Relationship between the potential difference (voltage), electric current , and resistance in an electric circuit .
In 1827 Georg Simon Ohm discovered that at constant temperature, the current I in a circuit is directly proportional to the potential difference V , and inversely proportional to the resistance R , or I = V / R . Resistance is generally measured in ohms ( 03A9; ). Ohm's law may also be expressed in terms of the electromotive force E of an electric energy source, such as a battery, or E = I R . In an alternating-current circuit, when the combination of resistance and reactance, called impedance Z , is constant, Ohm's law is applicable and V / I = Z .