Shortt-synchronome clock
Encyclopedia
The Shortt-Synchronome free pendulum clock was a complex precision electromechanical pendulum clock
invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK. They were the most accurate pendulum clocks ever commercially produced, and became the highest standard for timekeeping between the 1920s and the 1940s, when mechanical clocks were superseded by quartz
time standards. They were used worldwide in astronomical observatories, naval observatories, in scientific research, and as a primary standard
for national time dissemination services
. The Shortt was the first clock to be a more accurate timekeeper than the Earth itself; it was used in 1926 to detect tiny seasonal changes (nutation
) in the Earth's rotation rate. Shortt clocks achieved accuracy of around a second per year. About 100 were produced between 1922 and 1956.
Shortt clocks kept time with two pendulum
s, a master pendulum swinging in a vacuum tank and a slave pendulum in a separate clock, which was synchronized to the master by an electric circuit and electromagnet
s. The slave pendulum was attached to the timekeeping mechanisms of the clock, leaving the master pendulum virtually free of external disturbances.
in a copper vacuum
tank 26 cm diameter and 125 cm high attached to a wall, and a precision pendulum clock "slaved" to it, standing a few feet away. The slave clock was a modified version of a standard Synchronome precision regulator clock. The two components were linked by wires which carried electric pulses that operated electromagnets in the mechanisms to keep the two pendulums swinging in synchronism. The master pendulum rod and its 14-pound weight were made of the alloy invar
to reduce thermal expansion
and contraction of the pendulum, which causes the pendulum's period to vary with changes in temperature. The residual thermal expansion rate was compensated to zero with a metal insert under the bob. The vacuum tank was evacuated by a hand-operated pump to a pressure of around 20 mm Hg to prevent changes in atmospheric pressure
from affecting the rate of the pendulum, and also to eliminate aerodynamic drag on the pendulum, which increased its Q factor
from 25,000 to 110,000, thus increasing its accuracy by a factor of four. Both pendulums were seconds pendulum
s, about 1 meter (39 in
) long, with a period
of 2 seconds; each swing of the master took exactly one second, with the slave's natural rate very slightly longer. The pendulums received a push from the mechanism once every 30 seconds to keep them swinging. The slave clock had two clock dials on it, showing the time kept by each pendulum, to verify that they were synchronized. It also had electrical terminals which produced a 1 Hz
timing signal. Wires could be attached to these to transmit the clock's ultra-accurate time signal to clocks in other cities, or broadcast it by radio.
free of external disturbances theoretically keeps perfect time. However, pendulums in clocks have to be linked to the clock's mechanism, which disturbs their natural swing, and this was the main cause of error in precision clocks of the early 20th century. An ordinary clock's mechanism interacts with the pendulum each swing to perform two functions: first, the pendulum must activate some kind of linkage to record the passage of time. Second, the clock's mechanism, triggered by the linkage, must give the pendulum a push (impulse) to replace the energy the pendulum loses to friction, to keep it swinging. These two functions both disturb the pendulum's motion.
The advantage of the Shortt clock is that, first, it reduced the disturbance of the master pendulum due to the impulse by only giving the pendulums an impulse once every 30 seconds (30 pendulum swings). Secondly, it eliminated all other interaction with the master pendulum by generating the necessary precise timing signal to control the slave clock (and record the passage of time) from the impulse mechanism itself, leaving the pendulum to swing "free" of interference.
using a 15-tooth count wheel which was moved forward each pendulum swing by a pawl
attached to the pendulum. Every 15 swings (30 seconds), this released a gravity lever which gave the slave pendulum a push. This simultaneously impulsed the master pendulum, by closing a switch which sent a pulse of current to an electromagnet which released a gravity lever to give the master pendulum a push. The impulse was provided by the weight of the gravity lever (acting as a remontoire
) rolling off a wheel attached to the pendulum, this mechanism ensuring that the pendulum received an identical impulse, at precisely the same part of its stroke, each time.
