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 Introduction
 Mechanical watches
 Electric-powered and electronic watches

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watch
timekeeping device
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Also known as: timepiece
Written by

Jonathan D. Betts
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Last Updated: Dec 14, 2023 • Article History

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Category: Science & Tech
Key People:
 Thomas Tompion

Richard W. Sears

Aaron Lufkin Dennison

Thomas Mudge

Edward Howard
(Show more)
Related Topics:

self-winding watch

electronic watch

pin-pallet watch

wristwatch

galvanometer drive
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Typical components of a mechanical watch.

Watch, portable timepiece that has a movement driven either by spring or by electricity
and that is designed to be worn or carried in the pocket.
Mechanical watches
The first watches appeared shortly after 1500, early examples being made by Peter
Henlein, a locksmith in Nürnberg, Ger. The escapement used in the early watches was
the same as that used in the early clocks, the verge. Early watches were made notably in
Germany and at Blois in France, among other countries, and were generally carried in
the hand or worn on a chain around the neck. They usually had only one hand for the
hours.

The mainspring, the element that drives the watch, consists of a flat spring-steel band
stressed in bending or coiling; when the watch, or other spring-driven mechanism,
is wound, the curvature of the spring is increased, and energy is thus stored. This energy
is transmitted to the oscillating section of the watch (called the balance) by the
wheeltrain and escapement, the motion of the balance itself controlling the release of
the escapement and consequently the timing of the watch. A friction drive permits the
hand to be set.

One of the main defects of the early watches was the variation in the torque exerted by
the mainspring; that is, the force of the mainspring was greater when fully wound than
when it was almost run down. Since the timekeeping of a watch fitted with a verge
escapement was greatly influenced by the force driving it, this problem was quite
serious. Solution of the problem was advanced almost as soon as the mainspring was
invented (about 1450) by the application of the fusee, a cone-shaped,
grooved pulley used together with a barrel containing the mainspring. With this
arrangement, the mainspring was made to rotate a barrel in which it was housed; a
length of catgut, later replaced by a chain, was wound on it, the other end being coiled
around the fusee. When the mainspring was fully wound, the gut or chain pulled on the
smallest radius of the cone-shaped fusee; as the mainspring ran down, the leverage was
progressively increased as the gut or chain pulled on a larger radius. With correct
proportioning of mainspring and fusee radii, an almost constant torque was maintained
as the mainspring unwound.

The going barrel, in which the mainspring barrel drives the wheeltrain directly, is fitted
to all modern mechanical watches and has superseded the fusee. With better quality
mainsprings, torque variations have been reduced to a minimum, and with a properly
adjusted balance and balance spring, good timekeeping is ensured.

Up to about 1580, the mechanisms of German watches were made almost wholly of iron;
about this time, brass was introduced.

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In the earliest watches a plain wheel, known as the balance, was used to control the rate
of going of the mechanism. It was subjected to no consistent restoring force;
consequently, its period of oscillation and, hence, the rate of the timekeeper were
dependent on the driving force. This explains the great importance of the fusee.
Controlling the oscillations of a balance with a spring was an important step in the
history of timekeeping. English physicist Robert Hooke designed a watch with a balance
spring in the late 1650s; there appears to be no evidence, however, that the spring was in
the form of a spiral, a crucial element that would become widely employed. Dutch
scientist Christiaan Huygens was probably the first to design (1674–75) a watch with a
spiral balance spring. The balance spring is a delicate ribbon of steel or other suitable
spring material, generally wound into a spiral form. The inner end is pinned into a collet
(a small collar), which fits friction-tight on the balance staff, while the outer end is held
in a stud fixed to the movement. This spring acts on the balance as gravity does on
the pendulum. If the balance is displaced to one side, the spring is wound and energy
stored in it; this energy is then restored to the balance, causing it to swing nearly the
same distance to the other side if the balance is released.

If there were no frictional losses (e.g., air friction, internal friction in the spring
material, and friction at the pivots), the balance would swing precisely the same distance
to the other side and continue to oscillate indefinitely; because of these losses, however,
the oscillations in practice die away. It is the energy stored in the mainspring and fed to
the balance through the wheel train and escapement that maintains the oscillations.

The performance of the modern watch depends on the uniformity of the period of
oscillation of the balance—i.e., the regularity of its movement. The balance takes the
form of a wheel with a heavy rim, while the spring coupled to it provides the restoring
torque. The balance possesses inertia, dependent on its mass and configuration. The
spring should ideally provide a restoring force directly proportional to
the displacement from its unstressed or zero position.

