A bob is the mass on the end of a

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the ...

found most commonly, but not exclusively, in

pendulum clock A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: It swings back and forth in a precise time interval dependent on it ...

Reason for use

Although a pendulum can theoretically be any shape, any rigid object swinging on a

pivot Pivot may refer to: *Pivot, the point of rotation in a lever system *More generally, the center point of any rotational system *Pivot joint, a kind of joint between bones in the body *Pivot turn, a dance move Companies *Incitec Pivot, an Austra ...

, clock pendulums are usually made of a weight or ''bob'' attached to the bottom end of a rod, with the top attached to a pivot so it can swing. The advantage of this construction is that it positions the

centre of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

close to the physical end of the pendulum, farthest from the pivot. This maximizes the moment of inertia, and minimises the length of pendulum required for a given

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. Shorter pendulums allow the clock case to be made smaller, and also minimize the pendulum's

air resistance In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...

. Since most of the energy loss in clocks is due to air friction of the pendulum, this allows clocks to run longer on a given power source.

Use in clocks

Traditionally, a clock pendulum bob is a round flat disk, lens-shaped in

section Section, Sectioning or Sectioned may refer to: Arts, entertainment and media * Section (music), a complete, but not independent, musical idea * Section (typography), a subdivision, especially of a chapter, in books and documents ** Section sig ...

, to reduce its

aerodynamic drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fl ...

, but bobs in older clocks often have decorative carving and shapes characteristic of the type of clock. They are usually made of a dense metal such as iron or

brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...

Lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

is denser, but is usually avoided because of its softness, which would result in the bob being dented during its inevitable collisions with the inside of the clock case when the clock is moved. In most pendulum clocks the rate is adjusted by moving the bob up or down on the pendulum rod. Moving it up shortens the pendulum, making it beat more quickly, and causing the clock to gain time. In the most common arrangement, the bob is attached to the pendulum with an adjustment nut at the bottom, on the threaded end of the pendulum rod. Turning the nut adjusts the height of the bob. But some bobs have levers or dials to adjust the height. In some precision clocks there is a smaller auxiliary weight on a threaded shaft to allow more fine adjustment.

Tower clock Clock towers are a specific type of structure which house a turret clock and have one or more clock faces on the upper exterior walls. Many clock towers are freestanding structures but they can also adjoin or be located on top of another buildi ...

s sometimes have a tray mounted on the pendulum rod, to which small weights can be added or removed, to adjust the rate without stopping the clock. The weight of the bob itself has little effect on the

of the pendulum. However, a heavier bob helps to keep the pendulum moving smoothly until it receives its next push from the clock's

escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

mechanism. That increases the pendulum's Q factor, making the motion of the pendulum more independent of the escapement and the errors it introduces, leading to increased accuracy. On the other hand, the heavier the bob is the more energy must be supplied by the clock's power source and more friction and wear occurs in the clock's movement. Pendulum bobs in quality clocks are usually made as heavy as the clock's movement can drive. A common weight for the bob of a one second pendulum, widely used in

grandfather clock A grandfather clock (also a longcase clock, tall-case clock, grandfather's clock, or floor clock) is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower or waist of the case. Clocks of this style are common ...

s and many others, is around 2 kilograms.

References

{{reflist Pendulums

Reason for use

Use in clocks

See also

References