Resonance experiment. Barton's pendulums, used to demonstrate coupling and resonance. At far left is a heavy pendulum, called the driver pendulum. It is coupled (by the connecting string) to the other pendulums, which are lighter and vary in length (a pendulum's natural frequency). The apparatus is viewed from the left and the driver pendulum set swinging across the line of sight. The weight of the driver pendulum forces all the pendulums to swing at the driver frequency, but the pendulum with the same frequency (length) resonates with the driver pendulum, receives the most energy from it, and so swings with the largest amplitude. For a view from the side, see A400/113.