Any weight attached to a rod or wire so that it can swing freely
may be called a “pendulum.” But for the purpose of time-keeping,
a much more accurate instrument is required; the rate of vibration
or oscillation of the pendulum, does not depend upon the weight of
the ball or “bob” at the lower end, but upon the distance of this
from the point at which the upper end turns, nor does the rate
of oscillation depend upon the distance through which the weight
traverses, for every pendulum will vibrate at the exact rate (with
certain restrictions) at which it is set off, until it ceases,
although the distance through which it traverses, decreases at
every vibration; these facts are taken advantage of in adapting
the pendulum to the purposes of regulating the time a clock shall
keep--the longer the pendulum the slower the vibrations. Now, as
everything in nature is expanded by heat and contracted by cold,
so a pendulum is constantly varying in length by every change of
temperature, and, as a consequence, the rate of the clock to which
it is attached will also vary. Pendulums which have an arrangement
to obviate this variation, are called “compensating” pendulums;
the best in use are of two kinds, one called (from its appearance)
the “gridiron,” the other the “mercurial,” this last is the most
accurate, and is used in nearly all good astronomical clocks.
The gridiron pendulum is made of iron and brass, or zinc, and is
constructed as shown in fig. 1; the rod and outer frame, A, is
made of iron, the two rods inside this of zinc or brass, B B. The
principle of the instrument is this--brass or zinc contract and
expand much more than iron does, and the short bars of these metals
will expand or contract as much as the long bar of iron forming the
rod of the pendulum, so that as this expands and lets the “bob” down,
the short bars expand and draw it upwards so that it keeps its place
at any temperature; this requires very accurate adjustment. The
mercurial pendulum is shown at fig. 2; it is on the same principle,
but is easier to regulate, and more manageable, the vessel in the
centre being partly filled with mercury, and forming the weight
itself, and thus as the mercury expands upwards it compensates for
the elongation of the rod, the same as in the gridiron pendulum.
The nearer any pendulum is to the centre of the earth the more
quickly does it vibrate; this has been used by scientific men, to
determine by the difference of rate in one placed on a hill, and
another at the bottom of a deep mine, the amount of matter which
constitutes our globe; indeed by these trials the world may fairly
be said to have been weighed!