magnetic field. The technological revolution attributed to the development of electric power and modem communications can be traced to these three landmarks.
electromotive force Energy per unit electric charge that is imparted by an energy source, such as an electric generator or a battery. As the device does work on the electric charge being transferred within itself, energy is converted from one form to another. The work done on a unit of electric charge or the energy gained by the unit charge is the electro¬ motive force emf (or E ) and is characteristic of any energy source capable of driving electric charge around a circuit. A common unit of electromo¬ tive force is the volt V, a unit equal to the difference in electric potential between two points in a conductor carrying a current of one ampere and dissipating one watt of power between the two points.
electromyography U-.lek-tro-ml-'ag-ro-feN Process of graphically recording the electrical activity of muscle, which normally generates an electric current only when contracting or when its nerve is stimulated. Electrical impulses are shown as wavelike tracings on an oscilloscope and recorded as an electromyogram (EMG), usually along with audible sig¬ nals. The EMG can show whether muscle weakness or wasting is due to nerve impairment (as in amyotrophic lateral sclerosis and poliomyelitis) or muscle impairment or disease (myopathy).
electron Lightest electrically charged subatomic particle known. It car¬ ries a negative charge (see electric charge), the basic charge of electricity. An electron has a small mass, less than 0.1% the mass of an atom. Under normal circumstances, electrons move about the nucleus of an atom in orbitals that form an electron cloud bound in varying strengths to the posi¬ tively charged nucleus. Electrons closer to the nucleus are held more tightly. The first subatomic particle discovered, the electron was identi¬ fied in 1897 by J. J. Thomson.
electron microscopy \ml-'kras-k3-pe\ Technique that allows exami¬ nation of samples too small to be seen with a light microscope. Electron beams have much smaller wavelengths than visible light and hence higher resolving power. To make them more observable, samples may be coated with metal atoms. Because electrons cannot travel very far in air, the elec¬ tron beam and the sample must be kept in a vacuum. Two different instru¬ ments are used. In the scanning electron microscope, a moving beam of electrons is scanned across a sample; electrons scattered by the object are focused by magnetic “lenses” to produce an image of the object’s surface similar to an image on a television screen. The images appear three- dimensional; they may be of small organisms or their parts, of molecules such as DNA, or even of large individual atoms (e.g., uranium, thorium). In the transmission electron microscope, the electron beam passes through a very thin, carefully prepared sample and is focused onto a screen or photographic plate to visualize the interior structure of such specimens as cells and tissues.
electron paramagnetic resonance See electron spin resonance
electron spin resonance (ESR) or electron paramagnetic resonance (ERR) Technique of spectroscopic analysis (see spectros¬ copy) used to identify paramagnetic substances (see paramagnetism) and investigate the nature of the bonding within molecules by identifying unpaired electrons and their interaction with their immediate surround¬ ings. Unpaired electrons, because of their spin, behave like tiny magnets and can be lined up in an applied magnetic field; energy applied by alter¬ nating microwave radiation is absorbed when its frequency coincides with that of precession of the electron magnets in the sample. The graph or spectrum of radiation absorbed as the field changes gives information valuable in chemistry, biology, and medicine.
electronic banking Use of computers and telecommunications to enable banking transactions to be done by telephone or computer rather than through human interaction. Its features include electronic funds transfer for retail purchases, automatic teller machines (ATMs), and auto¬ matic payroll deposits and bill payments. Some banks offer home bank¬ ing, whereby a person with a personal computer can make transactions, either via a direct connection or by accessing a Web site. Electronic bank¬ ing has vastly reduced the physical transfer of paper money and coinage from one place to another or even from one person to another.
electronic mail See E-mail
electronic music Any music involving electronic processing (e.g., recording and editing on tape) and whose reproduction involves the use
© 2006 Encyclopaedia Britannica, Inc.
610 I electronics ► elephant bird
of loudspeakers. In the late 1940s, magnetic tape began to be used, espe¬ cially in France, to modify natural sounds (playing them backward, at dif¬ ferent speeds, etc.), creating the genre known as musique concrete. By the early 1950s, composers in Germany and the U.S. were employing assembled conglomerations of oscillators, filters, and other equipment to produce entirely new sounds. The development of voltage-controlled oscillators and filters led, in the 1950s, to the first synthesizers, which effectively standardized the assemblages and made them more flexible. No longer relying on tape editing, electronic music could now be created in real time. Since their advent in the late 1970s, personal computers have been used to control the synthesizers. Digital sampling—composing with music and sounds electronically extracted from other recordings—has largely replaced the use of oscillators as a sound source.
electronics Branch of physics that deals with the emission, behaviour, and effects of electrons and with electronic devices. The beginnings of electronics can be traced to experiments with electricity. In the 1880s Tho¬ mas Alva Edison and others observed the flow of current between elements in an evacuated glass tube. A two-electrode vacuum tube constructed by John A. Fleming (1849-1945) produced a useful output current. The Audion, invented by Lee De Forest (1907), was followed by further improvements. The invention of the transistor at Bell Labs (1947) initi¬ ated a progressive miniaturization of electronic components that by the mid 1980s resulted in high-density microprocessors, which in turn led to tremendous advances in computer technology and computer-based auto¬ mated systems. See also semiconductor.
electrophile Atom or molecule that in a chemical reaction seeks an atom or molecule containing an electron pair available for bonding or the nega¬ tive end of a polar molecule (see covalent bond; electric dipole). In the Lewis electron theory (see acid-base theory) advanced by the U.S. chem¬ ist Gilbert Lewis (1875-1946) in 1923, electrophiles are by definition Lewis acids. Examples include the hydronium ion (H 3 0 + ), boron triflu¬ oride (BF 3 ), and the halogens fluorine, chlorine, bromine, and iodine in molecular form. See also acid; nucleophile.
electrophoresis Ni-.lek-tro-fo-'re-sosX Movement of electrically charged particles in a fluid under the influence of an electric field. The par¬ ticles migrate toward the electrode of the opposite electric charge, often on a gel-coated slab or plate, sometimes in a fluid flowing down a paper. Originated about 1930 by Arne Tiselius (1902-1971) as a technique for analysis, electrophoresis is used to analyze and separate colloids (e.g., pro¬ teins) or deposit coatings.
electroplating Process of coating with metal by means of an electric current. Plating metal may be transferred to conductive surfaces (e.g., metals) or to nonconductive surfaces (e.g., plastics, wood, leather) if a conductive coating has been applied. Usually the current deposits a given amount of metal on the cathode (workpiece) and the anode (source of metal) dissolves to the same extent, maintaining a fairly uniform solution. Silver plating is used on tableware, electrical contacts, and engine bear¬ ings. The most extensive use of gold plating is on jewelry and watch cases. Zinc coatings prevent the corrosion of steel articles, and nickel and chro¬ mium plate are used on automobiles and household appliances. See also TERNEPLATE, SHEFFIELD PLATE.