- 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.
An electrical cell that converts the intrinsic chemical free energy of a fuel directly into direct-current electrical energy in a continuous catalytic process. As in the classical definition of catalysis, the fuel cell itself should not undergo change; that is, unlike the electrodes of a battery, its electrodes ideally remain unchanged. For most fuel–oxidant combinations, the available free energy of combustion is somewhat less than the heat of combustion. In a typical thermal power conversion process, the heat of combustion of the fuel is turned into electrical work via a Carnot heat-engine cycle coupled with a rotating electrical generator. Since the Carnot conversion rarely proceeds at an efficiency exceeding 40% because of heat source and sink temperature limitations, the efficiency of conversion in a fuel cell can be greater than in a heat engine, especially in small devices.
Industry:Science
An electrical component used to transfer electric energy from one alternating-current (ac) circuit to another by magnetic coupling. Essentially it consists of two or more multiturn coils of insulated conducting material, so arranged that any magnetic flux linking one coil will link the others also. This configuration creates mutual inductances between the coils. The mutual magnetic field acts to transfer energy from one input coil (or primary winding) to the other coils, which are then referred to as secondary windings. Under steady-state conditions, only one winding can serve as a primary.
Industry:Science
An electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an electric conductor at the surface of which a change occurs from conduction by electrons to conduction by ions. For gases and vacuum, the electrodes merely serve to conduct electricity to and from the medium.
Industry:Science
An electrical device that accelerates charged atomic or subatomic particles to high energies. The particles may be charged either positively or negatively. If subatomic, the particles are usually electrons or protons and, if atomic, they are charged ions of various elements and their isotopes throughout the entire periodic table of the elements. Before the advent of accelerators, the only sources of energetic particles for research were cosmic rays, or the naturally occurring radioactive atoms that emit subatomic particles such as electrons and alpha particles at various energies ranging from kilovolts to over 8 MeV.
Industry:Science
An electrical energy meter, that is, an electricity meter that measures and registers the integral, with respect to time, of the power in the circuit in which it is connected. This instrument can be considered as having two parts: a transducer, which converts the power into a mechanical or electrical signal, and a counter, which integrates and displays the value of the total energy that has passed through the meter. Either or both of these parts can be based on mechanical or electronic principles.
Industry:Science
An electrical instrument for measuring electric current. Currents are usually either unidirectional and steady (direct current or dc) or alternating in direction at a relatively low frequency (alternating current or ac). A current that is unidirectional but regularly fluctuating is a superposition of dc and ac. Higher-frequency ac is often referred to as radio-frequency or RF current. At frequencies above about 10 MHz, where the wavelength of the signal becomes comparable with the dimensions of the measuring instrument, current measurements become inaccurate and finally meaningless, since the value obtained depends on the position where the measurement is made. In these circumstances, power measurements are usually used.
Industry:Science
An electrical resistor with a relatively large temperature coefficient of resistance. Thermistors are useful for measuring temperature and gas flow or wind velocity. Often they are employed as bolometer elements to measure radio-frequency, microwave, and optical power. They also are used as electrical circuit components for temperature compensation, voltage regulation, circuit protection, time delay, and volume control. A common type of thermistor is a semiconducting ceramic composed of a mixture of several metal oxides. Metal electrodes or wires are attached to the ceramic material so that the thermistor resistance can be measured conveniently. The temperature coefficient of resistance is negative for these thermistors. Other types can have either negative or positive temperature coefficients.
Industry:Science
An electrically energized coil of insulated wire which produces a magnetic field within the coil. If the magnetic field produced by the coil is used to magnetize and thus attract a plunger or armature to a position within the coil, the device may be considered to be a special form of electromagnet and in this sense the words solenoid and electromagnet are synonymous. In a wider scientific sense the solenoid may be used to produce a uniform magnetic field for various investigations. So long as the length of the coil is much greater than its diameter (20 or more times), the magnetic field at the center of the coil is sensibly uniform, and the field intensity is almost exactly that given by the equation for a solenoid of infinite length.
Industry:Science
An electrically powered mechanical appliance for the dry removal of dust and loose dirt from rugs, fabrics, and surfaces. Portable vacuum cleaners are widely used in domestic and industrial cleaning of surfaces that cannot be wiped or brushed, such as carpets, upholstery, tapestry, or highly contoured surfaces. Small hand vacuum cleaners are used for cleaning the inside of passenger cars. Large industrial tank cleaners are used in cleaning shops. Power saws may be equipped with vacuum cleaners which collect sawdust as it is made. Schools and offices have systems of centrally located dust collectors and fans for creating vacuum; pipes lead to room outlets at which cleaning appliances are attached.
Industry:Science
An electroacoustic device containing a transducer which is actuated by sound waves and delivers electric signals proportional to the sound pressure. Microphones are usually classified with respect to the transducer principle used. Their directional characteristics are also of interest, that is, the voltage output as a function of the direction of incidence for constant sound pressure.
Industry:Science
