Читать книгу The Modern Clock. A Study of Time Keeping Mechanism; Its Construction, Regulation and Repair онлайн

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We shall first need to know the total length of our pendulum, so that we can calculate the expansion of the rod. A seconds pendulum measures 39.2 inches from the point in the suspension spring at the lower edge of the chops to the center of oscillation. With a lens-shaped bob the center of gravity will be practically at the center of the bob, if we use a light wooden rod and a steel adjusting screw and brass nut, as these metal parts, although short, will be heavy enough to nearly balance the suspension spring and that portion of the rod which is above the center. We shall also gain a little in balance if we leave the steel screw long enough to act as an index over the degree plate, in the case, at the bottom of the pendulum, by stripping the thread and turning the end to a taper an inch or so in length.

We shall only be able to use one-half of the expansion upwards of our bob, because the centers of gravity and oscillation will be practically together at the center of the bob. We shall find the center of gravity easily by balancing the pendulum on a knife edge and thus we will be able to make an exceedingly close guess at the center of oscillation.