 This proportionality is called Ohm's law, and materials that satisfy it are called "ohmic" materials.In other cases, such as a transformer, diode or battery, "V" and "I" are not directly proportional.The ratio "V" over "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.