Pocketwatch 101™ – Basic Information about Vintage and Antique Pocket Watches
What does "Adjusted to Temperature and Positions" mean?
It's not unusual to see a vintage watch movement marked "Adjusted" or "Adjusted to Temp. & Pos." or "Adj. 5 Pos." or some other proclamation of superior "adjusted" quality. What these markings are referring to is the way the watch was made and adjusted for proper time-keeping before it left the factory. Typically, the higher the number of "adjustments" the higher the quality of the watch.
Adjusted to Temperature
Temperature adjustment was designed to keep the watch running at a consistent rate across the widest possible range of temperatures. Since a mechanical watch uses a spring (the balance spring or "hairspring") as its primary time-keeping mechanism, the "stiffness" of that steel spring will vary at different temperatures, thus potentially altering the timekeeping rate of the watch.
A major breakthrough in temperature compensation was the bi-metallic "compensating" balance, typically made out of a layer of steel fused to a layer of brass. This type of compensating balance is also called a "cut-balance" because the balance rim is not continuous; it is "cut" at two points to allow the bi-metallic balance rim to expand and contract slightly with changes in temperature.
Because the brass has a greater coefficient of expansion than does steel, an increase in temperature will cause the "limbs" of the balance to curl inward a tiny amount, thus increasing the rate of the watch. In cold temperatures, when the balance spring is stiffer, the limbs of the balance will curl outward a tiny amount, thus decreasing the rate of the watch as compensation for the increased stiffness of the balance spring. Pretty ingenious! By changing the position of the weight-screws on the balance, a skilled watch adjuster could increase or decrease the amount of temperature compensation over a range of temperatures..
Adjusted to Positions
When a watch runs in different positions, there are different dynamic forces at work on the various moving parts of the watch, in particular the balance and hairspring. When the watch is lying flat on its back, the balance is running primarily on the end of one pivot. When the watch is held in a "vertical" position, say pendant up, then the balance is running on the sides of both pivots, which is an entirely different situation in terms of friction. Gravity treats the delicate mechanism of the watch differently depending on its position, and any poise error in the balance (heavy or light spots) will be evident as positional time-keeping errors. Thus, the goal of positional adjustment was to achieve the greatest possible time-keeping consistency across a range of positions.
The best watches were timed in six positions: DU - dial up, DD - dial down, PU - pendant up, PL - pendant left, PD - pendant down and PR - pendant right. Often five positions were used, typically omitting the PD position. Many factors can affect positional timing: hairspring pinning point, overcoil shape, balance poise and shape of balance pivots are all critical factors in achieving good positional time-keeping. A significant positional error could be a sign of a mechanical problem with the watch.
Adjusted for Isochronism
Isochronism refers to the tendency of a watch to run at different rates when under different amounts of spring tension. When a mainspring is fully wound, it will "pull harder" than it does when it is nearly run down. A watch with higher isochronal error will run at different rate when fully wound than when nearly unwound due to this difference in the pull of the spring.
Isochronism exists in all watches, but is more common in older watches with steel mainsprings and inefficient time-trains. Alloy mainsprings and more efficient, jeweled mechanisms have helped to make watches less susceptible to isochronal error. Most isochronal adjustments must be "designed-in" to the watch... there isn't a lot the watch repairer can do after the fact to make a watch have less isochronal error.
The stop-works and the fusee mechanism were both intended to address the problem of isochronism; the stop-works by limiting the mainspring to its "mid-range" i.e. not allowing either a fully-wound or fully-unwound condition, and the fusee by "equalizing" the pull of the mainspring by passing its power through a stepped fusee-barrel which compensated for the unequal pull of the spring.
Modern Adjustment of Vintage Watches
We are sometimes asked to bring an antique pocket watch back to original factory "adjusted" specifications. If a vintage watch is in excellent condition this may be possible but many times it is not. Watchmakers, or Railroad Time Inspectors, of years past had vastly greater access to spare parts than we do today. If a balance or hairspring wasn't behaving properly, the watchmaker could simply swap it out for a new one. If a jewel was cracked or a winding part showed signs of wear, it could be immediately replaced with a factory original. Thus, the skilled watchmaker of yesterday had a much larger "bag of tricks" in the form of factory-new parts they could draw upon in keeping the watch at original factory specifications. Today, we are usually forced to make the watch the best it can be using its existing parts, and while we often achieve excellent results, we don't claim to achieve "factory adjusted" accuracy on a vintage watch.