A Treatise on Staff Making and Pivoting | Page 4

hole has a like failing, which is often the case, it
is very evident that this want of truth in both the pivot and hole is very
detrimental to the good going of a watch.
[Illustration: Fig. 4.]
[Illustration: Fig. 5.]
There are two kinds of pivots, known respectively as straight and
conical pivots, but for the balance staff there is but one kind and that is
the conical, which is illustrated in Fig. 4. The conical pivot has at least
one advantage over the straight one, i. e., it can be made much smaller
than a straight pivot, as it is much stronger in proportion, owing to its
shape. All pivots have a tendency to draw the oil away from the jewels,
and particularly the conically formed variety, which develops a strong
capillary attraction. To prevent this capillary attraction of the oil, the
back-slope is formed next to the shoulder, although many persons seem
to think that this back-slope is merely added by way of ornament, to
make the pivot more graceful in appearance. It is very essential,
however, for if too much oil is applied the staff would certainly draw it
away if its thickness were not reduced, by means of the back-slope.
Before leaving the subject of capillarity let us examine the enlarged
jewel in Fig. 5; c is an enlarged pivot, b is the hole jewel and a is the
end stone. We observe that the hole jewel on the side towards the end

stone is convex. It is so made that through capillarity the oil is retained
at the end of the pivot where it is most wanted. It is, in my opinion,
very necessary that the young watchmaker should have at least a fair
understanding of capillarity, and should understand why the end stone
is made convex and the pivot with a back slope. For this reason I will
try and make clear this point before proceeding further. We all know
that it is essential to apply oil to all surfaces coming in contact, in order
to reduce the friction as much as possible, and if the application of oil
is necessary to any part of the mechanism of a watch, that part is the
pivot. Saunier very aptly puts it thus: "A liquid is subject to the action
of three forces: gravity, adhesion (the mutual attraction between the
liquid and the substance of the vessel containing it), and cohesion (the
attractive force existing among the molecules of the liquid and
opposing the subdivision of the mass.)"
We all know that if we place a small drop of oil upon a piece of flat
glass or steel and then invert the same the oil will cling to the glass,
owing to the adhesion of the particles; if we then add a little more to
the drop and again invert, it will still cling, although the drop may be
elongated to a certain degree. This is owing to the cohesion of the
molecules of the oil, which refuse to be separated from one another. If,
however, we again add to the drop of oil and invert the plate the drop
will elongate and finally part, one portion dropping while the other
portion clings to the main body of the liquid. The fall of the drop is
occasioned by gravity overcoming the cohesion of the molecules. Now
take a perfectly clean and polished needle and place a drop of oil upon
its point and we will see that the oil very rapidly ascends towards the
thicker portion of the needle. Now if we heat and hammer out the point
of the needle into the form of a small drill and repeat the operation we
find that the oil no longer ascends. It rises from the point to the extreme
width of the drill portion, but refuses to go beyond. It clings to that
portion of the needle which would correspond to the ridge just back of
the slope in a conical pivot. Water, oil, etc., when placed in a clean
wine glass, do not exhibit a perfectly level surface, but raise at the
edges as shown at a in Fig. 6. If a tube is now inserted, we find that the
liquid not only rises around the outside of the tube and the edges of the
vessel, but also rises in the tube far beyond its mean level, as shown at

b. These various effects are caused by one of the forces above
described, i. e., the adhesion, or mutual attraction existing between the
liquid and the substance of the vessel and rod. The word capillarity is
of Latin derivation, and signifies hair-like slenderness. The smaller the
tube, or the nearer the edges of a vessel are brought together, the higher
in proportion will the liquid rise above