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

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3² = 9.

39.1393×9=352.2537inches =29.3544feet.

A pendulum beating two-thirds of a second, or 90 beats:

(⅔)²=⁴⁄₉

39.1393×4

—————=17.3952inches.

9

A pendulum beating half-seconds or 120 beats:

(½)²=¼

39.1393×1

——————=9.7848inches.

4

Center of Oscillation.—Having now briefly considered the basic facts governing the time of oscillation of the pendulum, let us examine it still further. The pendulum shown in ssss1 has all its weight in a mass at its end, but we cannot make a pendulum that way to run a clock, because of physical limitations. We shall have to use a rod stiff enough to transmit power from the clock movement to the pendulum bob and that rod will weigh something. If we use a compensated rod, so as to keep it the same length in varying temperature, it may weigh a good deal in proportion to the bob. How will this affect the pendulum?

If we suspend a rod from its upper end and place along-side of it our ideal pendulum, as in ssss1, we shall find that they will not vibrate in equal times if they are of equal lengths. Why not? Because when the rod is swinging (being stiff) a part of its weight rests upon the fixed point of suspension and that part of the rod is consequently not entirely subject to the force of gravity. Now, as the time in which our pendulum will swing depends upon the distance of the effective center of its mass from the point of suspension, and as, owing to the difference in construction, the center of mass of one of our pendulums is at the center of its ball, while that of the other is somewhere along the rod, they will naturally swing in different times.