clock
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What is a clock?
A clock is a device that measures and displays time using hands on a dial or a periodic system. Clocks operate by linking a device that performs regular movements at equal intervals to a counting mechanism. The power is transmitted through a series of gears from a spring or weight to the hands.
The oldest surviving clock in England is at Salisbury Cathedral, dating back to 1386. Water clocks were also among the earliest time-measuring devices not dependent on celestial observation. Today, atomic clocks provide the most precise time scale, forming the basis for scientific and legal timekeeping.
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Why do people use clocks?
People use clocks to measure time and to orient themselves in time. Clocks help people to synchronize their daily activities, such as sleeping, eating, and working. Clocks have been used since humans began tracking the rising and setting of the Sun. Sundials were used to identify midday and other important times of the day. Mechanical clocks appeared in the towers of large Italian cities in the 14th century.
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How does a clock measure time?
A clock measures time by assigning a unique number to an epoch (instantaneous event) or interval (duration of a continued event). It relies on the progress of a regularly changing phenomenon, whether gravitation, electromagnetism, rotational inertia, or radioactivity.
Different types of clocks use different phenomena. For example, electric clocks use electric currents to signal time, while atomic clocks use resonance frequencies of atoms to keep time with extreme accuracy. Atomic clocks are so accurate that they serve as the basis of scientific and legal clock times.
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What are the main types of clocks?
Clocks are time-measuring devices with diverse designs, each utilizing different mechanisms to track and display the time. Here are some main types of clocks:
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Electric Clocks These clocks use electric currents as a power source, either replacing traditional weights or springs. They can be battery-driven or synchronous, relying on the frequency stability of the alternating current.
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Quartz Clocks These clocks use a quartz crystal oscillating at a specific frequency. The regularity of the crystal's vibrations makes these clocks highly accurate.
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Atomic Clocks Atomic clocks achieve extreme accuracy by using the resonance frequencies of atoms, such as cesium or rubidium, to measure time.
Other types of clocks include sundials, water clocks, and chronometers.
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How do mechanical and digital clocks work differently?
Mechanical and digital clocks operate on different principles. Mechanical clocks use a pendulum or balance wheel and a series of gears, called the wheel train, to measure time. The gears transmit motion from a spring or weight via an escapement to the clock's hands.
Digital clocks, on the other hand, use electronic oscillators to measure time. A quartz crystal is a common timekeeping element in digital clocks. The crystal oscillates at a specific frequency, and this frequency is divided and used to drive a digital display, showing the time in numerical format.
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How have clocks helped people organize daily life and history?
Clocks have revolutionized the organization of daily life and the recording of history. Early timekeeping devices, such as water clocks found in ancient Egypt around 1500 BCE, helped determine hours during the night and day. The development of mechanical clocks in 14th-century Europe marked a significant advancement, followed by the invention of spring-powered clocks in the early 16th century, which made timekeeping more portable.
The standardization of time became crucial with the advent of railways, leading to the adoption of standard time zones to avoid scheduling confusion. Standard time uses 24 standard meridians of longitude, which are 15° apart, starting at the prime meridian in Greenwich, England. This system helps coordinate time with daylight as the Earth rotates, impacting political, social, and economic activities.
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News •
clock, mechanical or electrical device other than a watch for displaying time. A clock is a machine in which a device that performs regular movements in equal intervals of time is linked to a counting mechanism that records the number of movements. All clocks, of whatever formâwhether 12-hour clocks or 24-hour clocksâare made on this principle. See also atomic clock; nuclear clock.
The origin of the all-mechanical escapement clock is unknown; the first such devices may have been invented and used in monasteries to toll a bell that called the monks to prayers. The first mechanical clocks to which clear references exist were large, weight-driven machines fitted into towers and known today as turret clocks. These early devices struck only the hours and did not have hands or a dial.
The oldest surviving clock in England is that at Salisbury Cathedral, which dates from 1386. A clock erected at Rouen, France, in 1389 is still extant, and one built for Wells Cathedral in England is preserved in the Science Museum in London. The Salisbury clock strikes the hours, and those of Rouen and Wells also have mechanisms for chiming at the quarter hour. These clocks are large, iron-framed structures driven by falling weights attached to a cord wrapped around a drum and regulated by a mechanism known as a verge (or crown wheel) escapement. Their errors probably were as large as a half hour per day. The first domestic clocks were smaller wall-mounted versions of these large public clocks. They appeared late in the 14th century, and few examples have survived; most of them, extremely austere in design, had no cases or means of protection from dust.
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About 1450, clockmakers working probably in southern Germany or northern Italy began to make small clocks driven by a spring. These were the first portable timepieces, representing an important landmark in horology. The time-telling dials of these clocks usually had an hour hand only (minute hands did not generally appear until the 1650s) and were exposed to the air; there was normally no form of cover such as a glass until the 17th century, though the mechanism was enclosed, and the cases were made of brass.
