The Test of Time

by Adrian Maneiro
THE CLOCK WATCH
Since the beginning of the 16th century, when clock watches were invented, they became an iconic accessory with a specific function. The clock watch was transitional in size, between a clock and a watch. These portable timepieces were made possible by the invention of the mainspring. These ornamental timepieces were worn as pendants, which were the first timepieces to be worn on the body.
These 'clock-watches' were fastened to clothing or worn on a chain around the neck. They were heavy drum-shaped cylindrical brass boxes several inches in diameter, engraved and ornamented. They had only an hour hand. The face was not covered with glass, but usually had a hinged brass cover, often decoratively pierced with grillwork so the time could be read without opening. The movement was made of iron or steel and held together with tapered pins and wedges, until screws began to be used after 1550. Many of the movements included striking or alarm mechanisms. They usually had to be wound twice a day.
THE POCKET WATCH

Styles changed in the 17th century and men began to wear watches in pockets instead of as pendants This is said to have occurred in 1675 when Charles II of England introduced waistcoats. This was not just a matter of fashion or prejudice; watches of the time were notoriously prone to fouling from exposure to the elements, and could only reliably be kept safe from harm if carried securely in the pocket. To fit in pockets, their shape evolved into the typical pocket watch shape, rounded and flattened with no sharp edges. Glass was used to cover the face beginning around 1610. Watch fobs began to be used, the name originating from the German word fuppe, a small pocket. Later in the 1800s Prince Albert, the consort to Queen Victoria, introduced the 'Albert chain' accessory, designed to secure the pocket watch to the man's outer garment by way of a clip. The watch was wound and also set by opening the back and fitting a key to a square arbor, and turning it. The timekeeping mechanism in these early pocket watches was the same one used in clocks, invented in the 13th century; the verge escapement which drove a foliot, a dumbbell shaped bar with weights on the ends, to oscillate back and forth. However, the mainspring introduced a source of error not present in weight-powered clocks. The force provided by a spring is not constant, but decreases as the spring unwinds. The rate of all timekeeping mechanisms is affected by changes in their drive force, but the primitive verge and foliot mechanism was especially sensitive to these changes, so early watches slowed down during their running period as the mainspring ran down. This problem, called lack of isochronism, plagued mechanical watches throughout their history. Efforts to improve the accuracy of watches prior to 1657 focused on evening out the steep torque curve of the mainspring. Two devices to do this had appeared in the first clock-watches: the stackfreed and the fusee. The stackfreed, a spring-loaded cam on the mainspring shaft, added a lot of friction and was abandoned after about a century. The fusee was a much more lasting idea. A curving conical pulley with a chain wrapped around it attached to the mainspring barrel, it changed the leverage as the spring unwound, equalizing the drive force. Fusees became standard in all watches, and were used until the early 19th century. The foliot was also gradually replaced with the balance wheel, which had a higher moment of inertia for its size, allowing better timekeeping.

THE BALANCE SPRING

 

A great leap forward in accuracy occurred in 1657 with the addition of the balance spring to the balance wheel, an invention disputed both at the time and ever since between Robert Hooke and Christiaan Huygens. Prior to this, the only force limiting the back and forth motion of the balance wheel under the force of the escapement was the wheel's inertia. This caused the wheel's period to be very sensitive to the force of the mainspring. The balance spring made the balance wheel a harmonic oscillator, with a natural 'beat' resistant to disturbances. This increased watches' accuracy enormously, reducing error from perhaps several hours per day to perhaps 10 minutes per day, resulting in the addition of the minute hand to the face from around 1680 in Britain and 1700 in France. The increased accuracy of the balance wheel focused attention on errors caused by other parts of the movement, igniting a two century wave of watchmaking innovation. The first thing to be improved was the escapement. The verge escapement was replaced in quality watches by the cylinder escapement, invented by Thomas Tompion in 1695 and further developed by George Graham in the 1720s. In Britain a few quality watches went to the duplex escapement, invented by Jean Baptiste Dutertre in 1724. The advantage of these escapements was that they only gave the balance wheel a short push in the middle of its swing, leaving it 'detached' from the escapement to swing back and forth undisturbed during most of its cycle. During the same period, improvements in manufacturing such as the tooth-cutting machine devised by Robert Hooke allowed some increase in the volume of watch production, although finishing and assembling was still done by hand until well into the 19th century.

 

TEMPERATURE COMPENSATION AND CHRONOMETERS

 

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