A Treatise on Staff Making and Pivoting | Page 4

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