In other cases, such as a transformer, diode or battery, V and I are not directly proportional. The ratio ⁠ V / I ⁠ is sometimes still useful, and is referred to as a chordal resistance or static resistance, since it corresponds to the inverse slope of a chord between the origin and an I – V curve. In other situations, the derivative d V d I may be most useful; this is called the differential resistance.