Pendulum Clocks

A pendulum is a relatively long, thin rod made of wood or metal that terminates in a disk. In the earliest versions of the pendulum clock, the disk weighed down the pendulum rod as it moved from one side to the other. An escapement gear, which converted the movement of a gear into the swing of a pendulum, allowed energy to go to the pendulum to overcome friction so that it could continue swinging. Later versions of the pendulum clock were powered by springs, and finally batteries.

Galileo is credited with coming up with the concept of a pendulum as an tool for measuring time, having studied its motion as early as 1582. Although he drew up design plans for a pendulum clock, Galileo died in 1642 before he could actually build one. Dutch mathematician and astronomer Christiaan Huygens is the man who is actually credited with the invention of the pendulum clock, in 1656
Huygens earliest pendulum clock measured time with an error of less than one minute per day. With later models, he was able to reduce that error down to less than 10 seconds. In 1721, George Graham increased pendulum clock accuracy by reducing error to one second per day. He achieved this by making adjustments for changes in the pendulum rod's length, which were caused by temperature variations.

Siegmund Riefler and W.H. Shortt increased pendulum clock accuracy even more. Riefler’s clocks, which used an escapement he perfected in 1889, became widely used in astronomical observatories, and were accurate down to a hundredth of a second per day. The Shortt clock, introduced in 1921, operated on the two-pendulum slave and master principle and replaced Riefler’s as clock of choice for observatories.

Quartz clock technology was introduced during the 1920s and became the standard for timekeeping in subsequent decades. The pendulum clock came to take on the status of a collector’s item, both in its antique and more contemporary forms. Three common types of pendulum clocks that might appear in a collection are the cuckoo clock, the grandfather clock, and the mantel clock. For cuckoo clocks and grandfather clocks, the pendulum typically swings twice every second, and once every two seconds, respectively.

The Water Clocks

A water clock is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured.
Water clocks, along with sundial, are likely to be the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.Where and when they were first invented is not known, and given their great antiquity it may never be. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China , also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, claim that water clocks appeared in China as early as 4000 BC.

The Greeks, Romans further advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy. Further advances were made in Byzantium and particularly the Islamic world, where increasingly accurate water clocks incorporated complex segmental and epicyclic gearing, water wheels, and programmability, advances which eventually made their way to Europe. Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, and water wheels, passing their ideas on to Korea and Japan.

Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. These early water clocks were calibrated with a sundial. While never reaching a level of accuracy comparable to today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by more accurate pendulum clocksin 18th century Europe.

Sundial Clocks

Sundial Clocks

The Sumerian culture was lost without passing on its knowledge, but the Egyptians were apparently the next to

formally divide their day into parts something like our hours. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 BCE. Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, additional markers around the base of the monument would indicate further subdivisions of time.

Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 BCE. This device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening. When the long stem with 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours."
The merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 BCE. A pair of merkhets was used to establish a north-south line (or meridian) by aligning them with the Pole Star. They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian.

In the quest for better year-round accuracy, sundials evolved from flat horizontal or vertical plates to more elaborate forms. One version was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer) and scribed with sets of hour lines for different seasons. The hemicycle, said to have been invented about 300 BCE, removed the useless half of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block. By 30 BCE, Vitruvius could describe 13 different sundial styles in use in Greece, Asia Minor, and Italy.

The History of Clock

The History of Clocks

The history of clocks is very long, and there have been many different types of clocks over the centuries. Not all historians agree on the history of the clock. The word clock was first used in the 14th century (about 700 years ago). It comes from the word for bell in Latin ("clocca").


Evolution of the clocks

- Using the Sun
- Sundial Clocks
- Water Clocks
- Pendulum clocks
- Quartz Crystal Clocks

Using the Sun
The first way that people could tell the time was by looking at the sun as it crossed the sky. When the sun was directly overhead in the sky, it was the middle of the day, or noon. When the sun was close to the horizon, it was either early morning (sunrise) or early evening (sunset). Telling the time was not very accurate.

Sundial Clocks
The oldest type of clock is a sundial clock, also called a sun clock. They were first used around 3,500 B.C. (about 5,500 years ago). Sundials use the sun to tell the time. The shadow of the sun points to a number on a circular disk that shows you the time. In the big picture below on the right, the shadow created by the sun points to 9, so it is nine o'clock.
Since sundials depend on the sun, they can only be used to tell the time during the day

Water Clocks
Around 1400 B.C. (about 3,400 years ago), water clocks were invented in Egypt. The name for a water clock is clepsydra (pronounced KLEP-suh-druh). A water clock was made of two containers of water, one higher than the other. Water traveled from the higher container to the lower container through a tube connecting the containers. The containers had marks showing the water level, and the marks told the time.
Water clocks were very popular in Greece, where they were improved many times over the years. Look at the picture below. Water drips from the higher container to the lower container. As the water level rises in the lower container, it raises the float on the surface of the water. The float is connected to a stick with notches, and as the stick rises, the notches turn a gear, which moves the hand that points to the time.
Water clocks worked better than sundials because they told the time at night as well as during the day. They were also more accurate than sundials

Pendulum Clocks
Before pendulum clocks were invented, Peter Henlein of Germany invented a spring-powered clock around 1510. It was not very precise. The first clock with a minute hand was invented by Jost Burgi in 1577. It also had problems. The first practical clock was driven by a pendulum. It was developed by Christian Huygens around 1656. By 1600, the pendulum clock also had a minute hand.
The pendulum swings left and right, and as it swings, it turns a wheel with teeth (see the picture to the right). The turning wheel turns the hour and minute hands on the clock. On the first pendulum clocks, the pendulum used to swing a lot (about 50 degrees). As pendulum clocks were improved, the pendulum swung a lot less (about 10 to 15 degrees). One problem with pendulum clocks is that they stopped running after a while and had to be restarted. The first pendulum clock with external batteries was developed around 1840. By 1906, the batteries were inside the clock.
As you already learned, a clock only shows 12 hours at a time, and the hour hand must go around the clock twice to measure 24 hours, or a complete day. To tell the first 12 hours of the day (from midnight to noon) apart from the second 12 hours of the day (from noon to midnight), we use these terms:
A.M.--Ante meridiem, from the Latin for "before noon"
P.M.-- Post meridiem, from the Latin for "after noon"

Quartz Crystal Clocks
Quartz is a type of crystal that looks like glass. When you apply voltage, or electricity, and pressure, the quartz crystal vibrates or oscillates at a very constant frequency or rate. The vibration moves the clock's hands very precisely. Quartz crystal clocks were invented in 1920.