Page:Popular Science Monthly Volume 90.djvu/739

 Popular Science Monthly

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���The framework removed to reveal the train, escapement and hairspring of a watch

The hairspring Is never-ceasing in its vigilance. If you move suddenly you may shorten the swing of the balance; but the hairspring instantly checkmates this by reducing the speed in exact proportion to the reduction in the extent of motion. Again, the balance may have its journey lengthened above the average by an increase of power, or by an external motion (as when you happen to give the watch a "twisting" motion). In this case, the hairspring instantly increases the speed of the balance in direct proportion to the increase of the extent in motion. Now, you understand how the hairspring helps to make good time- keeping possible, by making the balance "space off" portions of time which are equal under all conditions. In cheap watches the hairspring is not "adjusted to isochronism." In better grades the isochronal (uniform timing) adjustment is secured by forming the hairspring's outer coil into a certain shape, found for each watch individually by experiment and trial. The practical application of isochronal adjustment requires high intelligence and skill on the part of the watchmaker. Yet no watch keeps jjerfect time constantly

��with the changes in the hairspring at different temperatures. Cheap watches are not adjusted to temperatures.

Fine watches are put through a series of adjustments to positions, in addition to those for isochronism and temperatures. In position-adjusting, the mechanism of the watch is altered where necessary so that the frictions affecting the moving parts, hence the timekeeping, are alike whether the watch is run lying down or hanging, or in any other position.

Why Has a Good Watch Jewels?

What are the advantages of jewels in a watch? In the early days of watchmaking, the bearings for the pivots of the moving

��parts were simply holes drilled in the metal of the framework. This is still the case in cheap watches. But good modern watches have the bearings made by drilling holes in hard ruby or sapphire disks. These disks are mounted in the framework, and in their highly polished holes the pivots turn with much less friction than in metal bearings. A first-class watch can be expected to keep time to within a half minute a week, while in use by an active person. Worn by a sedate, careful person, a good watch may run even better. It then remains for the owner to have the good timekeeping quality of his watch maintained by having it cleaned and oiled by a reliable watchmaker, at least every two years.

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