To demonstrate coupling of motion between two pendula of the same length.
Coupled pendula, as photographed.
The pendula are hung from a rod which can rock back and forth to transfer the motion from one pendulum to another. If you start the pendulum at the left in motion (in and out of the picture), the motion will couple back and forth between the pendula of the same length, leaving the others with only a slight perturbation. It is of interest to note the phase of the two pendula as the motion is transferred back and forth.