clock: Electric and Other Clocks
Electric clocks were made in the second half of the 19th cent. but were not used extensively in homes until after c.1930. In an analog clock the hands of an electric clock are driven by a synchronous electric motor supplied with alternating current of a stable frequency. Digital clocks use LCDs (liquid crystal displays) or LEDs (light emitting diodes) to form the numbers indicating the time. The quartz clock, invented c.1929, uses the vibrations of a quartz crystal to drive a synchronous motor at a very precise rate. Some quartz clocks have an error of less than one thousandth of a second per day. The atomic clock , which is based upon the frequency of an atomic or molecular process, is even more precise; a state of the art atomic clock, such as NIST-F2 (which is one of two U.S. time frequency standard clocks), is accurate to one second in 300 million years.
- The Evolution of Mechanical Clocks
- Some Famous Clocks
- Electric and Other Clocks
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