In watchmaking, as in life, sometimes words don’t mean what they mean. Books on watch adjusting and the posts about our example watches talk about measuring the watch’s rates at “full wind.” Full doesn’t always mean full, however.
Getting Wound Up About Winding
When adjusting, a “fully wound watch” is perhaps 85% to 95% wound rather than wound up tight. Gideon Thisell, in his book Science of Watch Repairing Simplified (1946), discussed how to use the new-fangled timing machines that were appearing on the scene. He noted:
“A watch should not be tested on the machine when it is wound tight; a more accurate record results if it has run an hour or two” (p. 86).
Over a half-century later, we see similar advice from Witschi, makers of a popular line of equipment for timing and testing watches. In a 2010 training course, they suggest that a watch should first be fully wound, but “before testing, the watch should run for approximately 20 minutes” (p. 53).
Why “Full” Instead of Full?
Why do adjusters measure and modify a watch’s rates in different positions at 90% wind instead of 100% wind? As with many things, it comes down to amplitude. Because the balance wheel’s amplitude is a function of the mainspring’s force, positional timing is affected by the level of wind.
The mainspring’s force varies as a function of the level of winding: it generates progressively more force as it is wound. But this function isn’t perfectly linear. The force at 70% wind, for example, isn’t necessarily twice as high as the force at 35% wind.
Here’s an example of a torque curve from Bowman and Borer’s Modern Watch Repairing and Adjusting (3rd ed., p. 54). It shows how a mainspring’s force declines non-linearly over time:
The X-axis is time since winding; the Y-axis is force generated by the mainspring. The exact shape of the torque curve depends on many things, such as the mainspring’s shape and material, but the key point for now is this:
- the force slope is steeper at very high and very low levels of wind (A to B and C to D)
- the force slope is flatter and more consistent in the middle (B to C)
The watch will spend most of its time in the middle, flatter section of the torque curve. As a result, we should adjust the watch to keep good time across positions in that range.
If we adjust a watch to 5 positions when it is fully wound, 100% tight, it will keep the excellent time we tuned it to keep when it is fully wound, 100% tight. But after an hour, the mainspring is already generating notably less force, so the amplitude will be lower. And lower amplitude means different effects of small poise errors on timing.
Unless the watch is nearly always close to 100% wind—something that can happen if people compulsively wind their watches throughout the day—it won’t spend much time in the state in which it was adjusted. Adjusting in the flat part of the torque curve, where the watch will spend most of its time, means the rates on the wrist will resemble the rates on the timing machine.
(As an aside, this is why self-winding wristwatches were such an important innovation. The watch keeps better, more consistent time when it operates at a consistent and high level of wind. The convenience is incidental.)
So that is the short (or long, depending on your patience) version of why we adjust at less than 100%—“full” but not full wind. When measuring its rates and adjusting its poise, just fully wind the watch tight and then let the mainspring barrel unwind a bit, equivalent to roughly an hour or so of running time.