- 行业: Printing & publishing
- Number of terms: 178089
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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
A means for improving the performance of remote sensing and communication systems that rely on electromagnetic- or acoustic-wave propagation but must contend with wave reflections, diffraction, and scattering. In particular, for applications of remote sensing—including radar, sonar, biomedical imaging, and nondestructive evaluation—the main intent is the detection, localization, and identification of distant objects or features that either scatter or generate electromagnetic or acoustic waves. Although the disparity in objectives between military-radar and ultrasonic-imaging systems may be considerable, they have in common that random wave scattering and diffraction tend to limit the accuracy and confidence with which such systems can be used and the distances over which such systems can operate. For radar and sonar, turbulence and wave motions in the ocean or atmosphere, or rough terrestrial or ocean surfaces, may cause such random wave scattering and diffraction. In ultrasonic remote sensing, the random scattering and diffraction may be caused by grain structure within metals or by variations between and within tissues.
Industry:Science
A means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the required bandwidth of the data modulation signal. Spread spectrum signals are typically transmitted by electromagnetic waves in free space, with usage in both nonmilitary and military systems.
Industry:Science
A means of producing chemical changes through reactions at electrodes in contact with an electrolyte by the passage of an electric current. Electrolysis cells, also known as electrochemical cells, generally consist of two electrodes connected to an external source of electricity (a power supply or battery) and immersed in a liquid that can conduct electricity through the movement of ions. Reactions occur at both electrode-solution interfaces because of the flow of electrons. Reduction reactions, where substances add electrons, occur at the electrode called the cathode; oxidation reactions, where species lose electrons, occur at the other electrode, the anode. In the cell shown in the <b>illustration</b>, water is reduced at the cathode to produce hydrogen gas and hydroxide ion; chloride ion is oxidized at the anode to generate chlorine gas. Electrodes are typically constructed of metals (such as platinum or steel) or carbon. Electrolytes usually consist of salts dissolved in either water or a nonaqueous solvent, or they are molten salts.
Industry:Science
A means of simultaneous, two-way communication comprising both audio and video elements. Participants in a video telephone call can both see and hear each other in real time. Videotelephony is a subset of teleconferencing, broadly defined as the various ways and means by which people communicate with one another over some distance. Initially conceived as an extension to the telephone, videotelephony is now possible using computers with network connections. In addition to general personal use, there are specific professional applications, such as criminal justice, health care delivery, and surveillance that can greatly benefit from videotelephony.
Industry:Science
A means of storing a collection of data. Computer science is in part the study of methods for effectively using a computer to solve problems, or in other words, the entire process of determining exactly the problem to be solved. This process entails (1) gaining an understanding of the problem; (2) translating vague descriptions, goals, and contradictory requests, and often unstated desires, into a precisely formulated conceptual solution; and (3) implementing the solution with a computer program. This solution typically consists of two parts: algorithms and data structures.
Industry:Science
A measure of the “opposition to rotation” around a fixed axis that a rigid body presents to a torque applied about that axis. The concept is used in analyzing the dynamics of systems in which rotation occurs, for example, in attitude control of spacecraft. It extends naturally into quantum mechanics and the notion of quantization of rotational energy.
Industry:Science
A measure of the collecting or diverging power of a lens or an optical system. Focal length, usually designated <i>f</i><sup>′</sup> in formulas, is measured by the distance of the focal point (the point where the image of a parallel entering bundle of light rays is formed) from the lens, or more exactly by the distance from the principal point to the focal point.
Industry:Science
A measure of the effectiveness of an optical system in enlarging or reducing an image. For an optical system that forms a real image, such a measure is the lateral magnification <i>m</i>, which is the ratio of the size of the image to the size of the object. If the magnification is greater than unity, it is an enlargement; if less than unity, it is a reduction.
Industry:Science
A measure of the potential ability of seawater to support life. Fertility is distinguished from productivity, which is the actual production of living material by various trophic levels of the food web. Fertility is a broader and more general description of the biological activity of a region of the sea, while primary production, secondary production, and so on, is a quantitative description of the biological growth at a specified time and place by a certain trophic level. Primary production that uses recently recycled nutrients such as ammonium, urea, or amino acids is called regenerated production to distinguish it from the new production that is dependent on nitrate being transported by mixing or circulation into the upper layer where primary production occurs. New production is organic matter, in the form of fish or sinking organic matter, that can be exported from the ecosystem without damaging the productive capacity of the system.
Industry:Science
A measure of the power of a lens or a prism. The diopter (also called dioptrie) is usually abbreviated D. Its dimension is a reciprocal length, and its unit is the reciprocal of 1 m (39.4 in.). Thus a thin lens of κ diopters has a focal length of 1000/κ mm or 39.4/κ in. The lens is collecting for positive κ, diverging for negative κ, and afocal for κ = 0.
Industry:Science
