Introduction
A clock is a mechanical or electrical device other than a watch
for displaying time. A clock is a machine in which a device that
performs regular movements in equal intervals of time is linked
to a counting mechanism that records the number of movements.
All clocks, of whatever form, are made on this principle.
The origin of the all-mechanical escapement clock is unknown;
the first such devices may have been invented and used in
monasteries to toll a bell that called the monks to prayers. The
first mechanical clocks to which clear references exist were
large, weight-driven machines fitted into towers and known today
as turret clocks. These early devices struck only the hours and
did not have hands or a dial.
The oldest surviving clock in England is that at Salisbury
Cathedral, which dates from 1386. A clock erected at Rouen,
France, in 1389 is still extant (see photograph), and one built
for Wells Cathedral in England is preserved in the Science
Museum in London. The Salisbury clock strikes the hours, and
those of Rouen and Wells also have mechanisms for chiming at the
quarter hour. These clocks are large, iron-framed structures
driven by falling weights attached to a cord wrapped around a
drum and regulated by a mechanism known as a verge (or crown
wheel) escapement. Their errors probably were as large as a half
hour per day. The first domestic clocks were smaller
wall-mounted versions of these large public clocks. They
appeared late in the 14th century, and few examples have
survived; most of them, extremely austere in design, had no
cases or means of protection from dust.
About 1450, clockmakers working probably in southern Germany or
northern Italy began to make small clocks driven by a spring.
These were the first portable timepieces, representing an
important landmark in horology. The time-telling dials of these
clocks usually had an hour hand only (minute hands did not
generally appear until the 1650s) and were exposed to the air;
there was normally no form of cover such as a glass until the
17th century, though the mechanism was enclosed, and the cases
were made of brass.
About 1581 Galileo noticed the characteristic timekeeping
property of the pendulum. The Dutch astronomer and physicist
Christiaan Huygens was responsible for the practical application
of the pendulum as a time controller in clocks from 1656 onward.
Huygens's invention brought about a great increase in the
importance and extent of clock making. Clocks, weight-driven and
with short pendulums, were encased in wood and made to hang on
the wall, but these new eight-day wall clocks had very heavy
weights, and many fell off weak plaster walls and were
destroyed. The next step was to extend the case to the floor,
and the grandfather clock was born. In 1670 the long, or
seconds, pendulum was introduced by English clock makers with
the anchor escapement.
Mechanical clocks
The pendulum
The pendulum is a reliable time measurer because, for small
arcs, the time required for a complete swing (period) depends
only on the length of the pendulum and is almost independent of
the extent of the arc. The length of a pendulum with a period of
one second is about 39 inches (990 mm), and an increase in
length of 0.001 inch (0.025 mm) will make the clock lose about
one second per day. Altering the length of a pendulum is
therefore a sensitive means of regulation. The alteration is
usually carried out by allowing the bob to rest upon a nut that
can be screwed up or down the pendulum rod.
Any expansion or contraction of the rod caused by changes of
temperature will affect the timekeeping of a pendulum; e.g., a
pendulum clock with a steel rod will lose one second a day for a
rise in temperature of approximately 4 °F (2.2 °C). For accurate
timekeeping, the length of the pendulum must be kept as nearly
constant as possible. This may be done in several ways, some of
which use the differing coefficients of expansion (the amount of
expansion per degree change in temperature) of different metals
to obtain a cancelling-out effect. In one popular compensation
method, the bob consists of a glass or metal jar containing a
suitable amount of mercury. The gridiron pendulum employs rods
of different metal, usually brass and steel, while in the
zinc-iron tube the pendulum rod is made of concentric tubes of
zinc and iron. An improved method, however, is to make the
pendulum rod from a special alloy called Invar. This material
has such a small coefficient of expansion that small changes of
temperature have a negligible effect and can easily be
compensated for if required.
Photograph: The Houses of Parliament (a group of government
buildings in London) are famous for their large …
* The Houses of Parliament (a group of government buildings in
London) are famous for their large …
In a pendulum clock an escape wheel is allowed to rotate through
the pitch of one tooth for each double swing of the pendulum and
to transmit an impulse to the pendulum to keep it swinging. An
ideal escapement would transmit the impulse without interfering
with the free swing, and the impulse should be as uniform as
possible. The double three-legged gravity escapement, which
achieves the second of these but not the first, was invented by
Edmund Beckett, afterward Lord Grimthorpe, and used by him for
the great clock at Westminster, now generally known as Big Ben,
which was installed in 1859. It became the standard for all
really accurate tower clocks.
Escapement
The true innovation of the weight-driven clock was the
escapement, the system that mediated the transfer of the energy
of the gravitational force acting on the weights to the clock's
counting mechanism. The most common escapement was the
verge-and-foliot.
In a typical verge-and-foliot escapement, the weighted rope
unwinds from the barrel, turning the toothed escape wheel.
Controlling the movement of the wheel is the verge, a vertical
rod with pallets at each end. When the wheel turns, the top
pallet stops it and causes the foliot, with its regulating
weights, to oscillate. This oscillation turns the verge and
releases the top pallet. The wheel advances until it is caught
again by the bottom pallet, and the process repeats itself. The
actions of the escapement stabilize the power of the
gravitational force and are what produce the ticktock of
weight-driven clocks.
The wheelwork
The wheelwork, or train, of a clock is the series of moving
wheels (gears) that transmit motion from a weight or spring, via
the escapement, to the minute and hour hands. It is most
important that the wheels and pinions be made accurately and the
tooth form designed so that the power is transferred as steadily
as possible.
