The chief requirements of a pivot are that it shall be round and well
polished. Avoid the burnish file at all hazards; it will not leave the
pivot round, for the pressure is unequal at various points in the
revolution. A pivot that was not perfectly round might act fairly well in
a jewel hole that was round, but unfortunately the greater proportion of
jewel holes are not as they should be, and we must therefore take every
precaution to guard against untrue pivots. Let us examine just what the
effect will be if an imperfect pivot is fitted into an unround hole
jewel, and to demonstrate its action more clearly let us exaggerate the
defects. Suppose we pick a perfectly round jewel and insert into the
opening a three-cornered piece of steel wire, in shape somewhat
resembling the taper of a triangular file. We find that this triangular
piece of steel will turn in the jewel with the same ease that the most
perfect cylindrical pivot will. Now suppose we change the jewel for one
that is out of round and repeat the experiment. We now find that the
triangular steel soon finds the hollow spots in the jewel hole and comes
to a stand-still as it is inserted in the hole. The action of a pivot
that is not true, when in contact with a jewel whose hole is out of
round, is very similar, though in a less marked degree. If the pivot
inclines toward the elliptical and the jewel 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
the level. An ascent of a liquid, due to capillarity, also takes place,
where the liquid is placed between two separate bodies, as oil placed
between two pieces of flat glass. If the plates are parallel to one
another and perpendicular to the surface of the liquid it will ascend to
the same height between the plates, as shown at _c_ in Fig. 6. If the
plates were united at the back like a book and spread somewhat at the
front, the oil would ascend the higher as the two sides approach one
another, as shown at _d_, Fig. 6. If a drop is placed somewhat away from
the intersecting point, of the glasses, as shown at _m_ it will, if not
too far away, gradually work its way to the junction, providing the
glasses are level. If, however, the glasses are inclined to a certain
extent, the drop will remain stationary, since it is drawn in one
direction by gravity and in the other by capillarity. When a drop of oil
is placed between two surfaces, both of which are convex, or one convex
and the other plain, as shown at _g_, it will collect at the point _n_,
at which the surfaces nearest approach one another. We now see very
clearly why the hole jewel is made convex on the side towards the
end-stone and concave on the side towards the pivot.
[Illustration: _Fig. 6._]
Particular pains should be taken to polish those portions of the pivots
which actually enter the jewel hole and to see that all marks of the
graver be thoroughly removed, because if any grooves, no matter how
small, are left, they act as minute capillary tubes to convey the oil.
If the hole jewel be of the proper shape, the end-stone not too far from
the hole jewel and too much oil is not applied at one time, the oil will
not spread nor run down the staff, but a small portion will be retained
at the acting surface of pivot and jewel, and this supply will be
gradually fed to these parts from the reservoir between the jewel and
end-stone, by the action of capillarity.
Having examined into the requirements of the pivot and its jewel and
having gained an insight into what their forms should be, we are the
better able to perform that portion of the work in an intelligent
manner.