attached to the slave pendulum. If the slave pendulum lagged behind the master, the spring would catch on the vane (called a "hit"). The result was that the restoring force on the pendulum is increased for that swing (it is now the force of gravity plus the force from the leaf spring), which shortens the time for that swing. If it was ahead (a "miss") it would make its normal swing, without acceleration. The slave pendulum was set to a slightly slower rate than the master, so the slave would lag behind the master more each interval, until it received a "hit" which set it ahead again. Typically the acceleration resulting from a "hit" would be about twice the normal loss, so that "hit" and "miss" cycles would roughly alternate, hence the name of the mechanism. This cycle, repeated over and over, kept it precisely in step with the master over the long term. This feedback loop functioned as an electromechanical version of a phase locked loop, later used in electronics.
for a month. He found that it was stable to 200 microsecond
s per day, equivalent to an error rate of one second in 12 years, far more accurate than the 1 second per year that was previously measured. His data revealed the clock was so sensitive it was detecting the slight changes in gravity due to tidal distortions in the solid Earth caused by the passage of the Sun and Moon overhead.
Primary source for account of development of Shortt clock, as well as history of free pendulum clocks
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...
invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK. They were the most accurate pendulum clocks ever commercially produced, and became the highest standard for timekeeping between the 1920s and the 1940s, when mechanical clocks were superseded by quartz
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...
time standards. They were used worldwide in astronomical observatories, naval observatories, in scientific research, and as a primary standard
Primary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working...
for national time dissemination services
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
. The Shortt was the first clock to be a more accurate timekeeper than the Earth itself; it was used in 1926 to detect tiny seasonal changes (nutation
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
) in the Earth's rotation rate. Shortt clocks achieved accuracy of around a second per year. About 100 were produced between 1922 and 1956.
Shortt clocks kept time with two pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
s, a master pendulum swinging in a vacuum tank and a slave pendulum in a separate clock, which was synchronized to the master by an electric circuit and electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
s. The slave pendulum was attached to the timekeeping mechanisms of the clock, leaving the master pendulum virtually free of external disturbances.
Description
The Shortt clock consists of two separate units: the master pendulumPendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
in a copper vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
tank 26 cm diameter and 125 cm high attached to a wall, and a precision pendulum clock "slaved" to it, standing a few feet away. The slave clock was a modified version of a standard Synchronome precision regulator clock. The two components were linked by wires which carried electric pulses that operated electromagnets in the mechanisms to keep the two pendulums swinging in synchronism. The master pendulum rod and its 14-pound weight were made of the alloy invar
Invar
Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...
to reduce thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
and contraction of the pendulum, which causes the pendulum's period to vary with changes in temperature. The residual thermal expansion rate was compensated to zero with a metal insert under the bob. The vacuum tank was evacuated by a hand-operated pump to a pressure of around 20 mm Hg to prevent changes in atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
from affecting the rate of the pendulum, and also to eliminate aerodynamic drag on the pendulum, which increased its Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
from 25,000 to 110,000, thus increasing its accuracy by a factor of four. Both pendulums were seconds pendulum
Seconds pendulum
A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz....
s, about 1 meter (39 in
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
) long, with a period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of 2 seconds; each swing of the master took exactly one second, with the slave's natural rate very slightly longer. The pendulums received a push from the mechanism once every 30 seconds to keep them swinging. The slave clock had two clock dials on it, showing the time kept by each pendulum, to verify that they were synchronized. It also had electrical terminals which produced a 1 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
timing signal. Wires could be attached to these to transmit the clock's ultra-accurate time signal to clocks in other cities, or broadcast it by radio.
