Resonance (electronics), in electric circuits, condition in a circuit in which the combined impedances of the capacity and induction to alternating currents cancel each other out or reinforce each other to produce a minimum or maximum impedance (see Electric Circuit; Electricity).

Resonance occurs at a given frequency, called the resonant frequency, for each circuit, depending upon the amounts of inductance and capacitance in the circuit (see Capacitor). If an alternating voltage of the resonant frequency is applied to a circuit in which capacity and inductance are connected in series, the impedance of the circuit drops to a minimum and the circuit will conduct a maximum amount of current. When the capacitance and inductance are connected in parallel, the opposite is true: The impedance is extremely high and little current will pass. Resonant circuits are used in electric equipment, such as filters, to select or reject currents of specific frequencies. Filters of this type, in which either the capacity or the inductance of the circuit can be varied, are used to tune radio and television receivers to the frequency of the transmitting station so that the receiver will accept that frequency and reject others. See Electronics.

Resonance can also occur in mechanical, structural, and acoustical systems. If a system is excited by the continued application of external forces at the natural frequency, which is the frequency at which the system would vibrate if deflected once and then allowed to move freely, the amplitudes of the oscillation will build up and may lead to the destruction of the device or structure. The collapse of the Tacoma Narrows suspension bridge at Puget Sound, Washington, in 1940, for example, was caused by wind-excited vibrations at the natural frequency of the structure.

On the other hand, vibrations that are not at the natural frequency or its harmonics (the integer multiples of the natural frequency), tend to die out rapidly. A violin string, for example, is excited over a wide range of frequencies by the drawn bow. Only the basic frequency of the actual note heard and the various harmonics, which have a smaller amplitude, however, will persist at an audible level. Resonance of a structure is usually avoided by changing its stiffness or its mass. Increasing the stiffness increases the natural frequency, and increasing the mass decreases the frequency.

Resonance phenomena also occur in atomic and nuclear physics; for example, electromagnetic radiation at certain frequencies can excite atoms to higher energy levels while they remain unaffected by nonresonant radiation.