A duality between the two types of negative resistance is noted. It is demonstrated that with series or parallel ``resonant circuits'' one can form either class by using either a nonlinear choke or nonlinear condenser. This ac characteristic can be converted to a similar dc one by using a bridge rectifier. With such an arrangement one can excite oscillations in a resonant circuit or produce relaxation oscillations. Several quite general oscillatory circuits are described, as are some multistable and counter circuits. The interaction with mechanical systems are considered.

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