Fermi eased into the subject matter by comparing an atom and the electromagnetic field to a pendulum and a vibrating string connected by a thin elastic thread, which represented the coupling of the two. If the pendulum is at rest and the string starts to vibrate only slightly, the elastic thread will perturb the pendulum only slightly. But when the string vibrates in tune with the amplitude of the pendulum, the elastic thread carries that energy to the pendulum and causes the pendulum to swing in time with the vibrating string, creating a resonance between the pendulum and the string.
