- 行业: Printing & publishing
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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.
An electrochemical device that stores chemical energy which can be converted into electrical energy, thereby providing a direct-current voltage source. Although the term “battery” is properly applied to a group of two or more electrochemical cells connected together electrically, both single-cell and multicell devices are called battery.
Industry:Science
An electrochemical technique used in analytical chemistry. Polarography involves measurements of current-voltage curves obtained when voltage is applied to electrodes (usually two) immersed in the solution being investigated. One of these electrodes is a reference electrode: its potential remains constant during the measurement. The second electrode is an indicator electrode. Its potential varies in the course of measurement of the current-voltage curve, because of the change of the applied voltage. In the simplest version, so-called dc polarography, the indicator electrode is a dropping-mercury electrode, consisting of a mercury drop hanging at the orifice of a fine-bore glass capillary (usually about 0.08 mm inner diameter). The capillary is connected to a mercury reservoir so that mercury flows through it at the rate of a few milligrams per second. The outflowing mercury forms a drop at the orifice, which grows until it falls off. The lifetime of each drop is several seconds (usually 2 to 5). Each drop forms a new electrode; its surface is practically unaffected by processes taking place on the previous drop. Hence each drop represents a well-reproducible electrode with a fresh, clean surface.
Industry:Science
An electrode with an invariant potential. In electrochemical methods, where it is necessary to observe, measure, or control the potential of another electrode (denoted indicator, test, or working electrode), it is necessary to use a reference electrode, which maintains a potential that is practically unchanged during the course of an electrochemical measurement. Potentials of indicator or working electrodes are measured or expressed relative to reference electrodes.
Industry:Science
An electromagnetic incremental-motion actuator which converts digital pulse inputs to analog output motion. The device is also termed a step motor. When energized in a programmed manner by a voltage and current input, usually dc, a step motor can index in angular or linear increments. With proper control, the output steps are always equal in number to the input command pulses. Each pulse advances the rotor shaft one step increment and latches it magnetically at the precise point to which it is stepped. Advances in digital computers and particularly microcomputers revolutionized the controls of step motors. These motors are found in many industrial control systems, and large numbers are used in computer peripheral equipment, such as printers, tape drives, capstan drives, and memory-access mechanisms. Step motors are also used in numerical control systems, machine-tool controls, process control, and many systems in the aerospace industry.
Industry:Science
An electromechanical apparatus for establishing the watt as an SI electrical unit. Prior to January 1, 1990, there were voltage units in use in the United States, France, the United Kingdom, Russia, and so on which differed from each other (and, with hindsight, from the SI volt) by up to 9 parts per million (ppm). These units were based on results from various current balances. Then results obtained from a different kind of apparatus at the National Physical Laboratory in the United Kingdom and the National Institute for Science and Technology in the United States, which were based on the simple principle described below, enabled the electrical units to be put on a sound SI basis. The accuracy in deriving the SI unit of voltage in this way was considered to be better than 0.2 ppm. Since that date these apparatuses have continued to be refined, and others are being developed in metrological laboratories, with the more ambitious objective of defining the kilogram in terms of fundamental physical constants instead of having to rely on a carefully preserved artifact, the cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures. To do this, the accuracy achieved by the apparatus will have to be demonstrated to be of the order of 0.01 ppm.
Industry:Science
An electromechanical transducer capable of converting mechanical vibrations into electrical voltages. Depending upon their sensing element and output characteristics, such pickups are referred to as accelerometers, velocity pickups, or displacement pickups.
Industry:Science
An electromechanical, solid-state, or digital device operated by variations in the input that, in turn, operate or control other devices connected to the output. Relays are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers, motor and sequencing controls, and automation systems. Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power. In the home, relays are used in refrigerators, automatic washing machines and dishwashers, and heating and air-conditioning controls. Although relays are generally associated with electrical circuitry, there are many other types, such as pneumatic and hydraulic. Input may be electrical and output directly mechanical, or vice versa.
Industry:Science
An electron device in which use is made of the electrostatically or magnetically controlled flow of electrons in an evacuated space, or of the phenomena accompanying the electron emission, acceleration, and collection associated with the flow. In common usage, completely evacuated tubes are called hard tubes; tubes containing a small amount of deliberately added gas or vapor are called soft tubes, but they are usually included in the family of vacuum tubes.
Industry:Science
An electron microscope that builds up its image as a time sequence of points in a manner similar to that employed in television. While the idea of the scanning electron microscope (SEM) dates back to the 1930s, a great deal of the pioneering development of the instrument took place at Cambridge University, England, during the years following World War II. The SEM became commercially available in 1966 and has rapidly taken its place as a useful addition to the battery of electron optical instruments used in both biological and physical research. A part of the attractiveness of the SEM lies in its ability to utilize both analytic and subjective or intuitive channels in attempts to understand the microscopic world.
Industry:Science
An electron tube comprising a photocathode and an anode mounted within an evacuated glass envelope through which radiant energy is transmitted to the photocathode. A gas phototube contains, in addition, argon or other inert gas which provides amplification of the photoelectric current by partial ionization of the gas. The photocathode emits electrons when it is exposed to ultraviolet, visible, or near-infrared radiation. The anode is operated at a positive potential with respect to the photocathode.
Industry:Science