In a clock driven by a weight or a spring, the power is first
transmitted by the main, or great, wheel. This engages with a
pinion (a gear with a small number of teeth designed to mesh
with a larger wheel), whose arbor (a turning rod to which gears
are attached) is attached to the second wheel that, in its turn,
engages with the next pinion, and so on, down through the train
to the escapement. The gear ratios are such that one arbor,
usually the second or third, rotates once an hour and can be
used to carry the minute hand. A simple 12-to-1 gearing, known
as the motion work, gives the necessary step-down ratio to drive
the hour hand. The spring or weight is fitted with a mechanism
so it can be rewound when necessary, and the arbor carrying the
minute hand is provided with a simple slipping clutch that
allows the hands to be set to the correct time.
The timekeeping part of all weight-driven clocks, including
large tower clocks, is substantially the same. The frame is made
up of two plates that carry the pivots of the various wheels and
other moving parts and that are united and spaced by four
pillars. The driving weight hangs from a line coiled around a
barrel or sprocket, which is raised by turning the winding
square or, in some cases, by pulling on the line. The main wheel
engages with the centre pinion, on the arbor (axle) of which is
also mounted the centre wheel. The front pivot of this wheel and
pinion carries the minute hand and part of the gearing necessary
to drive the hour hand.
The centre wheel is also coupled through a suitable gear train
to the escape wheel, which engages with the pallets that are
fixed to the arbor between the front plate and the pendulum
suspension cock. Also fixed to the pallet arbor is the crutch,
which terminates at its lower end in a fork that embraces the
pendulum rod.
The motion work used for driving the hands is mounted between
the dial and the front plate of the frame. The cannon pinion,
which drives the motion work, rotates once an hour; it is
coupled to the centre arbor by a flat spring that acts as a
clutch and permits the hands to be set.
Electric clocks
Electric currents can be used to replace the weight or spring as
a source of power and as a means of signaling time indications
from a central master clock to a wide range of distant
indicating dials. Invented in 1840, the first battery electric
clock was driven by a spring and pendulum and employed an
electrical impulse to operate a number of dials. Considerable
experimental work followed, and it was not until 1906 that the
first self-contained battery-driven clock was invented.
In a master clock system, electricity is used to give direct
impulses to the pendulum, which in turn causes the clock's gear
train to move, or to lift a lever after it has imparted an
impulse to the pendulum. In various modern master clocks the
pendulum operates a light count wheel that turns through the
pitch of one tooth every double swing and is arranged to release
a lever every half minute. This lever gives an impulse to the
pendulum and is then restored to its original position by an
electromagnet. The pulse of current that operates the
electromagnet can also be transmitted to a series of distant
dials, or slave clocks, advancing the hands of each through the
space of a half minute. Thus, a master clock can control scores
of dials in a large group of buildings, as well as such other
apparatus as time recorders and sirens.
Electric master clocks of this type are good timekeepers, since
the impulse can be given symmetrically as the pendulum passes
through its middle position and the interference with its motion
is small.
With the application of the synchronous electric motor to clocks
in 1918, domestic electric clocks became popular. A synchronous
electric motor runs in step with the frequency of the electric
power source, which in most countries alternates at 60 hertz
(cycles per second). The electric motor is coupled to a
reduction gearing that drives the clock hands at the correct
rate.
The synchronous electric clock has no timekeeping properties in
itself and is wholly dependent upon the frequency stability of
the alternating current supplied. If this frequency changes, the
electric clock will not keep correct time.
The most accurate mechanical timekeeper is the Shortt pendulum
clock; it makes use of the movement described above for electric
master clock systems. The Shortt pendulum clock consists of two
separate clocks, one of which synchronizes the other. The
timekeeping element is a pendulum that swings freely, except
that once every half minute it receives an impulse from a gently
falling lever. This lever is released by an electrical signal
transmitted from its slave clock. After the impulse has been
sent, a synchronizing signal is transmitted back to the slave
clock that ensures that the impulse to the free pendulum will be
released exactly a half minute later than the previous impulse.
The pendulum swings in a sealed box in which the air is kept at
a constant, low pressure. Shortt clocks in observatories are
kept in a room, usually a basement, where the temperature
remains nearly constant, and under these conditions they can
maintain the correct time to within a few thousandths of a
second per day.
In 1929 the quartz crystal was first applied to timekeeping;
this invention was probably the single greatest contribution to
precision time measurement. Quartz crystals oscillating at
frequencies of 100,000 hertz can be compared and frequency
differences determined to an accuracy of one part in 1010.
The timekeeping element of a quartz clock consists of a ring of
quartz about 2.5 inches (63.5 mm) in diameter, suspended by
threads and enclosed in a heat-insulated chamber. Electrodes are
attached to the surfaces of the ring and connected to an
electrical circuit in such a manner as to sustain oscillations.
Since the frequency of vibration, 100,000-hertz, is too high for
convenient time measurement, it is reduced by a process known as
frequency division or demultiplication and applied to a
synchronous motor connected to a clock dial through mechanical
gearing. If a 100,000 hertz frequency, for example, is subjected
to a combined electrical and mechanical gearing reduction of
6,000,000 to 1, then the second hand of the synchronous clock
will make exactly one rotation in 60 seconds. The vibrations are
so regular that the maximum error of an observatory
quartz-crystal clock is only a few ten-thousandths of a second
per day, equivalent to an error of one second every 10 years.
