Cesium Atomic Clocks Kinds of cesium clocks, with pictures, plus a description of their operation and the science behind them. http://tycho.usno.navy.mil/cesium.html
How Stuff Works: Pendulum Clock Concise description using drawings and animation to show the operation of a pendulum clock. http://www.howstuffworks.com/clock.htm
Smithsonian Institute - On Time History of the clock and how Americans have measured time and been affected by it. http://americanhistory.si.edu/ontime/
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