Resonance experiment. Barton's pendulums, used to demonstrate coupling and resonance, seen from the side, swinging from side to side. At centre (thick line) is a heavy pendulum,the driver pendulum. It is coupled (by connecting strings, not seen) to the other pendulums, which are lighter and vary in length (a pendulum's natural frequency). When the driver pendulum is set moving, its weight makes the other pendulums move at the driver frequency. The pendulum with the nearest frequency (length) resonates with the driver pendulum, receives the most energy from it and so swings with the largest amplitude (seen just below the driver pendulum). For another view of this apparatus, see A400/056.