Clocks

The passage of time has had a lot of attention lately–which makes this the ideal time to honor the clock.

The earliest timekeeping device, used as far back as 3,500 B.C., was a vertical stick that casts a shadow. As the sun crossed the sky, the shadow moved; by measuring its movement, the ancients could also measure the passage of time.

Other early timekeeping methods included burning a notched candle or a knotted rope, the hour glass (still in use in some kitchens as an egg-timer) and the water clock, which measures time by how long it takes water to flow from place to place.

The first mechanical clocks appeared in the 14th century in church bell towers. (“Clock” originally meant “bell.” Fortunately, the term soon replaced “horologium,” or “hour teller.”) These huge, innacurate clocks had only one hand, which indicated the nearest quarter hour.

They were powered by falling weights, whose descent was controlled by an “escapement,” a device which allows only one tooth of a gear at a time to escape–and literally makes the clock tick.

In the late 1500s Galileo noticed that the time it takes a pendulum to complete a swing remains constant. Fifty years later, Dutch physicist Christiaan Huygens showed how a pendulum could be used to regulate a clock; English physicist Robert Hooke invented a new escapement which made it possible. Mechanical wall and grandfather clocks still use it.

As early as 1450 in Italy, someone got the idea to power clocks with coiled springs instead of falling weights. By the 17th century, craftsmen were making pocket watches with minute and second hands and protective crystals. Jeweled bearings (to reduce friction) appeared in the 18th century.

In the early 1900s, American Henry E. Warren invented the electric clock, in which a small motor runs in unison with the power-station generator, which delivers an alternating current at precisely 60 cycles per second (because Warren prevailed upon the generating companies to make it so).

In the quartz clock, invented in 1929, a ring of quartz connected to an electrical current vibrates at 100,000 cycles per second. This vibration is converted to an alternating current, which drives an electric motor. The best quartz clocks are accurate to within a second over 10 years.

Electric watches appeared in 1957, followed by electronic watches, which substituted tuning forks for escapements. In the ’60s, glowing red LED (light emitting diode) digital watches appeared; in the ’70s, LEDs lost favor to LCDs (liquid crystal displays).

For some scientific measurements, however, even quartz clocks aren’t accurate enough. Enter the atomic clock, developed in England in 1955.

The interval of time we know as a second has been defined, since 1972, in atomic terms: as the time it takes an atom of Cesium 122 to tick through 9,192,631,770 resonant cycles after passing through an electromagnetic field.

The latest version of the atomic clock, the cesium fountain atomic clock, is accurate to within on second every 20 million years. The U.S. showed off its version, NIST F-1, last week. Used to calibrate clocks across the country, it’s a cylindrical vacuum chamber about one and a half metres tall. A gas of cesium atoms is released inside the chamber. Six infrared lasers placed at right angles press the atoms into a ball, which slows their movement, dropping their temperature close to absolute zero. Then two vertical lasers push the ball up through a cavity filled with microwaves. The lasers are shut off, and gravity pulls the ball back through the cavity. The microwaves cause some of the atoms to fluoresce–emit light–when they return to their starting point.

This procedure is repeated while the microwave frequency is tuned until it makes most of the cesium atoms fluoresce. That frequency is the natural resonance frequency of the cesium atom–the definition of a second.

You won’t be wearing a wrist-sized atomic clock anytime soon, but technological advances continue to be made in everyday clocks, too. Today, mainsprings are made from high-tech materials that resist breakage and rust, and synthetic jewels have replaced precious stones. New sources of power include sunlight and body heat. We’ve even developed alarm watches that go off during movies and funerals.

Watches are so ubiquitous today they’ve become fashion statements. Once upon a time we couldn’t tell time at all; today, we wear high-tech timepieces so artistically designed that, when we look at them…we still can’t tell what time it is.

That’s progress!

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