Precision Clocks 精準的時鐘

[b][size=6][color=darkorange]Precision Clocks 精準的時鐘
[/color][/size][size=4][color=royalblue]十八世紀以來，一般機械鐘的精確度，足夠滿足廣大市民的個人計時需要。但在科學研究方面，就需要一些更高層次〔精確度更高〕的時計，在實驗室和觀察站使用。為了滿足這種需求，鐘匠需要發展出一些更精確的時鐘。
18世紀初，人們已知道溫度會使時鐘的金屬膨脹和收縮，從而影響計時精確度。
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[img]http://www.bmumford.com/mset/tech/hsntmp/Image7.gif[/img]
[color=darkorange]Figure 1 - Temperature effect on a Hermle movement[/color]
[img]http://www.bmumford.com/mset/tech/hsntmp/Image8.gif[/img]
[color=darkorange]Figure 2 - Temperature effect on a pocket watch
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[img]http://www.bmumford.com/mset/tech/hsntmp/Image9.gif[/img]
[color=darkorange]Figure 3 - Experimental pendulum[/color]
[b][size=4][color=royalblue]在1715年，喬治葛理翰George Graham做了大量有關不同金屬的熱反應研究，希望能找到兩種金屬，其的膨脹比率會相互補償或互相取消。他發現鋼材steel和粘性的金屬汞viscous metal mercury能造成這種關係，在1722年發明了溫度補償擺mercury-compensated pendulum。葛理翰Graham的工作，有助於開創一個新時代的精密計時。
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[img]http://www.theodoregray.com/PeriodicTable/Samples/080.3b/s12s.JPG[/img]
[img]http://www.theodoregray.com/PeriodicTable/Samples/080.10/s12s.JPG[/img]
[color=darkorange]Temperature compensating pendulum.
To keep accurate time, a pendulum clock needs a pendulum whose center of gravity remains constant. Unfortunately when the metal shaft of a pendulum warms up, it expands, shifting the center of gravity down and causing the clock to run slow. To compensate for this, glass vials containing mercury can be placed at the bottom of the pendulum. Not only is mercury nice and heavy, making for a good solid pendulum, it also expands more than most other metals on heating. Thus as the pendulum warms up the bottom of the vials moves down (because the shaft gets longer) but the mercury gets taller, shifting the center of gravity back up where it belongs. How much more accurate this made a clock I don't know: In any case a cheap electronic clock now is vastly more accurate than any of these ever were.
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[b][size=4][color=royalblue]在整個18世紀，工匠們對冶金及新工藝技術掌握越來越多，計時準確度也日漸在改善中。在1676年托馬斯湯皮恩Thomas Tompion為英國皇家格林威治天文台（Royal Observatory in Greenwich），製造了一個精確的時鐘，精確度能維持每天誤差在2至3秒。
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[img]http://www.nmm.ac.uk/searchstation/images/fsnegs/x2/d8930_2.jpg[/img]
[color=darkorange]Year-Going Pendulum Clock by Thomas Tompion, London, 1676
This clock is one of a pair that were originally built into the panelling of the Octagon Room in the Observatory and had 13-foot (almost 4 m) pendulums suspended above them. They were removed in 1719 by the widow of the first Astronomer Royal, John Flamsteed, and sold as domestic longcase clocks. One is preserved at the British Museum and the other has now returned to Greenwich. This picture shows a detail of the movement. The primary achievement of these two clocks was to prove that the earth spins on its axis at a uniform rate.
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[b][size=4][color=royalblue]大約50年後，約翰哈里森John Harrison建造了一個精準時鐘，其中採納了他新發明的柵形補償擺 (temperature-compensated gridiron pendulum)，將精確度推前達至每月誤差只有一秒。
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[img]http://www.arcadianclock.com/gallery/g_arcadianr/detailsmap.jpg[/img]
[img]http://www.arcadianclock.com/gallery/g_arcadianr/14pendulum.jpg[/img]
[color=darkorange]The relative expansion of the steel and brass rods compensate for changes in temperature
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[img]http://www.telegraph.co.uk/news/graphics/2006/02/13/nclock13.jpg[/img]
[color=darkorange]The priceless Late Regulator clock took John Harrison, the pioneer of longitude, 36 years to build and he was still calibrating it when he died at his home in London on March 24, 1776, his 83rd birthday.
[/color][b][size=4][color=royalblue]1900年，德國的裡夫勒公司（Riefler Company）製成了準確度達至每一天誤差0.01秒的計時器。這突破不僅是由於力學原理和金屬材料改進，而且也是由於發明了新的外殼設計。為了保持恆定的壓力、溫度和濕度，工匠們把精確時鐘機芯放在密不透風的玻璃罩內。以後的設計，也常常把這些玻璃罩放置在精準的時鐘外面。
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[img]http://www.antiqueclockspriceguide.com/priceguideimages/antiquorum1111/149.jpg[/img]
[img]http://tf.nist.gov/general/museum/riefler.jpg[/img] [img]http://tf.nist.gov/general/museum/shortt-big.jpg[/img]
[color=darkorange]In 1904 NIST purchased a very stable pendulum clock, the Riefler Clock (left) from Clemens Riefler in Germany. This clock served as a time interval standard until 1929 when it was replaced by the Shortt Clock (right), a double pendulum clock developed at Edinburgh Observatory and fabricated in London. This mechanical standard was replaced only a few years later by standards based on electronic methods. (NIST-National Institute of Standards and Technology)
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[b][size=4][color=royalblue]這些時鐘充分體會到正確的，令人難以置信的準確性〔由於當時的科學家尚未量度得到更微量的時間值〕。例如，邵特W.H. Shortt在1920年製作的邵特鐘Shortt Clock，被認為是保持時間誤差值在兩毫秒（ 0.002 ）一天，不過，在1984年進行一次測試時，人員發現這個邵特鐘保持的時間誤差值以200微秒（0.0002）一天，比原先認為的有十倍以上的準確度。
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[img]http://www.nmm.ac.uk/searchstation/images/fsnegs/x1/e0251.jpg[/img]
[color=darkorange]High Precision 'Free Pendulum' Clock by W.H. Shortt, London, No. 16, 1927[/color]
[img]http://hua.umf.maine.edu/China/astronomy/tianimage/0038_synchronome6745w.jpg[/img]
[color=darkorange]This Synchronome housed at the Beijing Ancient Observatory includes a second hand and a 24 hour dial.
[/color][b][size=4][color=royalblue]17世紀中葉，義大利科學家伽利略Galilei發現了單擺的等時性。
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[img]http://upload.wikimedia.org/wikipedia/commons/9/9d/Simple_pendulum_height.png[/img]
[b][size=4][color=royalblue]1656年，荷蘭物理學家惠更斯Christiaan Huygens利用這一性質制出了第一個實用的機械擺鐘，從此人類掌握了比較精確的測量時間的方法。[/color][/size][/b]
[img]http://www.cultuurwijzer.nl/sites/cultuurwijzer.nl/contents/i000484/tijd_huygens2.jpg[/img]
[b][size=4][color=royalblue]1658年英國物理學家胡克Robert Hooke發明了有擺輪的懷錶，1760年具有時、分、秒三個針的懷錶問世，機械表更加具有實用價值。最精確的機械鐘要數1920年問世的邵特鐘Shortt Clock，它一晝夜誤差只有千分之一秒，被當時的天文臺用來作天文鐘。
但是機械鐘怕震，一次小地震就可能使它停擺或產生較大的誤差，而且它的精度不能再提高了。20世紀30年代石英鐘問世了，它一晝夜誤差只有萬分之一秒，充當了天文鐘的角色。
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[img]http://www.explainthatstuff.com/quartz.jpg[/img]
[color=darkorange]Crystals of quartz[/color]
[img]http://www.timemuseum.com/ttmop/ATOMICCL.JPG[/img]
[color=darkorange]In 1928 at Bell Telephone Laboratories in New York, Warren A. Marrison developed the first quartz-crystal clock. The escapement and pendulum concept was replaced by the measurement of a standard frequency of the vibration of a quartz crystal. By the end of World War II, quartz-crystal clocks were accurate to one second in 30 years, and became the standard for time measurement until the arrival of the atomic clock.
[/color][img]http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/MIH-film27jpg.jpg/391px-MIH-film27jpg.jpg[/img]

