If a rod of moment of inertia I and mass m is hung from two threads of length L and separated by a distance d (Figure 1) and is oscillated in a horizontal plane about a vertical axis through its centre, then the period (T) is given by:

This arrangement is
called a bifilar suspension because the rod is supported by two threads.

It can be
used to calculate the value of the acceleration due to gravity (g) if the moment of inertia of
the rod is calculated.

For a rod being oscillated about its centre the moment of inertia (I) is: I = ML