With the quest for precision under way – so that navigators might plot their position at sea – watchmaking’s most brilliant minds set about rethinking the parts most apt to improve their watches’ capacity to mark time with greater exactitude. After escapements – mainly detent – and balance wheels – generally bimetallic split-rim balances – the most gifted chronometer-makers of the late eighteenth and early nineteenth centuries turned their attention to novel balance springs that would further improve the energy efficiency and precision of their timepieces.
These were instruments that would ply the high seas, and for this reason the first problem to be tackled was that of corrosion caused by salt air and humidity. A second problem lay with the rapid temperature changes which affected the rigidity of the metal and therefore overall precision. The foremost specialists of England and France experimented with a variety of materials including glass and gold, both non-corrosive. Glass balance springs had the advantage of being almost constant in length but proved far too fragile. Gold was inalterable but tended to stretch at the least exposure to heat. The verdict finally came down in favour of crucible steel which was drawn, cold-laminated then heat-treated (blued), a choice that continued to hold sway until the early years of the twentieth century.
Function follows form
As must now be abundantly clear, the balance spring has a vital role in ensuring the regular functioning of a watch. This paper-thin strip of metal could easily have remained flat and spiral-shaped; instead the most talented horologists understood that this modest component played its part most effectively when combined with a quality escapement and a finely crafted balance. This led them to discover that the point of attachment of flat balance springs prevented them from “breathing” evenly, and that each expansion or contraction had an impact, however infinitesimal, on the balance staff and therefore on the watch’s daily rate. Purists thus looked for a form that would enable the balance spring to work efficiently without causing peripheral disturbances to the system. Some opted for a cylindrical shape, others chose a spherical version. In either case, the objective was to enable the spring to develop as concentrically as possible around the staff and to reduce imbalance, thereby producing the most precise, or least disrupted, oscillation possible. These elevated springs were, however, unsuited to pocket watches that had to be slender enough to suit the day’s close-fitting fashions. Watchmakers such as John Arnold, and later his son John Roger, and Abraham-Louis Breguet worked on flat balance springs with a terminal curve or overcoil attaching the spring at its inner and outer ends. This ensured optimal concentricity for the 12 or 15 coils in an Archimedian spiral spring.
There followed a significant improvement in the precision of quality watches. In 1860, an engineer by the name of Edouard Phillips at Ecole Polytechnique in France established a simple relation between the period of oscillation, the balance’s moment of inertia, and the length and yield moment of the balance spring. He then derived the ideal shapes of terminal curves that would allow the balance spring to maintain its centre of gravity at the centre of the spring, i.e. theoretically on the balance staff. This way, there would be no pressure on the pivots in contact with the balance jewels while reducing friction at the tip of the said pivot (in theory, and for a watch in a flat position). Brands continue to use these curves in their quality watches on balance springs which are no longer fashioned from drawn, laminated and blued crucible steel, but from a complex, energy-efficient and anti-magnetic alloy.
Twentieth-century balance springs
Until the early years of the twentieth century, good-quality balance springs were made from flame-blued steel and mounted on a balance with a bimetallic rim in brass and steel, with gold or brass setting screws. These were followed by the first balance springs made from a nickel-iron alloy (36% nickel and 64% iron) invented by Charles-Edouard Guillaume and known as Invar (“invariable”), due to its relative anti-magnetic property and low coefficient of thermal expansion. These new balance springs were combined with monometallic balance wheels (bronze-beryllium) which were less costly to manufacture than their bimetallic cousins and resistant to magnetic fields.
Dr Reinhard Straumann (Precision Engineering) also developed various metal alloys in the early 1930s that would improve balance spring quality. These impressive combinations of infinitely small amounts of up to seven pure elements would give rise to the modern balance spring. Despite its simple appearance, this new balance spring is exceptionally complex to produce and requires a significant investment in precision tools, hence why so few companies have brought production in-house; a very small number of specialised companies continue to have a virtual monopoly on balance spring manufacturing.