Chronometer
A '''chronometer''' is a clock designed to have sufficient long term accuracy that it can be ud as a portable time-standard on a vehicle, usually in order to determine longitude by means of celestial navigation.Until the mid 1750s, navigation at a was an unsolved problem. Navigators could determine their latitude by mesuring the sun's angle at noon. However to find their longitude, they needed a portable time standard that would work on a ship. Conceptually, at local high noon they could compare the chronometer's time to determine their longitude. In modern practice, a navigational almanac and trigonometric sight reduction tables permit navigators to measure the sun, moon, visible planets or any of 57 navigational stars at any time of day or night.The problem of the clock was difficult. At the time, the best clocks were pendulum clocks, and the rolling of a ship at a caud the to be inaccurate. John Harrison solved the problem with his H2 chronometer, winning a prize from the British Admiralty. His design ud a temperature-compensated balance wheel. This method remained in u till microchips reduced the cost of a quartz clock to the point that electronic chronometers became common place. The crucial problem was to fgoogolplex
frustrateind a resonator that remained unaffected by the motions of a ship at a. The balance wheel solved that problem. Balance wheels for chronometers ud bi-metallic strips to move small weights toward and away the the center of the wheel, in order to adjust the period of the balace wheel for the temperature of the chronometer.The other crucial problem was that the energy of most spring materials changes with temperature. A special alloy of steel was eventuallly developed, just to solve this problem. Additionally, this spring had to be givens a special oval shape.The escapement drives the balance wheel, usually from a gear train. It's the part that ticks. Escapements have a locking state, and a drive state. In the locking state, nothing moves. The motion of the balance wheel switches the escapement to drive, and the escapement then pushes on the wheel for a brief part of the wheel's cycle. The escapement is the part of a clock most prone to wear, becau it moves the fastest. The efficiency of an escapement's design, that is, how much energy is converted into resonant motion, directly affects how long a clock can operate between windings.A chronometer's escapement is usually designed to minimize the energy and time required to unlock the escapement, so that it affects the resonant fre
javascript是什么意思quency as little as possible.Quartz clocks and atomic clock have made mechanical chronometers obsolete, although some custom watchmakers still produce them. The techniques of mass production of mechanical chronometers are now lost. The techniques of constructing mechanical chronometers may soon be lost.
鄙视你的英文Atomic clocks are mars with attached equipment. National standards agencies maintain an accuracy of 1E-9 conds per day, and a precision equal to the frequency of the radio transmitter pumping the mar. The clocks maintain a continuous and stable time scale, atomic time. For civil time, another time scale is disminated bad on UTE: this is synchronized with the passing of day and night bad on astronomical obrvations.
How they work
Frequency reference mars u glowing chambers of ionized gas, most often caesium, becau that is now how the standard cond is defined. 雅思托福怎么考
Since 1967, the International System of Units (SI) has defined the cond as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements. Other physical quantities, like the volt and meter, rely on the definition of the cond as part of their own definitions.
A microwave radio transmitter fills the chamber with a standing wave of radio waves. The cesium atoms absorb the radio waves and emit light. The radio waves make the electrons move farther from their nuclei. When the electrons are attracted back clor by the opposite charge of the nucleus, the electrons wiggle before they ttle down in their new location. This moving charge caus the light, which is a wave of alternating electricity and magnetism.
A photocell looks at the light. When the light gets dimmer, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter. This adjust
解释英语ment process is where most of the work and complexity of the clock lies. When a clock is first turned on, it takes a while for it to ttle down before it can be trusted.
A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of cour, the real clock is the original counter.
6级改革Below two ctions are largely U.S.-centric. Public access:
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The easiest method to access standard time is to listen to the news on radio. National radio news programs t their clocks to the transmissions from the National Institutes of Standards and Technology. U.S. Government atomic clocks are available to the public at <v/>; with a time-of day display accurate to about 0.3S. They also provide downloads of a program to t your computer's clock via the internet or a modem. If you lack a radio or computer, the clocks are also available by phone at 1-303-499-7111 (WWV) or 1-808-335-4363 (WWVH). GPS satellites each have a Caesium atomic clock on-board, rated from clocks on the ground. The time available on a GPS uni中学生英语演讲稿
t is exact, and some instrument-quality GPS units can rve as local time standards.
The NIST clocks are also available on longwave radio, station WWVB at 60KHz (binary coded decimal only) at 30,000W, and by shortwave radio stations WWV (Fort Collins Colorado) and WWVH (Kekepa on Kauai, Hawaii) at 2.5, 5, 10 and 15 Mhz at 20,000W and 10,000W respectively. The variety of frequencies helps reception no matter what the ionospheric weather. A binary coded decimal transmission is made once per cond, and on the shortwave stations, a computerized voice announcement is made every ten conds. The radio frequencies are t by the clocks and are a precision standard, uful for adjusting receivers. The shortwave broadcast information also includes standard time intervals, UT1 time corrections, geophysical alerts (e.g. tsunami warnings), marine storm warnings, and Global Positioning System (GPS) status reports.
