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Clocks require a relatively constant motion by which to measure time. The earliest clocks were sundials, which display a shadow as the Sun seems to cross the sky. Other clocks could work without sun by using gravity to provide a reasonably constant motion : these include sand hourglasses, sand clocks, and water clocks.

Later mechanical clocks use a pendulum or wound spring to turn gears in what is called an escapement. This is a way of accumulating the repeated constant motion and showing it as a visual display, such as a clock face.

Electric watches use either a regulated motor or a crystal vibration (a constant repetition). Some clocks used scintillation counters to measure the emissions of decay particles from an unstable radioactive element. The actual "atomic clocks" measure the oscillation frequency of excited gas molecules, and are accurate to tiny fractions of a second.

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Building clocks:

Sundial

You can construct a sundial by using a triangular "gnomon" (upright part) whose shadow crosses a measured bar or circle.

Sand Clock

A sand clock is not always as accurate, but can also be built. A large quantity of fine white sand is stored in a funnel-shaped container and trickled through a small hole into another container. Build a free-spinning wheel with equally spaced flat spokes that project from it (such as playing cards), so that the spokes are spun by the falling sand stream. The spinning motion can be shown as a moving hand on a large dial face, and calibrated to reflect the passage of a certain amount of time.

(see also the related link)

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7y ago

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