Fusee (horology) - Obsolescence

Obsolescence

The fusee was a good mainspring compensator, but it was also expensive, difficult to adjust, and had other disadvantages:

• It was bulky and tall, and made pocket watches unfashionably thick.
• If the mainspring broke and had to be replaced, a frequent occurrence with early mainsprings, the fusee had to be readjusted to the new spring.
• If the fusee chain broke, the force of the mainspring sent the end whipping about the inside of the clock, causing damage.

Achieving isochronism was recognised as a serious problem throughout the 500 year history of spring-driven clocks. Many parts were gradually improved to increase isochronism, and eventually the fusee became unnecessary in most timepieces.

The invention of the pendulum and the balance spring in the mid-17th century made clocks and watches much more isochronous, by making the timekeeping element a harmonic oscillator, with a natural "beat" resistant to change. The pendulum clock with an anchor escapement, invented in 1670, was sufficiently independent of drive force so that only a few had fusees.

In pocketwatches, the verge escapement, which required a fusee, was gradually replaced by escapements which were less sensitive to changes in mainspring force. In 1760, Jean-Antoine Lépine dispensed with the fusee, inventing a going barrel to power the watch gear train directly. This contained a very long mainspring, of which only a few turns were used to power the watch. Accordingly, only a part of the mainspring's 'torque curve' was used, where the torque was approximately constant. In the 1780s, pursuing thinner watches, French watchmakers adopted the going barrel with the cylinder escapement. By 1850, the Swiss and American watchmaking industries employed the going barrel exclusively, aided by new methods of adjusting the balance spring so that it was isochronous. England continued to make the bulkier full plate fusee watches until about 1900 They were inexpensive models sold to the lower classes and were derisively called "turnips". After this, the only remaining use for the fusee was in marine chronometers, where the highest precision was needed, until they became obsolete in the 1970s.