Reason for accuracy
A pendulum swinging in a vacuum at a constant amplitudeAmplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
free of external disturbances theoretically keeps perfect time. However, pendulums in clocks have to be linked to the clock's mechanism, which disturbs their natural swing, and this was the main cause of error in precision clocks of the early 20th century. An ordinary clock's mechanism interacts with the pendulum each swing to perform two functions: first, the pendulum must activate some kind of linkage to record the passage of time. Second, the clock's mechanism, triggered by the linkage, must give the pendulum a push (impulse) to replace the energy the pendulum loses to friction, to keep it swinging. These two functions both disturb the pendulum's motion.
The advantage of the Shortt clock is that, first, it reduced the disturbance of the master pendulum due to the impulse by only giving the pendulums an impulse once every 30 seconds (30 pendulum swings). Secondly, it eliminated all other interaction with the master pendulum by generating the necessary precise timing signal to control the slave clock (and record the passage of time) from the impulse mechanism itself, leaving the pendulum to swing "free" of interference.
How it works
The master and slave pendulums were linked together in a feedback loop which kept the slave synchronized with the master. The slave clock had a mechanical escapementEscapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
using a 15-tooth count wheel which was moved forward each pendulum swing by a pawl
Pawl
Pawl may refer to:* A common component of a ratchet* A part of the adjustable height locking mechanism of an extension ladder* Pawl , a former racing car constructor...
attached to the pendulum. Every 15 swings (30 seconds), this released a gravity lever which gave the slave pendulum a push. This simultaneously impulsed the master pendulum, by closing a switch which sent a pulse of current to an electromagnet which released a gravity lever to give the master pendulum a push. The impulse was provided by the weight of the gravity lever (acting as a remontoire
Remontoire
In mechanical horology, a remontoire, is a small secondary source of power, a weight or spring, which runs the timekeeping mechanism and is itself periodically rewound by the timepiece's main power source, such as a mainspring...
) rolling off a wheel attached to the pendulum, this mechanism ensuring that the pendulum received an identical impulse, at precisely the same part of its stroke, each time.
Hit and miss synchronizer
In return, a pulse from the master pendulum was used to keep the slave pendulum in phase with it through a device called a "hit and miss synchronizer". Every 30 swings, after the master pendulum was impulsed, the position of the two pendulums was compared. This was accomplished by contacts on the master gravity arm. After the master pendulum was impulsed by the gravity arm, its fall closed contacts, sending a signal to the slave, which moved a vane into the path of a leaf springLeaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...
attached to the slave pendulum. If the slave pendulum lagged behind the master, the spring would catch on the vane (called a "hit"). The result was that the restoring force on the pendulum is increased for that swing (it is now the force of gravity plus the force from the leaf spring), which shortens the time for that swing. If it was ahead (a "miss") it would make its normal swing, without acceleration. The slave pendulum was set to a slightly slower rate than the master, so the slave would lag behind the master more each interval, until it received a "hit" which set it ahead again. Typically the acceleration resulting from a "hit" would be about twice the normal loss, so that "hit" and "miss" cycles would roughly alternate, hence the name of the mechanism. This cycle, repeated over and over, kept it precisely in step with the master over the long term. This feedback loop functioned as an electromechanical version of a phase locked loop, later used in electronics.
Recent accuracy measurement
In 1984 Pierre Boucheron studied the accuracy of a Shortt clock preserved at the US Naval Observatory. Using modern optical sensors which detected the precise time of passage of the pendulum without disturbing it, he compared its rate to an atomic clockAtomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
for a month. He found that it was stable to 200 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s per day, equivalent to an error rate of one second in 12 years, far more accurate than the 1 second per year that was previously measured. His data revealed the clock was so sensitive it was detecting the slight changes in gravity due to tidal distortions in the solid Earth caused by the passage of the Sun and Moon overhead.
External links
- Michel Viredaz Shortt clock animation Detailed, high resolution animation of the entire mechanism, giving a good understanding of how it works.
See also
- Master clockMaster clockA master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...
- Pendulum clockPendulum clockA pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...
- EscapementEscapementIn mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
Primary source for account of development of Shortt clock, as well as history of free pendulum clocks