For most conductors, the resistance at a given temperature is almost constant and has a certain value. This type of resistance is called linear resistance.The resistance of some materials varies significantly with the current (or voltage). The volt-ampere characteristic is a curve. This type of resistance is called nonlinear resistance.Under the action of a given voltage (or current), the ratio of voltage to current is the static resistance at the working point, and the slope of the volt-ampere characteristic curve is the dynamic resistance.The expression of nonlinear resistance characteristics is complex, but these nonlinear relations have been widely used in electronic circuits.