What could be so special about a wheel turning back and forth, and a spring, returning it back each time? Why have legendary scientists dedicated hundreds of experiments and lots of theoretical research to this miniature couple? It turns out, that accuracy of any watch depends on the nuances of their work.
From the dozens of essential movement components ensuring the accurate timekeeping of your watch, the crucial part still goes to the balance wheel and the balance spring. Returning back the energy from the escapement, they function similar to a pendulum in a clock, forming the classical harmonic oscillator – when being off from its equilibrium position, it experiences a restoring force proportional to the displacement.
As a rule, the measurement abilities of any watch directly depends on the operational speed of the balance. Nowadays the most commonly used frequency is 28800 vibrations per hour or 4 Hz, which allows the accuracy of 1\8 of a second. However, this does not mean that stumbling into a 3 or 5-Hz figure when browsing the specs of a timepiece, one should immediately go on to the other catalog page. Any variant has its pros and cons: less speed usually allows more efficient energy use, and therefore provides a movement with additional power reserve. The faster one, on the contrary, spares no efforts in pursuit of the result up to 1/10 of a second and even more.
The balance wheel has been used in horology since the XIV century and originally it carried out its duties alone, returning back to the starting position due to the inertia of the escapement. The English scientist Robert Hooke equipped it with a metal spring in the mid XVII century, and the Dutch scientist Christian Huygens developed the idea further, by giving it the spiral shape and thus significantly increasing its effectiveness.
However, once engineering surpasses one fundamental issue, there’s usually another problem arising on the horizon. The balance turned out to be extremely sensitive to temperature: during the long-runs or external influences, the system behaved unstable. The solution was found thanks to French horologist Pierre Le Roy British watchmakers John Arnold and Thomas Earnshaw. Since metal changes it’s size when being heated or cooled, choosing the right materials, can at least make the process controllable.
The solution was found in bimetallic outer ring construction. When made half in steel and the other half in bronze, it bends in both directions following the temperature rises or falls. As a result, the inertia either decreases, or increases, compensating an arising error, and the movement accuracy increases significantly. The Swiss physicist and ironically the son of a watchmaker Charles Edouard Guillaume set a serious scintific milestone by presenting the alloy of nickel and iron called invar, which is extremely resistant to changes of temperature. His discovery won him the Nobel Prize in 1920 and had an enormous effect far beyond horology.
The another natural enemy of the balance system is, of course, magnetism. Considering the constantly growing number of electronic devices around us, this threat becomes more and more real every year, and if the Swiss brands have not started to build a defense in advance, who knows whether our favorite industry would exist now. In 1888, IWC created the first anti-magnetic piece, with balance and escapement made of palladium, bronze and gold. Half a century later, with a serial production for military pilots, the specialists of the same Schaffhausen brand came with an idea of placing the movement in a double case. Its inner part was made of soft iron, and thus played the role of a reliable barrier between the outer magnetic fields and the main wheel train. Though being constantly improved, this basic principle remains unchanged to this day.
Another sensible blow to precision comes from gravity. In 1801 Abraham Louis Breguet had to come up with the tourbillon to fight against a similar issues. But as your hand constantly changes its position, a tourbillon in a wristwatches mainly serves the aesthetic purposes. Still there are plenty of ways to stand your mechanical ground without going beyond the traditional wheel train. The increased number of balance wheels is one such way, and at least in theory, quite effective. Working together they correct possible errors of each other and transmit energy as evenly as possible. Needless to say, that such a construction looks really fascinating in action, think Audemars Piguet Royal Oak Openworked Double Balance Wheel with an automatic caliber 3132 or Arnold & Son DBG Skeleton, with double balances as well as a double wheel train powered from two independent barrels.
Hi-tech materials like silicon can also work wonders: because of its elasticity, silicon balance wheel stops earlier than its metal counterpart, makes the necessary micromotions, and gets back just due to the natural inertia. By the way, the Freak collection by Ulysse Nardin, which marked the arrival of silicon balances in the early noughties, is still produced, and is always in demand.