Clocks and wristwatches

When we think of a clock, we picture the familiar face with numbers, two hands and maybe a sweeping second hand, but that picture is too narrow. Over the centuries, people have developed all manner of machinery to tell time. The Chinese invented the incense clock between 960 and 1279, and its use spread throughout eastern Asia. In one type of incense clock, metal balls were attached to the incense with string. When the incense burned up, the ball would drop, sounding a gong that announced the hour.

Other clocks used color, and some used different scents at different times. Candle clocks had numbered markings down their lengths. When the candle burned down to a mark, the dial noted the time. Sometimes the lines were not numbered, and the person using the candle clock would need to know how long it took the candle to burn down to each mark.

The discovery in the 1400s that coiled springs could move the hands of a clock made smaller timepieces possible. Pocket watches were the order of the day for men for centuries, and wristwatches were considered jewelry - but for women only. All of those fashion rules changed during World War I, when taking out a pocket watch was impossible during battle. For most of the remainder of the 20th century, almost everyone - especially men - wore wristwatches. The gift of a watch symbolized your passage into manhood, and many companies presented you with a gold watch upon your retirement.

As we advance through the 21^st century, the ubiquitous wristwatch may once again be fading from style and use. While at work, we can check the time on the computer's clock, and while we're out, our cell phones or MP3 players display accurate times around the clock. Still, an informal survey by a "Houston Chronicle" reporter showed that many people say they will never give up their wristwatches.

Days, hours and minutes are the same in all countries, but how people view them varies. In some countries - Germany, for instance - punctuality is all-important. Being late for an appointment is the height of rudeness and may earn you a rebuke from your host. In laid-back Brazil, however, time often is ignored, and locals sometimes run hours late for meetings, doctor appointments, even college classes.

Springs or weights operate the gears inside a mechanical clock, but when the quartz clock came along, electricity of a sort began to move the hands. The mineral quartz - usually with the help of a battery - powers a quartz clock. Quartz is piezoelectric, meaning that when a quartz crystal is squeezed, it generates a small current of electricity, which causes the crystal to vibrate when the current passes through. All quartz crystals vibrate at the same frequency. Quartz clocks use a battery to create the crystal vibration and a circuit to count the vibrations. The circuit uses this information to generate one pulse every second. These pulses power the gears in a mechanical clock or power the display in a digital clock. Quartz clocks still dominate the market because of their accuracy and the low production cost.

Although the name sounds rather menacing, atomic clocks are not dangerous at all. They measure time by tracking how long it takes for an atom to switch its energy state from positive to negative and back again. The official time standard for the United States is set by NIST F-1, the cesium atomic clock at the National Institute of Science and Technology (NIST) in Boulder, Colo. NIST F-1 is a fountain clock, named for the movement of atoms. Scientists introduce cesium gas into the clock's vacuum chamber and then direct infrared laser beams at 90-degree angles to the center of the chamber. The lasers force the atoms together into a ball, which gets tossed through a microwave-filled area. The scientists measure the number of atoms with altered states and tune the microwaves to different frequencies until most of the atoms are altered. This final frequency is the natural resonance frequency for cesium atoms and constitutes the number of oscillations that define a second. It sounds complicated, but it results in a worldwide standard for seconds; clocks around the world can automatically set to the NIST standard by time zone. The atomic clock keeps track of time on the most miniscule level, and calendars help us cope with larger blocks of time.

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