How clocks work
The invention of the mechanical clock in the 13th century started a change in timekeeping methods from continuous processes, such as the motion of the gnomon's shadow on a sundial or the flow of liquid in a water clock, to repetitive oscillatory processes, like the swing of a pendulum or the vibration of a quartz crystal, which WINDOWS REMINDER were more accurate.[16] All modern clocks use oscillation.
Although the methods they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be divided into analogous parts.[17][18][19] They consist of an object that repeats the same motion over and over again, an oscillator, with a precisely constant time interval between each repetition, or 'beat'. Attached WINDOWS REMINDER to the oscillator is a controller device, which sustains the oscillator's motion by replacing the energy it loses to friction, and converts its oscillations into a series of pulses. The pulses are then added up in a chain of some type of counters to express the time in convenient units, usually seconds, minutes, hours, etc. Then finally some WINDOWS REMINDER kind of indicator displays the result in a human-readable form.
[edit] Power source
This provides power to keep the clock going.
* In mechanical clocks, this is either a weight on a cord suspended from a pulley, or a mainspring.
* In electric clocks, it is either a battery or the AC power line.
Since WINDOWS REMINDER clocks must run continuously, there is often a small secondary power source to keep the clock going temporarily during interruptions in the main power. In old mechanical clocks, a maintaining power spring provided force to turn the clock's wheels while the mainspring was being wound up.[20] In quartz clocks a backup battery or capacitor is often included to WINDOWS REMINDER keep the clock going if the power cord is unplugged.
[edit] Oscillator
The timekeeping element in every modern clock is a harmonic oscillator, a physical object (resonator) that vibrates or oscillates repetitively at a precisely constant frequency.[21]
* In mechanical clocks, this is either a pendulum or a balance wheel.
* In some early WINDOWS REMINDER electronic clocks and watches such as the Accutron, it is a tuning fork.
* In quartz clocks and watches, it is a quartz crystal.
* In atomic clocks, it is the vibration of electrons in atoms as they emit microwaves.
* In early mechanical clocks before 1657, it was WINDOWS REMINDER a crude balance wheel or foliot which was not a harmonic oscillator because it lacked a balance spring. As a result they were very inaccurate, with errors of perhaps an hour a day.[22]
The advantage of a harmonic oscillator over other forms of oscillator is that it employs resonance to vibrate at a precise natural resonant frequency or 'beat' WINDOWS REMINDER dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its Q,[23][24] or quality factor, which increases (other things being equal) with its resonant frequency.[25] This is why there has been a long term trend toward higher frequency oscillators in clocks.
Some clocks WINDOWS REMINDER rely for their accuracy on an external oscillator; that is, they are automatically synchronized to a more accurate clock:
* Slave clocks, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a master clock in the building, and periodically received a signal to WINDOWS REMINDER synchronize them with the master, often on the hour.[26]
* Synchronous electric clocks don't have an internal oscillator, but rely on the 50 or 60 Hz oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. This drives a synchronous motor in the clock which rotates once for every WINDOWS REMINDER cycle of the line voltage, and drives the gear train.
* Computer real time clocks keep time with a quartz crystal, but are periodically (usually weekly) synchronized over the internet to atomic clocks (UTC), using a system called Network Time Protocol.
* Radio clocks keep time with a quartz crystal, but are periodically (often daily) synchronized to atomic clocks (UTC) with time signals from government radio stations like WWV, WWVB, CHU, DCF77 and the GPS system.
