Watches,  small portable timepieces usually designed to be worn on the person. Other kinds of timepieces are generally referred to as clocks. At one time it was generally believed that the first watches were made in Nuremburg, Germany, c.1500. However, there is now evidence that watches may have appeared at an earlier date in Italy. Early watches were ornate, very heavy, and made in a variety of shapes, e.g., pears, skulls, and crosses; the faces were protected by metal latticework. Watch parts were made by hand until c.1850, when machine methods were introduced by watch manufacturers in the United States.  The introduction of machine-made parts not only cut manufacturing costs but increased precision and facilitated repairs. 

 In the 15th century, navigation and mapping increased the desire for portability in timekeeping. The latitude could be measured by looking at the stars, but the only way a ship could measure its longitude was by comparing the midday (high noon) time of the local longitude to that of a European meridian (usually Paris or Greenwich)—a time kept on a shipboard clock. However, the process was notoriously unreliable until the introduction of John Harrison´s marine chronometer. For that reason, most maps from the 15th century through the 19th century have precise latitudes but distorted longitudes.

The first reasonably accurate mechanical clocks measured time with simple weighted pendulums, which are unworkable when irregular movement of the fulcrum occur whether at sea or in watches. The invention of a spring mechanism was crucial for portable clocks.  In Tudor England, the development of "pocket-clockes" was enabled by the development of reliable springs and escapement mechanisms, which allowed clockmakers to compress a timekeeping device into a small, portable compartment.  In 1524, Peter Henlein created the first pocket watch. It is rumored that Henry VIII had a pocket clock which he kept on a chain around his neck.

However, these watches only had an hour hand—a minute hand would have been useless because of the inaccuracy of the watch mechanism. Eventually, miniaturization of these spring-based designs allowed for accurate portable timepieces (marine chronometers) which worked well even at sea.  In 1850, Aaron Lufkin Dennison founded Waltham Watch Company, which was the pioneer of the industrial manufacturing of pocket watches with interchangeable parts, the American System of Watch Manufacturing. Breguet developed the first self-winding watch known as the perpetual in 1780.

Parts

The first two are key mechanisms within any mechanical watch of classical design; the third is optional:

1. The escapement – a mechanism that controls and limits the unwinding of the watch, converting what would otherwise be a simple unwinding, into a controlled and periodic energy release. The escapement does this by interlocking with a gear in a simple manner that switches between a "driven" and a "free" state, with abrupt locking at each end of the cycle. The escapement also for the same reason produces the ticking noise characteristic of mechanical watches.


2. The balance wheel together with the balance spring (also known as Hairspring) – these form a simple harmonic oscillator, which controls the motion of the gear system of the watch in a manner analogous to the pendulum of a pendulum clock. This is possible because the moment of inertia of the balance wheel is fixed, and the wheel as a whole provides a regular motion of known period.


3. The tourbillon – a rotating frame for the escapement. It is intended to cancel out or reduce the effects of bias to the timekeeping of gravitational origin, which might result from the watch being kept in a particular position for much of the day. It is technically very challenging to create a high quality tourbillon, and those made by specialists and found in prestige watches are often very highly valued.

Watch movements

A movement in watchmaking is the mechanism that measures the passage of time and displays the current time (and possibly other information including date, month and day). Movements may be entirely mechanical, entirely electronic (potentially with no moving parts), or a blend of the two. Most watches intended mainly for timekeeping today have electronic movements, with mechanical hands on the face of the watch indicating the time.

Mechanical movements

 
Mechanical Omega Pocket Watch.

Purely mechanical watches are still popular, although they are most commonly seen among medium priced watches such as Fortis, Omega, Rolex and TAG Heuer and expensive watches like Patek Phillipe, Vacheron Constantin, Ulysse Nardin, Audemars Piguet and Rolex. Their superb craftsmanship accounts for much of the attraction of purely mechanical watches. Compared to electronic movements, mechanical watches keep less accurate time, often with errors of seconds per day. They are frequently sensitive to position and temperature, they are costly to produce, they require regular maintenance and adjustment. These finely crafted mechanical watches have remained popular as precision time pieces and in many cases more so because of their aesthetic value as jewelry.

