File:DifferentialChordalResistance.svg

English: The IV curve of a nonlinear (non-ohmic) electrical device (purple), showing the difference between static and differential resistance. The curve shows the current through it (I) for any given voltage (V). In a non-ohmic device, the resistance is not constant but varies depending on the voltage across it. At any point A two types of resistance can be defined. "Static resistance" (also called DC or chordal resistance) R_static = V/I is the inverse slope of line B from the point A through the origin. The "differential resistance" (also called incremental or small-signal resistance) R_diff = dV/dI is the inverse slope of the line C tangent to the curve at A.