Pocket Watch - Adjusted Movements

Adjusted Movements

Pocket watch movements are occasionally engraved with the word "Adjusted", or "Adjusted to n positions". This means that the watch has been tuned to keep time under various positions and conditions. There are eight possible adjustments:

• Dial up.
• Dial down.
• Pendant up.
• Pendant down.
• Pendant left.
• Pendant right.
• Temperature (from 34–100 degrees Fahrenheit).
• Isochronism (the ability of the watch to keep time, regardless of the mainspring's level of tension).

Positional adjustments are attained by careful poising (ensuring even weight distribution) of the balance-hairspring system as well as careful control of the shape and polish on the balance pivots. All of this achieves an equalization of the effect of gravity on the watch in various positions. Positional adjustments are achieved through careful adjustment of each of these factors, provided by repeated trials on a timing machine. Thus, adjusting a watch to position requires many hours of labor, increasing the cost of the watch. Medium grade watches were commonly adjusted to 3 positions (dial up, dial down, pendant up) while high grade watches were commonly adjusted to 5 positions (dial up, dial down, stem up, stem left, stem right) or even all 6 positions. Railroad watches were required, after 1908, to be adjusted to 5 positions. 3 positions were the general requirement before that time.

Early watches used a solid steel balance. As temperature increased, the solid balance expanded in size, changing the moment of inertia and changing the timing of the watch. In addition, the hairspring would lengthen, decreasing its spring constant. This problem was initially solved through the use of the compensation balance. The compensation balance consisted of a ring of steel sandwiched to a ring of brass. These rings were then split in two places. The balance would, at least theoretically, actually decrease in size with heating to compensate for the lengthening of the hairspring. Through careful adjustment of the placement of the balance screws (brass or gold screws placed in the rim of the balance), a watch could be adjusted to keep time the same at both hot (100°F) and cold (32°) temperatures. Unfortunately, a watch so adjusted would run slow at temperatures between these two. The problem was completely solved through the use of special alloys for the balance and hairspring which were essentially immune to thermal expansion. Such an alloy is used in Hamilton's 992E and 992B.

Isochronism was occasionally improved through the use of a stopworks, a system designed to only allow the mainspring to operate within its center (most consistent) range. The most common method of achieving isochronism is through the use of the Breguet overcoil. which places part of the outermost turn of the hairspring in a different plane from the rest of the spring. This allows the hairspring to "breathe" more evenly and symmetrically. Due to the difficulty with forming an overcoil, modern watches often use a slightly less effective "dogleg", which uses a series of sharp bends to place part of the outermost coil out of the way of the rest of the spring.