Tuning-fork movements

Tuning fork watches (introduced by Bulova in 1960) use a tuning fork at a precise frequency (most often 360 hertz) to drive a mechanical watch. Since the fork is used in place of a typical balance wheel, these watches naturally hum instead of tick. The inventor, Max Hetzel, was born in Basel, Switzerland, and joined the Bulova Watch Company of Bienne, Switzerland, in 1948.  Hetzel was the first to use an electronic device, a transistor, in a wristwatch. Thus, he developed the first watch that could be qualified as electronic. However, fork movements are actually more "electrical", like an old electrical wall clock, than electronic. The sweep second hand moves fluidly like that of an old electrical wall clock. Tuning fork movements are electromechanical. The task of converting electronically pulsed fork vibration into rotary movement is done via two tiny jeweled fingers, called pawls, one of which is connected to one of the tuning fork´s tines. As the fork vibrates, the pawls precisely ratchet a tiny index wheel. This index wheel has over 300 barely visible teeth and spins more than 38 million times per year. The tiny electric coils that drive the tuning fork have 8000 turns of insulated copper wire with a diameter of 0.015 mm and a length of 90 meters. This amazing feat of engineering was prototyped in the 1950s.

Electronic movements ~ Quartz

Electronic movements have few or no moving parts. Essentially, all modern electronic movements use the piezoelectric effect in a tiny quartz crystal to provide a stable time base for a mostly electronic movement: the crystal forms a quartz oscillator which resonates at a specific and highly stable frequency, and which can be used to accurately pace a timekeeping mechanism. For this reason, electronic watches are often called quartz watches. Most quartz movements are primarily electronic but are geared to drive mechanical hands on the face of the watch in order to provide a traditional analog display of the time, which is still preferred by most consumers. The best quartz movements are significantly more accurate than the worst, but the difference is much smaller than that found between mechanical movements and quartz movements. Quartz movements, even in their most inexpensive forms, are an order of magnitude more accurate than purely mechanical movements. Whereas mechanical movements can typically be off by several seconds a day, an inexpensive quartz movement in a child´s wristwatch may still be accurate to within 500 milliseconds per day—ten times better than a mechanical movement. Recently efforts have been made to combine the best features of quartz and mechanical movements. For example, the Seiko Spring Drive, introduced in 2005, uses a mainspring to power both a mechanical movement and, via a generator, a quartz regulator that controls it´s speed. The result is claimed to be a timepiece that operates as a mechanical watch, but with quartz accuracy.

Power sources

Springs

Traditional, purely mechanical watch movements generally use a wound spring as a power source. The spring must be rewound by the user periodically (usually once a day, or once every few days).

Self-winding watches

 Automatic watches have an eccentric weight, called a rotor, that swings with the movement of the wearer´s body and keeps the spring wound. A self-winding mechanism is one that rewinds the mainspring (power spring) of a mechanical movement through some means other than explicit winding by the user. The first self-winding mechanism was invented in 1770 by Abraham-Louis Breguet,  but the first "self-winding," or "automatic," wristwatch was the invention of a British watch repairer named John Harwood in 1923. This type of watch allows for a constant winding without special action from the wearer: it works by an eccentric weight, called a winding rotor, that rotates to the movement of the wearer´s body. The back-and-forth motion of the winding rotor couples to a ratchet to automatically wind the watch. The spring drives an escapement, which consists of a lever that moves back and forth against a gear, keeping the gear moving at a specific number of times per second, usually four or five. That gear, in turn, drives all of the other gears of the watch that turn the hands on the dial.

Kinetic power

Some watches are powered by the movement of the wearer of the watch. Kinetic powered quartz watches make use of the motion of the wearer´s arm turning a rotating weight, which in turn, turns a generator to supply power. The concept is similar to that of self-winding spring movements, except that electrical power is generated instead of mechanical motion alone.