[b][size=4][color=royalblue]後來人們發現某些物質的分子或原子有更為穩定的計時功能，於是出現了原子鐘。世界上第一台原子鐘1949年在美國造出。原子鐘運行3000多年才產生1秒的誤差，所以目前天文台使用的都是原子鐘。[/color][/size][/b]
[img]http://tf.nist.gov/general/museum/nbs-1.jpg[/img]
[color=darkorange]1952 -- NIST completes the first accurate measurement of the frequency of the cesium clock resonance. The apparatus for this measurement is named NBS-1.
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[img]http://tf.nist.gov/general/museum/nbs-6.jpg[/img]
[color=darkorange]1975 -- NBS-6 begins operation; an outgrowth of NBS-5, it is one of the world’s most accurate atomic clocks, neither gaining nor losing one second in 300,000 years.
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[img]http://tf.nist.gov/general/museum/nist-7a.jpg[/img]
[color=darkorange]1993 -- NIST-7 comes on line; eventually, it achieves an uncertainty of 5 x 10-15, or 20 times more accurate than NBS-6.
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[img]http://tf.nist.gov/general/museum/nist-f1.jpg[/img]
[color=darkorange]1999 -- NIST-F1 begins operation with an uncertainty of 1.7 x 10-15, or accuracy to about one second in 20 million years, making it one of the most accurate clocks ever made (a distinction shared with similar standards in France and Germany).[/color]

