A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, year and galactic year. Devices operating on several physical processes have been used over the millennia.
Some predecessors to the modern clock may be considered as “clocks” that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, a well-known example being the hourglass.
Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels.
Traditionally, in horology, the term clock was used for a striking clock, while a clock that did not strike the hours audibly was called a timepiece. This distinction is no longer made. Watches and other timepieces that can be carried on one’s person are usually not referred to as clocks. Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished.
Below, Pritish Kumar discusses different time measuring devices
The apparent position of the Sun in the sky moves over the course of each day, reflecting the rotation of the Earth. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a (usually) flat surface, which has markings that correspond to the hours. Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in ancient times.
Devices that measure duration, elapsed time and intervals
Many devices can be used to mark the passage of time without respect to reference time (time of day, hours, minutes, etc.) and can be useful for measuring duration or intervals. Examples of such duration timers are candle clocks, incense clocks and the hourglass.
Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less constant allowing reasonably precise and repeatable estimates of time passages. In the hourglass, fine sand pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined passage of time. The resource is not consumed but re-used.
The next development in accuracy occurred after 1656 with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to regulate the motion of a time-telling device earlier in the 17th century. Christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (about 99.4 cm or 39.1 inches for the one second movement) and had the first pendulum-driven clock made.
In mechanical clocks, the power source is typically either a weight suspended from a cord or chain wrapped around a pulley, sprocket or drum; or a spiral spring called a mainspring. Mechanical clocks must be wound periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store energy in the weight or spring to keep the clock running.
In electric clocks, the power source is either a battery or the AC power line. In clocks that use AC power, a small backup battery is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage. Battery-powered analog wall clocks are available that operate over 15 years between battery changes.
Balance wheel, the oscillator in a mechanical mantel clock.
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.
1)In mechanical clocks, this is either a pendulum or a balance wheel.
2)In some early electronic clocks and watches such as the Acutrim, it is a tuning fork.
3)In quartz clocks and watches, it is a quartz crystal.
4)In atomic clocks, it is the vibration of electrons in atoms as they emit microwaves.
5)In early mechanical clocks before 1657, it was 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.
Synchronized or slave clocks
1)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 synchronize them with the master, often on the hour. Later versions without pendulums were triggered by a pulse from the master clock and certain sequences used to force rapid synchronization following a power failure.
2)Synchronous electric clocks do not have an internal oscillator, but count cycles of the 50 or 60 Hz oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. The counting may be done electronically, usually in clocks with digital displays, or, in analog clocks, the AC may drive a synchronous motor which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train.
Although changes in the grid line frequency due to load variations may cause the clock to temporarily gain or lose several seconds during the course of a day, the total number of cycles per 24 hours is maintained extremely accurately by the utility company, so that the clock keeps time accurately over long periods.
3)Computer real time clocks keep time with a quartz crystal, but can be periodically (usually weekly) synchronized over the Internet to atomic clocks (UTC), using the Network Time Protocol (NTP).
4)Radio clocks keep time with a quartz crystal, but are periodically synchronized to time signals transmitted from dedicated standard time radio stations or satellite navigation signals, which are set by atomic clocks.
This has the dual function of keeping the oscillator running by giving it ‘pushes’ to replace the energy lost to friction, and converting its vibrations into a series of pulses that serve to measure the time.
1)In mechanical clocks, this is the escapement, which gives precise pushes to the swinging pendulum or balance wheel, and releases one gear tooth of the escape wheel at each swing, allowing all the clock’s wheels to move forward a fixed amount with each swing.
2)In electronic clocks this is an electronic oscillator circuit that gives the vibrating quartz crystal or tuning fork tiny ‘pushes’, and generates a series of electrical pulses, one for each vibration of the crystal, which is called the clock signal.
3)In atomic clocks the controller is an evacuated microwave cavity attached to a microwave oscillator controlled by a microprocessor. A thin gas of cesium atoms is released into the cavity where they are exposed to microwaves. A laser measures how many atoms have absorbed the microwaves, and an electronic feedback control system called a phase-locked loop tunes the microwave oscillator until it is at the frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by digital counters to become the clock signal.