Batteries

Electronic watches require electricity as a power source. Some mechanical movements and hybrid electronic-mechanical movements also require electricity. Usually the electricity is provided by a replaceable battery. The first use of electrical power in watches was as substitute for the mainspring, in order to remove the need for winding. The first electrically-powered watch, the Hamilton Electric 500, was released in 1957 by the Hamilton Watch Company of Lancaster, Pennsylvania. Batteries  for watches are specially designed for their purpose. They are very small and provide tiny amounts of power continuously for very long periods (several years or more). In most cases, replacing the battery requires a trip to a watch-repair shop or watch dealer; this is especially true for watches that are designed to be water-resistant, as special tools and procedures are required to ensure that the watch remains water-resistant after battery replacement. Silver-oxide and lithium batteries are popular today; mercury batteries, formerly quite common, are no longer used, for environmental reasons.

Light-powered watches

Some electronic watches are powered by light. A photovoltaic cell on the face (dial) of the watch converts light to electricity, which in turn is used to charge a rechargeable battery or capacitor. The movement of the watch draws its power from the rechargeable battery or capacitor. As long as the watch is regularly exposed to fairly strong light (such as sunlight), it never needs battery replacement, and some models need only a few minutes of sunlight to provide weeks of energy (as in the Citizen Eco-Drive).  Some of the early solar watches of the 1970s had innovative and unique designs to accommodate the array of solar cells needed to power them (Synchronar, Nepro, Sicura and some models by Cristalonic, Alba, Seiko and Citizen). As the decades progressed and the efficiency of the solar cells increased while the power requirements of the movement and display decreased, solar watches began to be designed to look like other conventional watches.

Thermal power

A seldom used power source is the temperature difference between the wearer´s arm and the surrounding environment (as applied in the Citizen Eco-Drive Thermo).

Displaying the time

There are two main ways in which watches display the time: analog and digital.

Analog display

Traditionally, watches have displayed the time in analog form, with a numbered dial upon which are mounted at least a rotating hour hand and a longer, rotating minute hand. Many watches also incorporate a third hand that shows the current second of the current minute. Watches powered by quartz have second hands that snap every second to the next marker. Watches powered by a mechanical movement have a "sweep second hand", the name deriving from its uninterrupted smooth (sweeping) movement across the markers, although this is actually a misnomer; the hand merely moves in smaller steps, typically 1/6 of a second, corresponding to the beat of the balance wheel. All of the hands are normally mechanical, physically rotating on the dial, although a few watches have been produced with “hands” that are simulated by a liquid-crystal display. Analog display of the time is nearly universal in watches sold as jewelry or collectibles, and in these watches, the range of different styles of hands, numbers, and other aspects of the analog dial can be very elaborate and detailed with enamel, diamonds or gems.

Digital display

Since the advent of electronic watches that incorporate small computers, digital displays have also been available. A digital display simply shows the time as a number, e.g., 12:40 AM instead of a short hand pointing towards the number 12 and a long hand pointing towards the number 8 on a dial. The first digital watch, a Pulsar prototype in 1970, was developed jointly by Hamilton Watch Company and Electro-Data. John Bergey, the head of Hamilton´s Pulsar division, said that he was inspired to make a digital timepiece by the then-futuristic digital clock that Hamilton themselves made for the 1968 science fiction film 2001: A Space Odyssey. On April 4, 1972 the Pulsar was finally ready, made in 18-carat gold and sold for $2,100 at retail. It had a red light-emitting diode (LED) display.

Another early digital watch innovator, Roger Riehl´s Synchronar Mark 1, provided an LED display and used solar cells to power the internal nicad batteries.  Most watches with LED displays required that the user press a button to see the time displayed for a few seconds, because LEDs used so much power that they could not be kept operating continuously.

