Theoretical pendulum

Pendulum \Pen"du*lum\, n.; pl. {Pendulums}. [NL., fr. L.
   pendulus hanging, swinging. See {Pendulous}.]
   A body so suspended from a fixed point as to swing freely to
   and fro by the alternate action of gravity and momentum. It
   is used to regulate the movements of clockwork and other
   machinery.
   [1913 Webster]

   Note: The time of oscillation of a pendulum is independent of
         the arc of vibration, provided this arc be small.
         [1913 Webster]

   {Ballistic pendulum}. See under {Ballistic}.

   {Compensation pendulum}, a clock pendulum in which the effect
      of changes of temperature of the length of the rod is so
      counteracted, usually by the opposite expansion of
      differene metals, that the distance of the center of
      oscillation from the center of suspension remains
      invariable; as, the {mercurial compensation pendulum}, in
      which the expansion of the rod is compensated by the
      opposite expansion of mercury in a jar constituting the
      bob; the {gridiron pendulum}, in which compensation is
      effected by the opposite expansion of sets of rods of
      different metals.

   {Compound pendulum}, an ordinary pendulum; -- so called, as
      being made up of different parts, and contrasted with
      {simple pendulum}.

   {Conical pendulum} or {Revolving pendulum}, a weight
      connected by a rod with a fixed point; and revolving in a
      horizontal circle about the vertical from that point.

   {Pendulum bob}, the weight at the lower end of a pendulum.

   {Pendulum level}, a plumb level. See under {Level}.

   {Pendulum wheel}, the balance of a watch.

   {Simple pendulum} or {Theoretical pendulum}, an imaginary
      pendulum having no dimensions except length, and no weight
      except at the center of oscillation; in other words, a
      material point suspended by an ideal line.
      [1913 Webster]