In order to make a precise adjustment, the watchmaker requires exact details about the rate. In the past, movements would spend hours being measured against a reference clock and any deviations noted. Today, timing machines use a contact microphone to record the sounds generated by the balance and spring, such as that of the impulse pin making contact with the pallet lever. This way, any deviation in the rate or the amplitude of the balance can be identified within minutes. Modern timing machines measure these parameters with the watch in six different positions: with the dial up and down, and with the crown to the right, left, up and down. “We can’t possibly make allowance for every aspect of the owner’s habits, but we do obtain data that helps with our adjustments,” say Stiewe, explaining the advantages of this method.
If there is a rate error, the balance vibrates too quickly or too slowly, and the watch is either fast or slow. The required correction can be made either by adjusting the effective length of the balance spring or the moment of inertia in the balance rim. In IWC’s 98000 calibres, the active length of the spring is adjusted using the index, which is attached to the balance spring by two tiny pins. Moving the index in one direction or the other causes the balance to vibrate faster or more slowly. Other IWC calibre families feature an indexless balance. Their frequency is adjusted using four weight screws on the balance rim. Screwing them outwards causes the vibrations to slow. Screwing them inward speeds up the rim. “Depending on the position of a rating error, a specialist knows where and what to turn,” says Stiewe.