About 1581 Galileo noticed the characteristic timekeeping property of the pendulum. The Dutch astronomer and physicist Christiaan Huygens was responsible for the practical application of the pendulum as a time controller in clocks from 1656 onward. Huygensâs invention brought about a great increase in the importance and extent of clock making. Clocks, weight-driven and with short pendulums, were encased in wood and made to hang on the wall, but these new eight-day wall clocks had very heavy weights, and many fell off weak plaster walls and were destroyed. The next step was to extend the case to the floor, and the grandfather clock was born. In 1670 the long, or seconds, pendulum was introduced by English clock makers with the anchor escapement.
Mechanical clocks
The pendulum
The pendulum is a reliable time measurer because, for small arcs, the time required for a complete swing (period) depends only on the length of the pendulum and is almost independent of the extent of the arc. The length of a pendulum with a period of one second is about 39 inches (990 mm), and an increase in length of 0.001 inch (0.025 mm) will make the clock lose about one second per day. Altering the length of a pendulum is therefore a sensitive means of regulation. The alteration is usually carried out by allowing the bob to rest upon a nut that can be screwed up or down the pendulum rod.
Any expansion or contraction of the rod caused by changes of temperature will affect the timekeeping of a pendulum; e.g., a pendulum clock with a steel rod will lose one second a day for a rise in temperature of approximately 4 °F (2.2 °C). For accurate timekeeping, the length of the pendulum must be kept as nearly constant as possible. This may be done in several ways, some of which use the differing coefficients of expansion (the amount of expansion per degree change in temperature) of different metals to obtain a cancelling-out effect. In one popular compensation method, the bob consists of a glass or metal jar containing a suitable amount of mercury. The gridiron pendulum employs rods of different metal, usually brass and steel, while in the zinc-iron tube the pendulum rod is made of concentric tubes of zinc and iron. An improved method, however, is to make the pendulum rod from a special alloy called Invar. This material has such a small coefficient of expansion that small changes of temperature have a negligible effect and can easily be compensated for if required.
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In a pendulum clock an escape wheel is allowed to rotate through the pitch of one tooth for each double swing of the pendulum and to transmit an impulse to the pendulum to keep it swinging. An ideal escapement would transmit the impulse without interfering with the free swing, and the impulse should be as uniform as possible. The double three-legged gravity escapement, which achieves the second of these but not the first, was invented by Edmund Beckett, afterward Lord Grimthorpe, and used by him for the great clock at Westminster, now generally known as Big Ben, which was installed in 1859. It became the standard for all really accurate tower clocks.
Escapement
The true innovation of the weight-driven clock was the escapement, the system that mediated the transfer of the energy of the gravitational force acting on the weights to the clockâs counting mechanism. The most common escapement was the verge-and-foliot.
In a typical verge-and-foliot escapement, the weighted rope unwinds from the barrel, turning the toothed escape wheel. Controlling the movement of the wheel is the verge, a vertical rod with pallets at each end. When the wheel turns, the top pallet stops it and causes the foliot, with its regulating weights, to oscillate. This oscillation turns the verge and releases the top pallet. The wheel advances until it is caught again by the bottom pallet, and the process repeats itself. The actions of the escapement stabilize the power of the gravitational force and are what produce the ticktock of weight-driven clocks.
The wheelwork
The wheelwork, or train, of a clock is the series of moving wheels (gears) that transmit motion from a weight or spring, via the escapement, to the minute and hour hands. It is most important that the wheels and pinions be made accurately and the tooth form designed so that the power is transferred as steadily as possible.
In a clock driven by a weight or a spring, the power is first transmitted by the main, or great, wheel. This engages with a pinion (a gear with a small number of teeth designed to mesh with a larger wheel), whose arbor (a turning rod to which gears are attached) is attached to the second wheel that, in its turn, engages with the next pinion, and so on, down through the train to the escapement. The gear ratios are such that one arbor, usually the second or third, rotates once an hour and can be used to carry the minute hand. A simple 12-to-1 gearing, known as the motion work, gives the necessary step-down ratio to drive the hour hand. The spring or weight is fitted with a mechanism so it can be rewound when necessary, and the arbor carrying the minute hand is provided with a simple slipping clutch that allows the hands to be set to the correct time.
The timekeeping part of all weight-driven clocks, including large tower clocks, is substantially the same. The figure shows the mechanism of a simple weight-driven timepiece with a pendulum. The frame is made up of two plates that carry the pivots of the various wheels and other moving parts and that are united and spaced by four pillars. The driving weight hangs from a line coiled around a barrel or sprocket, which is raised by turning the winding square or, in some cases, by pulling on the line. The main wheel engages with the centre pinion, on the arbor (axle) of which is also mounted the centre wheel. The front pivot of this wheel and pinion is lengthened to the right of the illustration; it carries the minute hand and part of the gearing necessary to drive the hour hand.
The centre wheel is also coupled through a suitable gear train to the escape wheel, which engages with the pallets that are fixed to the arbor between the front plate and the pendulum suspension cock. Also fixed to the pallet arbor is the crutch, which terminates at its lower end in a fork that embraces the pendulum rod.
The motion work used for driving the hands is mounted between the dial and the front plate of the frame. The cannon pinion, which drives the motion work, rotates once an hour; it is coupled to the centre arbor by a flat spring that acts as a clutch and permits the hands to be set.