The best mechanical timekeeper known is a pendulum swinging freely under
gravity, but in order to convert such a pendulum into a practical clock,
the pendulum must be sustained in motion so that the oscillations do not
die down, and the swings must be counted. In ordinary clocks, the
sustaining and counting function are both carried out by the escapement and
the clock mechanism, but the free motion of the pendulum is considerably
interfered with thereby, with a consequent loss of accuracy in
What is so special about the Shortt Free Pendulum Clock. Is that a free pendulum became a reality, and a time during the period between the two world wars, it served as the pre-eminent time-keeper, and was to be found in all the worlds observatories.
It is a mechanism of exquisite delicacy. It relies for its extra-ordinary time keeping on the simple fact that a master pendulum is kept vibrating in a near vacuum, in a sealed room, bolted to a concrete wall, and with strict temperature control. The astonishing fact is that the pendulum has absolutely no work to do. It vibrates in solitary isolation and locks and unlocks no mechanisms. If this is so, how is time recorded, and to what does it owe its accuracy? It is a free pendulum in almost every sense of the word. Almost, because there is no such thing as a free pendulum, as there is no such thing as perpetual motion. In some way, it has to receive a maintaining force to keep it vibrating, and this is a gravity arm. If this force were arranged to be of a strictly uniform quantity, and was delivered in phase with the pendulum's vibration at zero position, it would have extra-ordinary time-keeping properties.
The success of the clock is due to the electro-mechanical age, which almost exactly lasted for 100 years during the 20th century. The clock is not an electric clock, because the impulse delivered to the pendulum does not rely on electricity but to gravity. But the reset force for the gravity arm is electric, and so are the signals that unlock the gravity arms, carry out phase comparison and generate the signal for time keeping. This is important, because the sealed pendulum chamber will only allow the passage of pairs of wires.
What is the nature of the slave, and how is the phase comparison achieved. To serve the master, Shortt enlisted the services of an established firm in London known as the Synchronome company. It director was Frank Hope Jones, and his clock had gained wide acceptance for its ability to deliver an electrical signal on time to a large number of clock dials in series with its electro-magnets. Thus it found use in public buildings,
and especially on the railways.
After much experimentation, Shortt approached Hope Jones and was allowed the use of his workshops in Clerkenwell, London to develop his ideas. Shortt wanted the synchronome to give the timed impulse for unlocking the master gravity arm, and become the slave for a higher master for the Synchronome was itself a highly accurate clock.
The first Shortt clock was tested in the Royal Observatory in Edinburgh in 1922, and the report on its behavior by the then Astronomer Royal for Scotland, Professor R. A. Sampson, was so favourable that others were soon installed at Greenwich (they subsequently had four), and in the principle observatories of the world. As an example of their performance, the maximum rate of change of Shortt No. 3 during its first year at Greenwich was .01 seconds and that of No. 41, during a continuous run of three years, from January 1943 to January 1946, in the Naval Observatory in Washington, was under 0.2 seconds.
The Shortt Clock has justly been described as the one of the most ingenious precision instruments ever made, dating from an age when imaginations were fired by electrical inventions . It has been regarded as the best timekeeper in its day, and has deservedly earned its place in the Horological Hall of Fame, being the first clock to allow accurate observation of the earth's nutation (the precession of the Earth's pole about the pole of the ecliptic). The Shortt clock turned out to be a great success. By 1956, the Synchronome company had made just about one hundred and exported them to observatories the world over. Over long periods, the variation of the Shortt clock might be as little as a second or two a year, which is probably close to the limit of what is possible for any clock whose resonant frequency depends on the uncertain forces of the Earth's gravity. It is a fitting climax to the era of the pendulum.
This diagram shows the master clock on the left and the slave on the right. The master pendulum is entirely free and has no function to perform such as counting or releasing an escapement. It oscillates in solitary splendor. It would eventually come to rest of course as there is no perpetual motion, and therefore it must receive an impulse to keep it in motion. This is delivered every thirty seconds and just in time by a light gravity arm G1 shown in the diagram. This itself is re-set by a heavier gravity arm, and is one of the most ingenious and precise of electro-mechanical inventions. The electrical reset of the heavy gravity arm closes an electrical contact and it is this impulse which is fed back to the slave.
The slave is an electric clock in its own right, entirely self contained and is known as the Hope Jones Synchronometer. It is also a highly accurate clock, and needs to be so in the context of the Shortt Synchroniser, because it must feed an electrical impulse to unlock the master light gravity arm and just in time. The phase of the master and slave pendulums are directly compared, and if the slave is late, it is given an extra force to push it back in phase with the Master. If it is early, no action occurs, but the slave is deliberately set to run 7 seconds slow a day, so that hits and misses occur regularly, thus ensuring that the phase of the slave is locked to the master. This is an incredibly sensitive action, with a timing interval of only 2.5 mili-seconds. The slave is kept in synchrony with the master in a phase locked loop.
This is the impulse counter. It receives an electrical impulse from the master reset every 30 seconds, and an electromagnet and ratchet and pawl are geared to the minute and hour hands.
This is the master clock in its Meccano incarnation. The heavy reset solenoids can be seen as well as the catch and tail of the light gravity arm.
Here is the Slave mechanism. The pendulum can be seen as well as the gravity arm above.. The two solenoids on the right reset the gravity arm. The smaller solenoid on the left is part of the phase comparator.
Here is the Shortt Clock housed in a special enclosure in my apartment. One the left is the master clock, on the right the slave, and between them both and above, the impulse counter which tells the time.
The Meccano version of the Shortt Clock is a complex construction. Like
the prototype, it has three sections - the Master with a seconds free pendulum,
the slave, which is a Hope Jones Synchronometer and also seconds pendulum,
and the impulse counter which is solenoid driven ratchet mechanism.. The
complete clock is housed in its own clock case. This clock is believed to
be the most complex and accurate ever built in Meccano and has become a ModelPlan (MP 130), and is available from MW Models.
It has never been shown in public apart from a brief one day exhibition in my
apartment and to a few traveling Meccano friends. It is believed to
be the most accurate Meccano timepiece ever built.
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