Assuming, however, that the motion is good in one position and drops off in the other, it is quite probable that only an ordinary position correction will be required and the immediate problem to be considered is that of causing the short arc of motion to accelerate enough to equal the longer arc. The precise correction required will most probably be found among the causes listed in No. 46, this Chapter.
52. _Which Rate to Use as the Unit for Comparison._
The horizontal position which has the slower rate of the two should be considered as the unit which is correct and it will always have the longer motion of the two, barring the occasional exception as described in No. 50.
This longer arc of motion is universally due to a better condition, while the shorter motion indicates that something is wrong, and it should always be the aim of the adjuster to improve some condition that is below standard, rather than to make some good condition a little worse in order to equalize the rates.
It may be possible to equalize horizontal rates by flattening the ends of pivots, but it does not require much more time to improve the motion in one position than it does to make it a little worse in another. The advantage is all one way and results either good or bad depend entirely upon the viewpoint of the worker and how he applies himself to the situation.
53. _Damaged Pivots, Pitted End Stones and Methods of Correction._
In the examination of pivots, end stones and jewels, it is necessary to use a stronger glass than the one used for ordinary work.
Damaged pivots can often be detected by looking through the end stone with a strong glass while the balance is moving. If imperfect they will appear dark or display a slight waver or flash and if they are in good condition they will appear bright and seem to stand still. They can also be examined in the lathe and a good true enclosed balance chuck is of immense value in detecting burrs, chipped edges, rings on the sides, slight bends and poorly shaped ends. The complete balance and spring can be inserted and the pivots can be refinished without disturbing the roller or hairspring. The chuck should be revolving very slowly when making the examination and moving the belt with the hand will enable one to see more than can be seen when the lathe is running at regular speed. Some watchmakers use small bow lathes for examining and finishing pivots, or the Jacot lathe, which is excellent for this kind of work. An end stone that has been deeply pitted should always be discarded and a new one supplied. If the hole is very slight, however, it can be removed entirely and the surface of the stone re-polished on a lap charged with No. 5 diamond powder, but the stone and setting should be thoroughly cleansed by brushing and pithing before replacement.