Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I = V / R where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor. More specifically, Ohm's law states that the R in this relation is constant, independent of the current. If the resistance is not constant, the previous equation cannot be called Ohm's law, but it can still be used as a definition of static / DC resistance. Ohm's law is an emperical relation which accurately describes the conductivity of the vast majority of electically conducive materials over many orders of magnitude of current. However, some materials do not obey Ohm's law; these are called non-ohmic. The law was named after the German physicist