Eileen goes to Switzerland to find out what makes clocks tick.
Segment Length: 6:45

Eileen's Photo

How do clocks keep time?

What was the world like without clocks?

How did people keep appointments?

 Overview   Connections   Activity   Try This   Key Concepts   Resources 

Getting Started

Without looking at the clock or your watch, look out the window and guess what time it is. How close were you to the "correct" time? What clues did you use to tell what time it was? Are there other things you could use? How do you know it's lunchtime without looking at a clock?

How does a clock measure time? What would happen if a minute were 100 seconds and an hour were 100 minutes? How would that change the way you schedule your day? How long would this class last? What about your school day?


The alarm clock rings in the morning and, even in your drowsy fog, you look to see what time it is. As you get dressed, you check the clock again and again to make sure you're not late for school.

We look at clocks all the time because these devices help us regulate our lives, telling us not only when to get up, but when to eat, sleep, play, and work. They are so much a part of our lives that we rarely think about what clocks really do.

Whether they are highly accurate atomic clocks or slightly less accurate quartz watches, electric alarm clocks or grandfather clocks with slowly swinging pendulums, all clocks have one thing in common - they consistently count precise units of time. Those units could be anything we want them to be, but for the world to function in harmony, we have a timekeeping standard based upon three units of time - seconds, minutes, and hours.

To measure these units, all clocks must have two things: a regular, repetitive resonator, or oscillator, to mark off equal units of time; and a way of displaying those units in an understandable form.

Most clocks and watches today keep time by applying electric energy to a quartz crystal, a system developed in the 1930s. The energy makes the crystal vibrate or oscillate at a constant frequency and produce regular electric pulses that regulate a motor. The motor advances the watch hands or, in a digital watch, the number display, by one-second increments.

Mechanical watches use a coiled mainspring for power. The mainspring drives gears that cause a hairspring to oscillate, rocking a lever to and fro. The lever drives other gears that move the clock hands.

Atomic clocks, the world's most accurate timekeepers, use the natural vibration, or oscillation, of the cesium atom as their resonator. Cesium atoms vibrate exactly 9,192,631,770 times a second, driving a clock that is accurate to within a millionth of a second per year. In ancient times, people used the rising and setting sun to keep track of time. The first devices to measure time, invented in about 3500 B.C., were small towers called obelisks. The changing length and position of their shadows divided the day into morning and afternoon. Then came sundials, which split the day into hours; water clocks, which measured even smaller units of time; mechanical clocks that were much more accurate; and finally, in about 1510, spring-driven clocks that led the way to clocks and watches accurate to within a minute or two a day.


  1. How would your day be different if nobody had clocks or watches? What other ways could you tell time?

  2. Why is it that time seems to pass quickly when you're doing something interesting or fun, and slowly when you're bored?

Main Activity

Waterclock Image Water Clock:

Build a clock that uses dripping water to measure how much time has passed.

For many centuries, the best technology available for keeping time was the water clock. While these clocks weren't very reliable, they worked indoors, at night, and on cloudy days, so they were much more useful than the sundial, the only other clock in use at the time. Over time, many styles of water clocks were invented. Here's an activity that lets you find out just how accurate an "inflow" water clock is.


  1. Screw the eye screws into the 30-cm stick, the first an inch or so above the level of the cans, the other an inch or so below the top of the stick.

  2. Run the thin, round stick through the openings in the eye screws and insert the lower end of the stick into a cork.

  3. Fasten the large stick to the outside of one of the cans with the two rubber bands. Make sure the cork at the bottom of the thin stick doesn't rub against the inside of the can.

  4. Glue a small paper or cardboard pointer to the thin stick so that it points at, but doesn't touch, the large stick.

  5. Use the can opener to make a tiny hole in the side of your second can as close to the bottom as possible. You want the hole small enough so the water only drips out.

  6. Fill the second can with water and set it on a platform so water drips from it into the first can. As the water slowly fills the first can, the cork will rise and push the thin stick and the pointer upward. Mark the starting level for the pointer on the large stick. Then every five minutes, as the water drips in, make another mark across from the rising pointer. At the end of the class period, you will have calibrated your clock.

Now, try it again and see if it remains accurate as it counts off the five-minute segments. There are many different designs for water clocks. Look for ideas on building other types of water clocks or come up with your own design. Compare the accuracy of different designs.


  1. Does the clock run slower or faster if you use very cold water? Why?
  2. Can you build two water clocks that measure time at the same rate? If not, why not?
  3. What disadvantages are there to using this type of clock?

Look at the sweep hand of a clock and note exactly when a minute begins. Without looking at the clock again, try to silently count off 60 seconds in your head. How close did you come? Try again, but this time have someone talk to you and interrupt your concentration.
Build your own sundial with a medium-sized flowerpot filled with sand. Put a 30-cm (12") stick in the center of the pot and set it in the sun. Every hour, mark where the shadow of the stick falls on the edge of the pot with a piece of masking tape. If you don't move the pot, you can keep track of time on sunny days.
Galileo realized that a pendulum oscillates, or swings, back and forth for the same unit of time, even as the arc of each swing decreases. Tie a weight to one end of a string and tie the other end to a stick. Lay the stick down on a table so the string is hanging free, then swing the string like a pendulum. Was Galileo right?

Key Concepts

atomic clock
a very accurate clock that keeps time by measuring the natural oscillations of a cesium atom
the science of measuring time
an upright, four-sided pillar that tapers to a pyramid at its top
to swing back and forth; with electricity, to switch between a high and a low charge
crystal when placed in an electric field, a quartz crystal vibrates and generates a regular electric signal that is a good resonator for running clocks and watches
spring-driven clocks
a tightly wound spring provides the energy for these clocks, but they slow down as the spring unwinds
weight-driven clocks
old mechanical clocks that used the pull of a heavy weight to provide energy to run the clock


Dohrn-van Rossum, G. (1996). History of the hour: Clocks and modern temporal orders. Chicago: University of Chicago Press.

Macaulay, D. (1988). The way things work. Boston: Houghton Mifflin Company.

Suplee, C. (1994, Nov 16). A brief history of time-keeping: How the mechanical clock set a new tempo for society. The Washington Post Horizons learning section, p.H1.

Tait, H. (1983). Clocks and watches - An illustrated history of clocks since the Middle Ages. Cambridge: British Museum Publications/Harvard University Press.

Index of Horology: http://www.horology.com/horology

National Institute of Standards and Technology: http://physics.nist.gov/lab.html
(Click on "General Interest" in main menu, then click on "A walk through time.")

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