- 行业: Printing & publishing
- Number of terms: 178089
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- Company Profile:
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.
Any of the deoxyribonucleic acid (DNA) polymerases present in particles of retroviruses, that is, in virions of ribonucleic acid (RNA) tumor viruses and related viruses. These DNA polymerases are coded by the retroviruses and are able to carry out DNA synthesis using an RNA template. This reaction is called reverse transcription since it is the opposite of the usual transcription reaction, which involves RNA synthesis using a DNA template. Thus, the viral DNA polymerases that carry out this reverse reaction have been called reverse transcriptases, and the viruses with this enzyme are called retroviruses because they reverse the usual flow of genetic information. Such viruses are much discussed now, since the etiologic agent of acquired immune deficiency syndrome (AIDS) is a retrovirus.
Industry:Science
Attempting to listen to another person talking in a crowded party room is more difficult than in a quiet room. The ability of the listener to understand the speaker (or of even recognizing that someone is speaking) is clearly diminished by the noise of the party. Because it is generally believed that noise degrades signals, much effort has been put into filtering out the noise. However, recent research has unveiled a wide class of systems that use noise to detect weak signals via a mechanism known as stochastic resonance (SR). Stochastic resonance was discovered in an electronic circuit and then in other physical systems. It is currently of interest in sensory biology and in psychophysics as a method of enhancing the ability of individuals to detect weak signals received from the environment.
Industry:Science
Control and stability of the electric supply network can be achieved by rapid deployment of hydrogenerated electricity. However, the ease and flexibility with which a hydrogenerating unit can be controlled compounds the equipment deterioration associated with normal energy conversion and the attendant forces acting on the large structure. Frequent changes in operating conditions produce cyclic stresses on the active components of the generator. These conditions often result in the onset of undetectable failures, emphasizing the limitations of conventional monitoring technology for immense generators. Because most hydrogenerator deterioration is associated with the stationary structure, a new monitoring scheme was implemented by mounting a complement of sensors on the rotor to scan the stator.
Industry:Science
Defined as a graphic art that occurs in time, animation is a dynamic visual statement that evolves through various interval changes in the display. Early examples of cartographic animation on film and video could not be easily duplicated, transported, or displayed, thus severely limiting their distribution and use. As a result, the making of these early animations was time-consuming and expensive. Computer technology, particularly storage devices such as the CD-ROM and advanced forms of data communications such as the World Wide Web, spurred new interest in this method of mapping during the 1990s. However, animation is still not widely used in cartography, has not been integrated in software for Geographic Information Systems (GIS), and faces a number of conceptual and methodological hurdles.
Industry:Science
Because of the sulfur dioxide (SO<sub>2</sub>) emissions when coal is burned, sulfur levels in coal are important. Coal supplies 32% of the world's energy. In 1995, United States electric utilities used coal to generate 55% of the nation's net electric power, and this approach is projected to remain virtually the same through 2005. In the United States, the utility and industrial sectors are the largest emitters of sulfur oxides, and in 1985 accounted for 22.3 million tons, or 96%, of the total SO<sub>2</sub> emitted. In the presence of particulate matter, sulfur oxides have been related to irritation of the human respiratory system, reduced visibility, materials corrosion, and varying effects on vegetation. The reaction of sulfur oxides with moisture in the atmosphere contributes to acid rain.
Industry:Science
Determination of the rate at which energy is transmitted by microwave propagation. Power is one of the fundamental parameters measured at microwave frequencies. Since the physical dimensions of the microwave system are comparable with the wavelength, the voltage along a conductor is no longer constant. Further, for energy transmitted inside a hollow, uniconductor waveguide, the voltage difference between two points is not uniquely defined and varies as a function of the path taken to pass from one point to the other. Thus voltage standards, basic to low-frequency electrical measurements, give way to power standards at microwaves. Power meters are connected either temporarily, to measure all the power, in place of the normal termination, or permanently to measure a fixed portion of the power.
Industry:Science
Disorders involving excessive or deficient blood levels of parathyroid hormone caused by abnormal functioning of the parathyroid gland. Parathyroid hormone is responsible for keeping the concentration of calcium in blood within a narrow normal range. If the blood calcium concentration falls, the parathyroid glands respond by secreting hormone which tends to increase the concentration of calcium. Conversely, an increase in blood calcium concentration above the normal range normally suppresses parathyroid hormone secretion. Parathyroid hormone acts directly on bone and kidney, and indirectly on the intestine, to increase the concentration of calcium in blood. It also acts on the kidney to increase excretion of phosphate in the urine, causing a lowering of the concentration of phosphorus in blood.
Industry:Science
Any of a group of aluminosilicate minerals whose crystal structures are composed of corner-sharing (AlO<sub>4</sub>) and (SiO<sub>4</sub>) tetrahedra linked in an infinite three-dimensional array, with charge-balancing cations (primarily sodium (Na), potassium (K), and calcium (Ca)) occupying large, irregular cavities in the framework of the tetrahedra. Collectively, the feldspars constitute about 60% of the outer 8–10 mi (13–17 km) of the Earth's crust. They are nearly ubiquitous igneous and metamorphic rocks, and are a primary constituent of arkosic sediments derived from them. The importance of the many feldspars that occur so widely in igneous, metamorphic, and some sedimentary rocks cannot be underestimated, especially from the viewpoint of a petrologist attempting to unravel earth history.
Industry:Science
Communication networks that are regional, nationwide, or worldwide in geographic area, with a minimum distance typical of that between major metropolitan areas. Smaller networks include metropolitan and local-area networks. A communication network provides common transmission, multiplexing, and switching functions that enable users to transport data between many sources and many destinations. Under ideal circumstances, the data that arrive at the destination are identical to the data that were sent. The rate of arrival of bits at any point in the network is said to be the data rate at that point and is typically measured in bits per second. These bits may come from one source or from a multiplicity of sources. The capacity of a network to transmit at a cerain data rate is known as its bandwidth.
Industry:Science
Electromagnetic radiations produced primarily by lightning strokes from thunderstorms. It is estimated that globally there occur about 2000 thunderstorms at any one time, and that these give rise to about 100 lightning strokes every second. The radiations are short impulses that usually last a few milliseconds, with a frequency content ranging from the low audio well into the gigahertz range. Sferics (short for atmospherics) are easily detected with an ordinary amplitude-modulation (AM) radio tuned to a region between radio stations, especially if there are thunderstorms within a few hundred miles. These sounds or noises have been identified and characterized with specific names, for example, hiss, pop, click, whistler, and dawn chorus. They fall into what is generally known as radio noise.
Industry:Science
