The History of Mechanical Clocks

The History of Mechanical Clocks During most of the Middle Ages, from roughly 500 to 1500 A.D., technological advancement was at a virtual standstill in Europe. Sundial styles evolved, but they didnt move far from ancient Egyptian principles.   Simple Sundials   Simple sundials placed above doorways were used to identify midday and four tides of the sunlit day in the Middle Ages. Several types of pocket sundials were being used by the 10th century one  English model identified tides and even compensated for seasonal changes of the suns altitude.   Mechanical Clocks In the early to mid-14th century, large mechanical clocks began to appear in the towers of several Italian cities. There is no record of any working models preceding these public clocks that were weight-driven and regulated by verge-and-foliot escapements. Verge-and-foliot mechanisms reigned for more than 300 years with variations in the shape of the foliot, but all had the same basic problem: The period of oscillation depended heavily on the amount of driving force and the amount of friction in the drive so the rate was difficult to regulate. Spring-Powered Clocks   Another advancement was an invention by Peter Henlein, a German locksmith from Nuremberg, sometime between 1500 and 1510. Henlein created  spring-powered clocks.  Replacing the heavy drive weights resulted in smaller and more portable clocks and watches. Henlein nicknamed his clocks Nuremberg Eggs. Although they slowed down as the mainspring unwound, they were popular among wealthy individuals because of their size and because they could be placed on a shelf or table instead of hung from a wall. They were the first portable timepieces, but they only had hour hands. Minute hands didn’t appear until 1670, and clocks had no glass protection during this time. Glass placed over the face of a watch didn’t come about until the 17th century. Still, Henleins advances in design were precursors to truly accurate timekeeping.   Accurate Mechanical Clocks   Christian Huygens, a Dutch scientist, made the first pendulum clock in 1656. It was regulated by a mechanism with a natural period of oscillation. Although Galileo Galilei  is  sometimes credited with inventing the pendulum and he studied its motion as early as 1582, his design for a clock was not built before his death. Huygens pendulum clock had an error of less than one minute a day, the first time such accuracy had been achieved. His later refinements reduced his clocks errors to less than 10 seconds a day.   Huygens developed the balance wheel and spring assembly sometime around 1675  and it’s still found in some of todays wristwatches. This improvement allowed 17th-century watches to keep time to 10 minutes a day. William Clement began building clocks with the new anchor or recoil escapement in London in 1671. This was a substantial improvement over the verge because it interfered less with the motion of the pendulum.   In 1721, George Graham improved the pendulum clocks accuracy to one  second a day by compensating for changes in the pendulums length due to temperature variations. John Harrison, a carpenter and self-taught clockmaker, refined Grahams temperature compensation techniques and added new methods of reducing friction. By 1761, he had built a marine chronometer with the spring and a balance wheel escapement that had won the British governments 1714 prize offered for a means of determining longitude to within one-half  a degree. It kept time aboard a rolling ship to about one-fifth of a second a day, nearly as well as a pendulum clock could do on land, and 10 times better than required.   Over the next century, refinements led to Siegmund Rieflers clock with a nearly free pendulum in 1889. It attained an accuracy of a hundredth of a second a day and became the standard in many astronomical observatories. A true free-pendulum principle was introduced by R. J. Rudd around 1898, stimulating the development of several free-pendulum clocks. One of the most famous, the W. H. Shortt clock, was demonstrated in 1921. The Shortt clock almost immediately replaced Rieflers clock as a supreme timekeeper in many observatories. This clock consisted of two pendulums, one a slave and the other a master. The slave pendulum gave the master pendulum the gentle pushes it needed to maintain its motion, and it also drove the clocks hands. This allowed the master pendulum to remain free from mechanical tasks that would disturb its regularity. Quartz Clocks   Quartz crystal clocks replaced the Shortt clock as the standard in the 1930s and 1940s, improving timekeeping performance far beyond that of pendulum and balance-wheel escapements.   Quartz clock operation is based on the piezoelectric property of quartz crystals. When an electric field is applied to the crystal, it changes its shape. It generates an electric field when squeezed or bent. When placed in a suitable electronic circuit, this interaction between mechanical stress and electric field causes the crystal to vibrate and generate a constant frequency electric signal that can be used to operate an electronic clock display.  Quartz crystal clocks were better because they had no gears or escapements to disturb their regular frequency. Even so, they relied on a mechanical vibration whose frequency depended critically on the crystals size and shape. No two crystals can be precisely alike  with exactly the same frequency.   Quartz clocks continue to dominate the market in numbers because their performance is excellent and they are inexpensive. But the timekeeping performance of quartz clocks has been substantially surpassed by atomic clocks.   Information and illustrations provided by the National Institute of Standards and Technology and the U.S. Department of Commerce.