[b][size=5][color=darkorange]Mercury pendulum水銀鐘擺[/color][/size][/b]
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水銀溫度補償擺錘的運作原理，當金屬擺桿遇熱膨脹向下伸展。水銀擺錘玻璃柱內密封的水銀會向上擴展，使擺錘重心向上提升，從而抵銷了金屬擺桿遇熱膨脹，向下伸延的長度。[/color][/size][/b]
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早期的標準時鐘(regulator clocks)會採用水銀補償的擺錘。大部份法國時鐘都是的。美國製造的標準時鐘(regulator clocks)通常會使用鍍銀的金屬柱代替，或用打磨得光滑亮麗的鋼子彈插入玻璃管模仿這種形貌。一些美國標準時鐘也會用水銀小瓶，不過相信是後來改用的。

由於水銀玻璃柱很易損壞，價格也很高，當不小心摔破時，需要定製才有得更換。人們便想到使用金屬柱代替，一來可以做到接近的溫度補償效果，另一方面損壞的機會也減少。發展至後來，只有較名貴的時鐘才使用真水銀柱。
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Pictures borrowed
[img]http://www.medfordclock.com/mercpend.jpg[/img]

[img]http://www.medfordclock.com/hgvial.jpg[/img]

[img]http://www.medfordclock.com/mercpen.jpg[/img]

[b][size=5][color=darkorange]Temperature-compensated gridiron pendulum
柵形溫度補償擺[/color][/size][/b]
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根據鐘擺原理(isochronous property of the pendulum)，鐘擺的運動時間，取決於擺桿的長度。擺桿越長，運動的時間越長；擺桿越短，運動的時間便快。

早期的時鐘，多採用金屬鐘擺，擺桿易受溫度變化影響，導致走時不準確。在1715年，英國人George Graham發明了水銀擺錘，改善了溫度對時鐘的影響。

在1725年，英國人約翰哈里遜John Harrison發明了柵形鐘擺，用另一種方法，解決溫度對金屬擺桿膨脹的補償問題。他採用黃銅和鐵組合製成的標尺，用兩種金屬計算出各自不同的溫度膨脹系數，設計成柵形的組合作擺桿，有效地改善了溫度對擺桿的影響，在大範圍的溫度改變時，也能有很好的表現。

[/color][/size][/b]Pictures borrowed

[img]http://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/BanjoPendulum.png/453px-BanjoPendulum.png[/img]

[img]http://www.snellenburg.com/images/clocks/portico/aa028portico_pendulum_500.jpg[/img]

[b][size=5][color=darkorange]Invar pendulum因鋼鐘擺[/color][/size][/b]
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因鋼，因瓦(鐵鎳)合金invar alloys，又名不脹鋼，恒範鋼。這是一種新金屬，特性是不受溫度變化影響，溫度膨脹係數0.0000012(接近零的溫度系數)。[/color][/size][/b]
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用這種合金製成的鐘擺，當溫度變化攝氏一度時，只會影響每天走時0.052秒。

這種合金的另一種特性是抗磁性(nonmagnetic)，是十分適宜用製作鐘擺的材料。[/color][/size][/b]

[b][size=4][color=royalblue][size=5][color=darkorange]Wood pendulum 木材制鐘擺
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差點兒漏左介紹這種最原始的鐘擺。[/color][/size][/b]
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木材的膨脹係數比一般金屬細，在小範圍的溫度改變下，比簡單的銅金屬鐘擺又勝一些。

所以有些平民化的鐘會用木材做擺杆。成本低而效果好。[/color][/size][/b]

Picture borrowed
[img]http://i53.photobucket.com/albums/g41/sampeter_2006/Watch%20Photo/Clock/8fa5_1.jpg[/img]

[img]http://i53.photobucket.com/albums/g41/sampeter_2006/Watch%20Photo/Clock/38ac_1.jpg[/img]

[img]http://i53.photobucket.com/albums/g41/sampeter_2006/Watch%20Photo/Clock/101572945_o.jpg[/img]

[img]http://i53.photobucket.com/albums/g41/sampeter_2006/Watch%20Photo/Clock/GarageSale_1187660168_1932.jpg[/img]