Jam besar berdiri
Sebuah jam besar berdiri, juga "jam peti panjang" atau "jam datuk", ialah sebuah jam batu ladung berdiri sendiri yang digerakkan oleh pemberat, dengan batu ladung digantung di dalam menara, atau pinggang peti jam. Peti jenis ini umumnya sekitar 1.8-2.4m (6-8 kaki) tinggi. Peti ini sering dihias secara terperinci dengan ukiran pada hud, atau bonet, yang mengelilingi dan membingkai dail, atau muka jam. Pembuat jam Inggeris William Clement dikreditkan sebagai pengembang bentuk ini pada 1670. Kebanyakan jam besar berdiri merupakan jam berbunyi, yang bermakna mereka membunyikan waktu pada setiap jam atau pecahan satu jam.
Istilah "jam datuk", "jam nenek", dan "jam cucu perempuan" telah digunakan untuk merujuk kepada jam besar berdiri. Walaupun tiada perbezaan khusus di kalangan istilah-istilah ini, pandangan umum menyatakan bahawa sebuah jam yang lebih kecil daripada 1.5 m (5 kaki) ialah jam cucu perempuan; 1.5m (5 kaki) ke atas ialah jam nenek; dan 1.8m (6 kaki) ke atas ialah jam datuk.
The advent of the longcase clock is due to the invention of the anchor escapement mechanism around 1670. Prior to that, pendulum clock movements used an older verge escapement mechanism, which required very wide pendulum swings of about 100°. Pendulum panjang dengan ayunan lebar semacam iyu tidak dapat dimuatkan dengan sebuah peti, jadi kebanyakan jam mempunyai pendulum pendek. Mekanisme anchor mengurangkan ayunan pendulum ke sekurangnya 4° to 6°, allowing clockmakers to use longer pendulums, which had slower "beats". These needed less power to keep going, caused less friction and wear in the movement, and were more accurate. Most longcase clocks use a "seconds" (or "Royal") pendulum, meaning that each swing takes one second. These are about a metre (39 inches) long (to the centre of the bob). This requirement for height, along with the need for a long drop space for the weights which power the clock, gave rise to the design of the long narrow case.
Modern longcase clocks use a more accurate variation of the anchor escapement called the deadbeat escapement.
Traditionally, longcase clocks were made with two types of movement: eight-day and one-day (30-hour) movements. A clock with an eight-day movement required winding only once a week, while generally less expensive 30-hour clocks had to be wound every day. Eight-day clocks are often driven by two weights - one driving the pendulum and the other the striking mechanism, which usually consisted of a bell or chimes. Such movements usually have two keyholes on either side of the dial to wind each one (as can be seen in the Thomas Ross clock above). By contrast, 30-hour clocks often had a single weight to drive both the timekeeping and striking mechanisms. Some 30-hour clocks were made with false keyholes, for customers who wished that guests to their home would think that the household was able to afford the more expensive eight-day clock. All modern striking longcase clocks have eight-day movements. Most longcase clocks are cable-driven, meaning that the weights are suspended by cables. If the cable was attached directly to the weight, the load would cause rotation and untwist the cable strands, so the cable wraps around a pulley mounted to the top of each weight. The mechanical advantage of this arrangement also doubles the running time allowed by a given weight drop.
Cable clocks are wound by inserting a special crank (called a "key") into holes in the clock's face and turning it. Others, however, are chain-driven, meaning that the weights are suspended by chains that wrap around gears in the clock's mechanism, with the other end of the chain hanging down next to the weight. To wind a chain-driven longcase clock, one pulls on the end of each chain, lifting the weights until the weights come up to just under the clock's face.
Pengilang dahulu [sunting]
- William Barrow, London
- Thomas Bullock, Bath
- Richard Donisthorp (fl. 1797), of Loughborough
- Peter Fearnley, Wigan
- John Harrison, Wakefield/Barrow/London
- Edward Harrison, Warrington
- William Lassell (1758-1790), Toxteth Park, Liverpool
- Thomas Ross, Hull
- John Snelling, Alton
- Thomas Worswick, Lancaster
- Henry Young, Swaffham
- Allam & Clements
- Thomas Cartwright
- Matthew & Thomas Dutton
- Thomas Johnson
- Daniel Quare
- Thomas Wright
Clockmakers in the United States
Pengilang kini [sunting]
Pautan luar [sunting]
- Charles Edwin on the evaluation of antique clocks
- Care guide for valuable antique longcase clocks
- P A Oxley discuss the longcase clock
- P A Oxley-How to set up an antique longcase/grandfather clock
Nota kaki [sunting]
- Headrick, Michael (2002). "Origin and Evolution of the Anchor Clock Escapement". Control Systems magazine, (Inst. of Electrical and Electronic Engineers) 22 (2). http://www.geocities.com/mvhw/anchor.html. Capaian 2007-06-06.
- Nelthropp, H. Leonard (1873). A Treatise on Watch-Work, Past and Present. London: E.& F.N. Spon. m/s. p.84. http://books.google.com/books?id=7DcDAAAAQAAJ&pg=PA84.
- Chappell, Jessica (2000). "The Long Case Clock: The science and engineering that goes into a grandfather clock". Illumin (Viterbi School of Engineering, USC) 1 (0): p.4. http://illumin.usc.edu/article.php?articleID=64&page=4. Capaian 2008-06-19.