The ratio of the potential difference between the ends of a conductor to the current flowing through it is constant. This constant is the resistance of the conductor, i.e. V = IR, where V is the potential difference in volts, I is the current in amperes, and R is the resistance in ohms. The law was discovered in 1827 by Georg Ohm. Most materials do not obey this simple linear law; those that do are said to be ohmic but remain so only if physical conditions, such as temperature, remain constant. Metals are the most accurately ohmic conductors.