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) Rstatic = 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) Rdiff = dV/dI is the inverse slope of the line C tangent to the curve at A.
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