Adjustment
The manipulation of the balance, its spring and staff, for the purpose of improving the time keeping qualities of a watch. Three adjustments are usually employed for this purpose, viz.: positions, isochronism and compensation.
Adjustment to Positions. The manipulation of the hairspring and balance so that the movement keeps time in the different positions. In ordinary watches two positions are taken, viz.: pendant up or vertical and dial up or horizontal. In the finer grade of work adjustments are made in the quarters, that is, with 3 up and 9 up. This adjustment is a delicate and often a difficult operation and it is only by constant study and application that the watchmaker can hope for success. Several excellent essays on this subject are in print, among which may be mentioned Modern Horology in Theory and Practice and the Watchmaker's Hand Book by Claudius Saunier, the Watch Adjuster's Manual by Charles Edgar Fritts, and Adjustments to Positions, Isochronism and Compensation, published by G. K. Hazlitt & Co., Chicago. Isochronal adjustments are thoroughly reviewed in an excellent little work by Moritz Immisch entitled Prize Essay on the Balance Spring. The object of timing or adjusting to positions is to ascertain how far a change of position modifies the compensation and isochronism and to verify the poising of the balance. Saunier says the balance can not possibly be accurately poised in all positions if the pivots and pivot holes are not perfectly round, and the poising will be modified with a change of temperature if the two arms do not act identically; as will be the case when the metals are not homogeneous, when one or both arms have been strained owing to want of skill on the part of the workman, or careless work, etc. After accurately timing in a vertical position with XII. up, make it go for twelve hours with VI. up and the same number of hours with III. and Ix. up. Observe with care both the rates and the amplitude of the arcs and note them down. Assuming the pivots and pivot holes to be perfectly round and in good condition and that the poising of the balance has been previously tested with care by the ordinary means, if the variations in the four positions are slight the poising may be regarded as satisfactory. As a general, but not invariable rule, a loss in one position on the rate observed in the inverse position may be taken to indicate that the weight of the upper part of the balance is excessive when it does not vibrate through an arc of 360 deg. or the lower part if the arcs of motion exceed this amount. Independently of the balance this loss may be occasioned by excessive friction of the pivots due to a too great pressure owing to the caliper being faulty, or to a distortion of the hairspring causing its center of gravity to lie out of the axis of the balance. If these influences become at all considerable their correction will be beyond the power of the isochronal hairspring, and indeed it will be impossible to counteract them. Changes in the rate on changing from the vertical to the horizontal position may also arise from the following causes: I. The action of the escape wheel, which is different according as it tends to raise the balance staff or to force it laterally. 2. A hairspring that starts to one side and so displaces its center of gravity, a balance that is not well poised, pivots or pivot holes that are not perfectly round, faults which although of but little importance in the vertical position of the balance staff become serious when it is horizontal. 3. The more marked portion of the friction of the pivots may take place against substances of different degrees of hardness in the two cases, the end stones being frequently harder than the jewels. Saunier further says that satisfactory results will be obtained in most cases by employing the following methods, either separately or two or more together, according to the results of experiments or the rates, the experience and the judgment of the workman: I. Flatten slightly the ends of the balance pivots so as to increase their radii of friction; when the watch is lying flat the friction will thus become greater. 2. Let the thickness of the jewel holes be no more than is absolutely necessary. It is sometimes thought sufficient to chamfer the jewel hole so as to reduce the surface on which friction occurs; but this does not quite meet the case since an appreciable column of oil is maintained against the pivot. 3. Reduce the diameters of the pivots, of course changing the jewel holes. The resistance due to friction, when the watch is vertical, increases rapidly with any increase in the diameters of pivots. 4. Let the hairspring be accurately centered, or it must usually be so placed that the lateral pull tends to lift the balance when the watch is hanging vertical. In this and the next succeeding case it would sometimes be advantageous to be able to change the point at which it is fixed, but this is seldom possible. 5. Replace the hair-spring by one that is longer or shorter but of the same strength; this is with a view to increase or diminish the lateral pressure in accordance with the explanation given in the last paragraph. 6. Set the escapement so that the strongest impulse corresponds with the greatest resistance of the balance. 7. Replace the balance. A balance that is much too heavy renders the timing for positions impossible. 8. Lastly, when these methods are inapplicable or insufficient there only remains the very common practice of throwing the balance out of poise. Adjustment to Isochronism. The manipulation of the hair-spring so that the long and short arcs of the balance are performed in the same time. The theory of isochronism advanced by Dr. Robert Hooke and more commonly known as Hooke's law, "as the tension so is the force," is an axiom in mechanics with which everybody is, or should be familiar. This law has like nearly all others its exceptions, and it is only partially true as applied to hair-springs of watches; "otherwise," says Glasgow, "every spring would be isochronous." Pierre Le Roy says that there is in every spring, of a sufficient extent, a certain length where all the vibrations, long or short, great or small, are isochronous, and that this length being secured, if you shorten the spring the great vibrations will be quicker than the small ones; if, on the contrary, it is lengthened, the small arcs will be performed in less time than the great ones. Glasgow says that a hair-spring of whatever form to be isochronous must satisfy the following conditions: Its center of gravity must always be on the axis of the balance, and it must expand and contract in the vibrations concentrically with that axis. When these conditions are secured in a properly made spring it will possess the quality of isochronism, that is, its force will increase in proportion to the tension, and it will not exert any lateral pressure on the pivots.
Britten says, it should be remembered that if the vibrations of a balance are to be isochronous the impulse must be delivered in the middle of its vibration, and that therefore no spring will be satisfactory if the escapement is defective in this particular.
The recognized authorities conflict considerably in their various theories in regard to adjustment to isochronism and particularly in regard to the length of sprig. Immisch says that mere length has nothing to do with isochronism. Glasgow contends that length has everything to do with it, and that a spring too short, whatever its form, would make the short arcs of the balance vibration be performed in a less time than the long arcs, and a spring too long would have just the contrary effect. Charles Frodsham advanced the theory that every length of spring has its isochronous point. Britten declares the length is all important; that a good length of spring for one variety of escapement is entirely unfitted for another variety. Saunier says that the discussion of the question whether short springs are preferable to long ones is a mere waste of time and can result in no good. In horology everything must be relative. Whatever be the escapement under consideration, it requires neither a long nor a short hair-spring, but one that is suited to its nature and mode of action, that is to say, the length must bear a definite relation to the extent of the arcs of vibration, etc.
Owing to the conflict of opinion it is advisable that the student read the various arguments set forth in the works referred to above and form his own conclusions.