great practical value in connection with wireless and other methods of communication, and also for other pur poses. In 1905, J. A. Fleming invented the thermionic valve con sisting of a hot filament and an electrode enclosed in an exhausted tube. This valve allows a current to flow through it from the elec trode to the filament, but not in the reverse direction, and so can be used as a current rectifier. Fleming used it for rectifying the small rapidly alternating currents received in wireless teleg raphy, and so was able to measure them with a galvanometer or to detect them with a telephone. In 1907 Lee de Forest invented the thermionic amplifier, which is a Fleming valve with a grating or grid between the electrode or plate and the filament. The cur rent carried by the electrons between the plate and the filament depends on the potential of the grid, so that small changes of this potential may be made to produce considerable changes in the current. The thermionic amplifier rapidly came into use in long distance telephony and in wireless. By the use of these am plifiers continuous electrical oscillations can be maintained, and wireless telephony is made possible, and is now in general use. Programmes of an entertainment and of an educational character are now regularly broadcast, and are received by listeners pro vided with wireless telephone receiving sets. The thermionic amplifier plays an essential part in the systems of television recently developed. (See THERMIONICS ; THERMIONIC VALVE; BROADCASTING : Technical Aspects.) Coolidge X-ray and Cathode Ray Tubes.—The universal use of X-rays for medical and dental examinations and other pur poses has made the manufacture and improvement of X-ray apparatus of commercial importance. An improved form of X-ray tube was invented by Coolidge. In this tube the cathode is a tungsten spiral heated by a current. The tube is almost per fectly exhausted, and the electrons emitted by the hot spiral are accelerated by potentials up to 200,000 volts, or more, and fall on an anti-cathode of tungsten or of molybdenum. Coolidge has also invented a cathode ray tube in which the electrons from a tungsten spiral fall on a thin nickel window and escape into the air outside the tube. Such a tube using 900,00o volts has been constructed. (See X-RAv.) Domestic Applications.—In a modern home electricity is used for many purposes. The home is lit by incandescent electric lamps, and is provided with telephones and a wireless receiving set. Various machines driven by small electric motors are also in common use; e.g., vacuum cleaners, sewing machines, washing machines and small refrigerating plants. Cooking by means of electrically heated ovens and stoves is becoming common, and electric toasters and irons are widely used. House heating by electricity is economical only where electricity is unusually cheap.

The manufacture of electric ignition, lighting and starting sys tems for motor cars now forms a large industry, especially in the United States where over 25 million cars are in operation. Each car has a generator, motor, secondary battery, and ignition and lighting accessories. The extraordinary perfection of modern elec trical apparatus is well illustrated by the fact that it is not un common for the electrical apparatus in a motor car to require no repairs or adjustments while the car runs 5,000 or even 10,000 miles.

Industrial Applications.

Electro plating with copper and silver was introduced commercially soon after electrolysis was dis covered, and has been popular ever since. Nickel or cobalt plating is now much used for protecting iron and steel, and chromium plating, which does not tarnish, has recently been introduced for the same purpose. (See ELECTROPLATING.) The commercial preparation of many elements and compounds by electrolytic methods has been an important industry for many years; e.g., copper and aluminum are made in large quantities by electrolysis. The manufacture of nitrogen compounds from at mospheric nitrogen, by means of long alternating current arcs in a current of air, is carried out on a large scale. The nitrogen and oxygen combine in the arc, and the oxides give nitric and nitrous acids with water. (See ELECTROCHEMISTRY ; and NITRO GEN, FIXATION OF.) Electric furnaces have been used for commercial purposes ever since large supplies of electric power became available, and are rapidly increasing in favour. Such furnaces are of three types: arc furnaces, resistance furnaces and induction furnaces. In the arc furnaces, an arc, between carbon poles, is produced inside a box of refractory material. The substances which it is desired to heat are placed in the box, in the path of the arc. In this way all known solid bodies can be fused or volatilized easily. In re sistance furnaces a current is passed, through the mixture to be heated, between graphite or other electrodes. The mixture may be made sufficiently conducting when cold by the addition of powdered graphite, or other suitable conductor, if necessary. All solids and liquids become conducting at very high temperatures. Such furnaces are used for the manufacture of calcium carbide and carborundum. In another type of resistance furnace, the current is passed through coils of wire surrounding the space to be heated. Such furnaces can be used only for temperatures below the melting point of the wire, but with tungsten wires tempera tures above 2,500° C may be reached. In induction furnaces a current is induced in the material to be heated, by means of an alternating current in a coil surrounding the material but separated from it. The coil itself is kept cool. Sometimes the material to be heated is made to form a single conducting turn round the core of a transformer. By using high frequency alternating currents, metals like platinum and tungsten can be melted quickly in a crucible placed inside a coil of a few turns, through which the current is passed while the coil remains cool. Electric furnaces are much used for the manufacture of special alloy steels. (See