The invention of the portable timepiece or, as we know it today, the watch, is attributed to Peter Henlein, a locksmith from the city of Nuremburg, Germany. He introduced the mainspring as a replacement for weights, enabling the small size and portability of the watch.
During Henlein's time the role of locksmith extended well past locks. Such a locksmith was also an expert mechanic, similar to a modern toolmaker. The medieval locksmith, like the medieval blacksmith, was involved in producing complex and detailed devices. As a result, many locksmiths and blacksmiths were involved in the development and construction of time-keeping devices.
Around 1500, Henlein began to make small clocks that were driven by a spring. These were the first portable timepieces and, designed to be carried by hand, were frequently circular or oval in shape. Because of this oval shape, and a mistranslation of the German word Ueurlein (little clocks) for Eierlein (little eggs), these timepieces were called Nuremburg eggs. The dials of these clocks were placed on top of the device and featured only an hour hand. A record of 1511 indicates that Henlein's watches included iron movements that were mounted in musk balls. This ball was a decorated and perforated sphere in which musk was placed.
The watches invented by Henlein were both desirable, fashionable ornaments and devices indicative of an increased societal reliance on technology. As Henlein's contemporary Johannes Coeulus wrote in 1511, "every day produces more ingenious inventions. A clever and comparatively young man—Peter Henlein—creates works that are the admiration of leading mathematicians, for, out of a little iron he constructs clocks with numerous wheels, which, without any impulse and in any position, indicate time for forty hours and strike, and which can be carried in the purse as well as in the pocket." These devices were also indicative of a new kind of future.
However, despite the fascination that these devices elicited from mathematicians and intellectuals, the Nuremburg eggs were far from accurate or reliable. In essence, they could not be moved and still keep accurate time. Likewise, they kept time unevenly: the force of the mainspring was greater when fully wound than when it was nearly run down. As the design possibilities of the Nuremburg egg increased, so did hostility to the inaccuracy of the device. In Shakespeare's Love's Labour's Lost, for example, Shakespeare compares an inconstant woman to "A German clock / Still a-repairing, ever out of frame, / And never going aright, being a watch."
Around 1525, however, Jacob Zech, a Swiss mechanic who lived in the city of Prague (in what is now the Czech Republic), began to study the problem of equalizing the pressure of the mainspring. Zech developed the fusee, a cone-shaped grooved pulley that was used together with a barrel containing the mainspring. Because the mainspring rotates the barrel in which it is housed, the leverage of the mainspring is progressively increased as it runs down. This device improved the accuracy of the watch, and ensured its continuation and development beyond the failings of Peter Henlein's Nuremburg egg.