By HAROLD C. KELLY
Member National Technical Board, U. H. A. of A.
A watch may be mechanically perfect, that is, its construction from barrel to balance may be as exact as human skill knows how to make it; and yet, in spite of such perfection there will be a variation of from fifteen to thirty seconds in twenty-four hours between some two vertical positions due to the condition of the balance spring alone. In watches that are less perfect, the error is frequently as high as forty seconds or more.
Action of the Balance Spring and Its Poise Error-The oscillation of the inner portion of the spring corresponds very nearly to that of the collet, that is, when the collet travels three-fourths of a circle the first coil in the center travels nearly an equal distance. Thus it is evident that each of the several coils, as they tend to become more distant from the center, will travel a shorter path until the movement ceases altogether at the regulator pins. If the coils are marked in a straight line from collet to pins, we would readily observe the distance traveled by the several coils and the extent of their path under different arcs of motion.
It is impossible to poise a spiral spring. Therefore it is at once evident that it is the unpoised inner portion of the spring when acted upon by the pull of gravity that causes position error in the vertical positions. The effect is similar to that of a balance which is out of poise. Slightly varied effects under different arcs of motion result, however, due to the fact that the greater mass of the unpoised inner portion of the spring vibrate a shorter path than does the balance proper.
Experimental Demonstration - A demonstration entirely at the command of every watchmaker is to take several watches and run them, first with the figure one up and following with the figures two, three, four, etc.; continuing the experiment around the dial with all figures up, running the watches in each of the twelve positions for twenty-four hours and taking note of the rate in each position.
Table I shows the result in an experiment at stated above, using four popular makes of American watches. All watches were in excellent condition with the balances perfectly poised, fitted with Breguet balance springs and the grades ranged from fifteen to twenty-one jewels.
The arc of motion of the balance of all watches was about 540 degrees when fully wound and a little more than 450 after twenty-four hours of running.
In watch Number 1 the rate was fastest at the time when the figure eleven was up; in watch Number 2 the rate was fastest at the time when the figure three was up. The watches Number 3 and Number 4 had definite fast positions also, and in all watches the slow position was opposite or nearly opposite that of the fast position.