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The Shortt clock had two pendulums.  The first, known as the master, swung freely in an evacuated case.  Its only job was to synchronize the swing of the second pendulum, called the slave, which was housed in a neighboring cabinet.  Every 30 seconds the slave sent an electrical signal to give a nudge to the master.  In return, via an elaborate electromechanical link, the master ensured that the slave never got out of step.

 Shortt clocks were standard provision in astronomical observatories of the 1920s and 1930s, and are credited with keeping time to better than 2 milliseconds a day.  Many were on record as losing or gaining no more than 1 second a year – a stability of one part in 30 million.  The first indications of seasonal variations in the Earth’s rotation were gleaned by the use of Shortt clocks.

 In 1984 Pierre Boucheron carried out a study of a Shortt clock which had survived in the basement of the US Naval Observatory since 1932.  Using the modern optical sensing equipment instead of the electromechanical coupling, he measured its rate against the observatory’s atomic clocks for a month.  He found that it was stable to 200 microseconds a day over this period, equivalent to two to three parts in a billion.  What is more, the data also reveal that the clock was sensing the distortion of the Earth due to tides from the Moon and the Sun.

joywu1221

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linlang

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饔牛

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nicky_yao198

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austinsun

从wiki查的一点资料

Free pendulum clock

In the 20th century William Hamilton Shortt invented a free pendulum clock, patented in September 1921 and manufactured by the Synchronome Company, with an accuracy of one hundredth of a second per day. In this system the time keeping "master" pendulum, whose rod is made from a special steel alloy with 36% nickel called Invar whose length does not change very much with temperature, swings as free of external influence as possible sealed in a vacuum chamber and does no work. It is in mechanical contact with its escapement for only a fraction of a second every 30 seconds. A secondary "slave" pendulum turns a ratchet, which triggers an electromagnet every thirty seconds. This electromagnet releases a gravity lever onto the escapement above the master pendulum. A fraction of a second later, the motion of the master pendulum releases the gravity lever to fall farther. In the process, the gravity lever gives a tiny impulse to the master pendulum, which keeps that pendulum swinging. The gravity lever falls onto a pair of contacts, completing a circuit that does several things:

1. energizes a second electromagnet to raise the gravity lever above the master pendulum to its top position,
2. sends a pulse to activate one or more clock dials, and
3. sends a pulse to a synchronizing mechanism that keeps the slave pendulum in step with the master pendulum.

Since it is the slave pendulum that releases the gravity lever, this synchronization is vital to the functioning of the clock. The slave clock is set to run slightly slow and the re-set circuit for the gravity arm activates a pivoted arm which just engages with the tip of a blade spring on the pendulum of the slave clock. If the slave clock has lost too much time its blade spring pushes against the arm and this accelerates the clock. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became a standard for use in observatories, and was the first clock capable of detecting small variations in the speed of Earth's rotation.

zhuhuizhi008

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