Clocks can be classified by the type of time display, as well as by the method of timekeeping.
Time display methods
Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and often also as a scale of 60 seconds – though many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. Of these alternative versions, the 24 hour analog dial is the main type in use today. The 10 hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch chiming 24 times uses more power).
Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon. Sundials use some or part of the 24 hour analog dial.
Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:
* the 24-hour notation with hours ranging 00–23;
* the 12-hour notation with AM/PM indicator, with hours indicated as 12AM, followed by 1AM–11AM, followed by 12PM, followed by 1PM–11PM (a notation mostly used in the United States).
Most digital clocks use an LCD or LED display; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, digital clocks without a backup battery or capacitor either start counting from 00:00, or stay at 00:00, often with blinking digits indicating that time needs to be set. Some newer clocks will actually reset themselves based on internet-time servers which in turn are tuned to national atomic clocks.
For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. The sound is either spoken natural language, (e.g. "The time is twelve thirty-five"), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour like the clock Big Ben).
Most types of clocks are built around some form of oscillator, an arrangement that goes through an endless sequence of periodic state changes, designed to provide a continuous and stable reference frequency. The periods of this oscillator are then counted and converted into the desired clock display.
* Mechanical clocks use a pendulum as their oscillator, which controls the rotation of a system of gears that drive the clock display.
* Electrical clocks use electrical current to run, rather than requiring manual winding and weights.
* Crystal clocks use an electronic quartz crystal oscillator and a frequency divider or counter. Most battery-powered crystal clocks use a 215 Hz = 32.768 kHz oscillator.
* Atomic clocks use a microwave oscillator (maser) tuned by the energy transitions of elements such as caesium, rubidium or hydrogen. These are the most precise clocks available. Atomic clocks based on caesium are used as the official definition of time today.
* Mains power clocks count the 50 or 60 hertz periods of their AC power.
* Radio clocks receive time signal broadcasts from a radio transmitter (which may be hundreds of kilometres away). The clock can decode the transmission and adjust its hands or display for perfect accuracy. The broadcast radio signals received are generated by an atomic clock. These clocks are used extensively by mariners, especially short-wave radio clocks which use simultaneous bursts of time-signals, often encoded or encrypted – not to be confused with number stations.
* Sundials observe the apparent rotation of the Sun around the Earth as their reference oscillation. They are observed with a solar tempometer.
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