Watches with LED displays were popular for the next few years, but soon the LED displays were superseded by liquid crystal displays (LCDs), which used less battery power. Digital watches were very expensive and out of reach to the common consumer up until 1975, when Texas Instruments started to mass produce LED watches inside a plastic case. From the 1980s onward, technology in digital watches vastly improved. In 1982 Seiko produced a watch with a small TV screen built in and Casio produced a digital watch with a thermometer and another watch that could translate 1,500 Japanese words into English. In 1985, Casio produced the CFX-400 scientific calculator watch. In 1987 Casio produced a watch that could dial your telephone number and Citizen revealed one that would react to your voice. In 1995 Timex release a watch which allowed the wearer to download and store data from a computer to their wrist.

Since their apex during the late 1980´s to mid 1990´s high technology fad, digital watches have mostly devolved into a simpler, less expensive basic time piece with little variety between models. Despite these many advances, almost all watches with digital displays today are not considered an impressive form of jewelry and most are seen as being in the category of simple timekeeping watches. Expensive watches for collectors rarely have digital displays since there is little demand for them. Less craftsmanship is required to make a digital watch face and most collectors find that analog dials (especially with complications) vary in quality more than digital dials due to the details and finishing of the parts that make up the dial (thus making the differences between a cheap and expensive watch more evident).

Watch functions

All watches provide the time of day, giving at least the hour and minute, and usually the second. Most also provide the current date, and often the day of the week as well. However, many watches also provide a great deal of information beyond the basics of time and date. Some watches include alarms. Other elaborated and more expensive watches, both pocket and wrist models, also incorporate striking mechanisms or repeater functions, so that the wearer could learn the time by the sound emanating from the watch. This announcement or striking feature is an essential characteristic of true clocks and distinguishes such watches from ordinary timepieces.

Complicated watches

A complicated watch has one or more functionalities beyond the basic function of displaying the time and the date; such a functionality is called a complication. Two popular complications are the chronograph complication, which is the ability of the watch movement to function as a stopwatch, and the moonphase complication, which is a display of the lunar phase. Other more expensive complications include, Tourbillion, Perpetual calendar, Minute repeater and Equation of time. A truly complicated watch has many of these complications at once. Among watch enthusiasts, complicated watches are especially collectible.

Chronographs and chronometers

The similar-sounding terms chronograph and chronometer are often confused, although they mean altogether different things. A chronograph is a type of complication, as explained above. A chronometer watch is an all-mechanical watch or clock whose movement has been tested and certified to operate within a certain standard of accuracy by the COSC (Contrôle Officiel Suisse des Chronomètres). The concepts are different but not mutually exclusive; a watch can be a chronograph, a chronometer, both, or neither.

Water resistance

Watches may be crafted to become water resistant. These watches are sometimes called diving watches. The International Organization for Standardization issued a standard for water resistant watches which also prohibits the term waterproof to be used with watches, which many countries have adopted. Water resistance is achieved by the gaskets which form a watertight seal, used in conjunction with a sealant applied on the case to help keep water out. The material of the case must also be tested in order to pass as water resistant.  The watches are tested in still water, thus a watch with a 50 meter rating will be water resistant if it is stationary and under 50 meters of still water. For normal use, the ratings must then be translated from the pressure the watch can withstand to take into account the extra pressure generated by motion. Watches are classified by its degree of water resistance, which roughly translates to the following:

• Water resistant - Will tolerate splashes of water or rain


• 50 meter - Usable while showering, bathing, dishwashing, and swimming in shallow water


• 100 meter - Usable while swimming, and snorkeling


• 150 meter - Usable during general water sports


• 200 meter - Usable during general water sports, including free diving
  
• Diver´s 150 meter - ISO standard for scuba diving

Some watches use bar instead of meters, which may then be multiplied by 10 to be approximately equal to the rating based on meters. Therefore, a 10 bar watch is equivalent to a 100 meter watch. Some watches are rated in atmospheres (atm), which are roughly equivalent to bar.