Maintained oscillations

Left to itself, a damped oscillator's amplitude keeps shrinking until it eventually stops -- a child on a swing, for instance, slows down after a few cycles purely because of damping, unless somebody keeps giving it a push. If a source of energy from outside supplies, once every cycle, exactly the amount of energy that damping has removed, the amplitude of oscillation can be held constant indefinitely instead of decaying. Such oscillations, sustained at constant amplitude by a compensating external energy supply (rather than being genuinely free of dissipation), are called maintained oscillations. A pendulum clock is a classic real-world example: a small periodic impulse delivered through the escapement mechanism once per swing exactly replaces the …