Before looking at the Montres KF Spirograph Sport, let’s dive deeper into marine chronometers. Two prominent features contribute to keeping marine chronometers so incredibly precise. First and foremost is their use of massive balance wheels. These balance wheels are both heavier and larger, compared to what we are normally see in wristwatches. Large adjustable weights on the balance wheel rim further increase the moment of inertia. The larger the moment of inertia of a balance wheel, the more stable its oscillations. Small shocks or bumps won’t disturb it as much as they would if the moment of inertia were less. Second, the balance wheels we see on marine chronometers are usually split, as in not complete and solid circles. This type of balance is referred to as a compensation balance. The compensation balance is a sandwich of steel and brass, and it changes its diameter as temperature changes, to compensate for the temperature-induced changes in the elasticity of the steel balance springs then in use. As temperature increases, a watch with a plain steel balance spring will tend to lose time; a compensation balance’s outer arms move inward as temperature increases, which tends to cause a gain in time, “compensating” for the change in elasticity of the balance. Such balances were found in marine chronometers but also in high grade pocket watches as well, until the invention of modern Nivarox-type alloys in the 1920s.
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