The balance is mounted on a staff with pivots, and, in watches of good quality, these run
in jewels. Two jewels are used at each end of the balance staff, one pierced to provide
a bearing, the other a flat end stone providing axial location by bearing against the
domed end of the pivot. Frictional effects at the pivots influence the performance of the
watch in various positions—for example, lying and hanging.

The balance and spring can be brought to time, or “regulated,” by varying either the
restoring couple provided by the spring or the moment of inertia of the balance. In the
first case (by far the more common), this is generally effected by providing a pair
of curb pins mounted on a movable regulator index that lengthen or shorten the balance
spring as needed.

In the second instance, screws are provided at opposite points on the rim of the balance;
these screws are friction-tight in their holes and thus can be moved in or out so as to
adjust the inertia of the balance. In “free-sprung” watches no regulator index is
provided, and the only adjusters are the screws on the balance rim.

Many modern mechanical watches use a lever escapement, invented in England about
1755 by Thomas Mudge, that leaves the balance free to oscillate, coupling to it only while
delivering the impulse, taken from the mainspring via the wheel train and while being
unlocked by the balance. It was developed into its modern form with the club-tooth
escape wheel at the beginning of the 19th century but was not universally adopted until
the early 20th century. In good-quality watches the club-tooth escape wheel is made of
hardened steel, with the acting surfaces ground and polished. An improved form of the
lever escapement is characterized by a double-roller safety action in which the
intersection between the guard pin and roller, which takes place underneath the roller,
is much deeper than in early single-roller watches; thus, any friction caused by jolts
encountered in wear causes less constraint on the balance and less endangerment of the
timekeeping properties of the watch. By far the most important watch escapement today
is the lever escapement; it is used in its jeweled form in watches of moderate to excellent
quality, and it is used with steel pallet pins and a simplified fork-and-roller action in
cheaper watches (known as pin-pallet watches).

In the wheel train of a modern watch, it is necessary to achieve a step-up ratio of

approximately 1 to 4,000 between barrel and escape wheel. This involves four pairs of
gears, the ratio per pair commonly being between 6 to 1 and 10 to 1. Because of space
considerations, the pinions must have a low number of leaves (teeth), commonly 6 to 12.
This entails a number of special gearing problems, aggravated by the fineness of the
pitch. Any error in centre distance, form, or concentricity is therefore proportionately
more important than in larger gear trains.
Typical components of a quartz watch.
The first patent covering the application of jewels in watches was taken out in London in
1704; diamonds and sapphires were used. Synthetic jewels made from fused powdered
alumina (aluminum oxide) are now commonly used. Watch jewels are given a very high
polish; a uniform outside diameter for the jewel bearings is highly important, because
they are pressed into accurately sized holes smaller than the jewels themselves and held
there by friction.

The first patent on the self-winding pocket watch was taken out in London in 1780. An
English invention patented in 1924, the self-winding wristwatch by Louis Recordon,
contains a swinging weight pivoted at the centre of the movement, coupled to the barrel
arbor through reduction wheels and gears. A more modern self-winding watch is fitted
with a weight or rotor swinging 360 degrees and winding in both directions.

Electric-powered and electronic watches

Typical components of an electrically powered watch.

Electric-powered watches use one of three drive systems: (1) the galvanometer drive,
consisting of the conventional balance-hairspring oscillator, kept in motion by
the magnetic interaction of a coil and a permanent magnet, (2) the induction drive, in
which an electromagnet attracts a balance containing soft magnetic material, or (3)
the resonance drive, in which a tiny tuning fork (about 25 mm [1 inch] in length), driven
electrically, provides the motive power. Both galvanometer and induction drive types
use a mechanical contact, actuated by the balance motion, to provide properly timed
electric-drive pulses. Each oscillation of the balance operates a time-
indicating gear train by advancing a toothed wheel one tooth. First produced in 1953,
the resonance drive type, properly called an electronic watch, is inherently more
accurate since it operates at a frequency higher than that customarily used with balance-
type watches, and the tuning fork is a fairly stable source of frequency. The higher
frequency requires the replacement of a mechanical contact by a transistor. The minute
and rapid motion of the tuning fork moves forward an extremely fine-
toothed ratchet wheel. There is very little friction in the electronic watch; only tiny
amounts of oil are needed. When the battery is too weak to operate the tuning fork, the
watch simply stops, without deterioration. Miniature high-energy-density batteries are
used as power sources in all three types.

The progressive miniaturization of electronic components in the late 20th century made
possible the development of all-electronic watches, in which the necessary transistors,
resistors, capacitors, and other elements were all on one or several miniature integrated
circuits, or chips. The complex circuitry of such watches enabled them to perform a
variety of timekeeping functions and also made possible digital readouts of the time in
place of the traditional second, minute, and hour hands.
Jonathan D. Betts