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Pendulum Clocks

The search for more accuracy in timekeeping, which had gone on for several thousand years, came to an end in 1656 when Christian Huygens in Holland applied the principles of physics inherent in a pendulum to a clock. The main principle useful to the accuracy of time keeping is that the length of time it takes for a pendulum to go through an arc is only variable by the length of the pendulum. The weight of the bob at the end of the pendulum makes no difference, nor does the distance of the arc: only the length of the shaft matters to how long it takes for a pendulum to swing through an arc. With this knowledge and some rather complex mathematical computations, Huygens was able to fashion a pendulum which went through an arc once a second, giving unprecedented accuracy to the measurement of time. Weights had been used to provide the power for clocks since they were first put into Town Halls and Church Towers; so using those to provide the power for a pendulum clock was relatively easy. Several challenges had to be faced after the initial success of devising a pendulum that would move a clock through one minute in sixty seconds. Before getting into the resolution of those issues and additions to the clock of minute and second hands, strike chime for the hour, and music box playing, it will be useful to look at a diagram of the very basic and somewhat simplified parts of a Pendulum clock.

It all begins with the weight(s), here only one illustrated for simplification. The weight is wound over a cylinder either with a strand of brass or a chain to allow gravity to pull it down thus providing power to the clock. Obviously, without some means of controlling that drop it would occur rapidly, which would be useless for a clock. Escapements were used in the tower clocks for that purpose, as it is used here, labeled the "escape wheel" which monitors the release of energy from the descending weight at a specific rate. The teeth of the escapement are angled in such a way to provide a precise release and a slight push to the pendulum. It is this checking of teeth between the anchor and escapement that provides the familiar "tick/tock" of a clock: the "tick" on one end of the anchor and the "tock" when the next tooth engages. The main wheel gear takes the energy from the descending weight and through gear reduction directly powers the escapement wheel which, controlled by the anchor to a specific rate, moves the pendulum through its arc, yielding the second accuracy, which determines the advancement of the other hour and minute hands and anything else attached to the movement. The process of gear reduction, shown here simply, extended the time between rewinds (i.e. lifting the weight) to eight days as an average.

Issues, which were resolved over a period of fifty years after the application of the pendulum to timekeeping, were the effects of such things as friction, temperature, humidity and gear reduction. While the escapement regulated the movement of the pendulum in a precise manner when constructed, the placement of the clock in the owner's residence might change that accuracy with temperature (heat expanding the parts, cold contracting them). If the clock were not exactly level, and many buildings were not, the pendulum could just slightly touch the case and that friction reducing its accuracy. Finally, the use of gear reduction to both expand the time between lifting the weights for another cycle and adding additional time displays (such as months) required complex mathematical computations and manufacture of gears meeting those requirements: incorrect gearing could adversely effect the clock's accuracy.

While the earliest pendulum clocks required opening the case and manually pulling the chord or chain to reset the weights for another cycle, later a winding aperture was added to the face of the clock, which was wound by a key to raise the weight. The first addition to simply time keeping was the strike, a chime or bell, to strike the hour. A second weight was added to power this, which was connected by gears to the hour hand, along with a second winding aperture. Next to be added was a music box, or series of bells, to play a tune, usually on the quarter hours, in addition to the hour strike. Perhaps the most well known of those is the Westminster Chime taken from the tower of Parliament in London, which we call "Big Ben". An additional weight was added for the music, along with a third winding aperture. Most pendulum clocks produced from about the middle of the 18th century on have had three weights. Additional displays were added to the face or above it, some for tides, some for phases of the moon, some for Zodiac signs, most of which were connected by gears to the weight used for striking, as that provided an accurate measure of time for those longer durations. One interesting aside, the phases of the moon displayed on many "Grandfather Clocks" is set by gears to rotate one full rotation every fifty-six days. There are two complete depictions of the phases of the moon on the dial so that when one is through the four phases, the next one starts to show in the clock's face. Click here to view clocks.

Since the basic principle of a pendulum meant that the length was the main standard that mattered, escapement and anchor settings were changed to accommodate smaller clocks by simply increasing the frequency of the arc of the pendulum, along with appropriate adjustments to the gears. A large floor clock might have the pendulum swing once every two seconds, so an escapement wheel would be fashioned to advance one tooth every two seconds and a complete cycle in 30 movements. A wall or mantel clock could be made with a shorter pendulum going through an arc every second. Most cuckoo clocks complete an arc every half-second. In this fashion, smaller clocks could be made which still had the accuracy of a pendulum. Many antique clocks from the 19th century are available and still working with the same accuracy as when the craftsman fashioned them.
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