【導讀】供水是一個關系國計民生的重要產(chǎn)業(yè)。傳統(tǒng)的人工供水方式，勞動強度大，工作效。此很有必要對水塔水位進行自動控制。為了達到節(jié)能的目的，提高供水系統(tǒng)的質量，考。慮采用單片機技術，設計出一套實用水位自動控制方案。護、自動聲光報警功能的控制系統(tǒng)，完成水塔水位控制系統(tǒng)的設計。機控制部分、時鐘顯示部分、數(shù)碼顯示部分、電機控制部分、報警部分等構成。系統(tǒng)的調試和仿真。因為它具有電路簡單、操作方便、性能良好、可靠性高等優(yōu)點，因。此該設計具有一定的實用性。

　　

【正文】 onductors, such as asbestos, have been used as insulators to impede heat flow (see insulation). Liquids and gases have their molecules farther apart and are generally poor conductors of heat. Conduction of electricity consists of the flow of charges as a result of an electromotive force, or potential difference. The rate of flow, ., the electric current, is proportional to the potential difference and to the electrical conductivity of the substance, which in turn depends on the nature of the substance, its crosssectional area, and its temperature. In solids, electric current consists of a flow of electrons。 as in the case of heat conduction, metals are better conductors of electricity because of their greater freeelectron density, while nonmetals, such as rubber, are poor conductors and may be used as electrical insulators, or dielectrics. Increasing the crosssectional area of a given conductor will increase the current because more electrons will be available for conduction. Increasing the temperature will inhibit conduction in a metal because the increased thermal motions of the electrons will tend to interfere with their regular flow in an electric current。 in a nonmetal, however, an increase in temperature improves conduction because it frees more electrons. In liquids and gases, current consists not only in the flow of electrons but also in that of ions. A highly ionized liquid solution, ., saltwater, is a good conductor. Gases at high temperatures tend to bee ionized and thus bee good conductors (see plasma), although at ordinary temperatures they tend to be poor conductors. See electrochemistry。 electrolysis。 superconductivity. Almost everyone has experienced the Doppler effect, though perhaps without knowing what causes it. For example, if one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it es closer. Then, as it passes, the pitch suddenly lowers perceptibly. This is an example of the Doppler effect: the change in the observed frequency of a wave when the source of the wave is moving with respect to the observer. The Doppler effect, which occurs both in sound and electromagic waves— including light waves— has a number of applications. Astronomers use it, for instance, to gauge the movement of stars relative to Earth. Closer to home, principles relating to the Doppler effect find application in radar technology. Doppler radar provides information concerning weather patterns, but some people experience it in a less pleasant way: when a police officer uses it to measure their driving speed before writing a ticket. Sound and light are both examples of energy, and both are carried on waves. Wave motion is a type of harmonic motion that carries energy from one place to another without actually moving any matter. It is related to oscillation, a type of harmonic motion in one or more dimensions. Oscillation involves no movement, only movement in place。 yet individual points in the wave medium are oscillating even as the overall wave pattern moves. The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves— waves in which a uniform series of crests and troughs follow each other in regular succession. A period (represented by the symbol T ) is the amount of time required to plete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f ) 蘭州交通大學博文學院 畢業(yè)設計（論文） 1 is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nieenthcentury German physicist Heinrich Rudolf Hertz (18571894), and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz。 103 or 1,000 cycles per second)。 megahertz (MHz。 106 or 1 million cycles per second)。 and gigahertz (GHz。 109 or 1 billion cycles per second.) Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength. Amplitude, though mathematically independent from the parameters discussed, is critical to the understanding of sound. Defined as the maximum displacement of a vibrating material, amplitude is the size of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity, which, in the case of sound waves, is manifested as what people monly call volume. Similarly, the amplitude of a light wave determines the intensity of the light. electromagic radiation,energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magic field, and if the motion is changing (accelerated), then the magic field varies and in turn produces an electric field. These interacting electric and magic fields are at right angles to one another and also to the direction of propagation of the energy. Thus, an electromagic wave is a transverse wave. If the direction of the electric field is constant, the wave is said to be polarized (see polarization of light). Electromagic radiation does not require a material medium and can travel through a vacuum. The theory of electromagic radiation was developed by James Clerk Maxwell and published in 1865. He showed that the speed of propagation of electromagic radiation should be identical with that of light, about 186,000 mi (300,000 km) per sec. Subsequent experiments by Heinrich Hertz verified Maxwell39。s prediction through the discovery of radio waves, also known as hertzian waves. Light is a type of electroma