Ohm's Law, a law of physics. It states that in an electrical conductor the ratio of potential difference (voltage) to current is constant. For example, if the terminals of an electric battery are connected to an electric lamp and the voltage output of the battery is then decreased by 20 per cent, the amount of current flowing through the lamp will also be reduced by 20 per cent.

Ohm's Law was derived experimentally by the German physicist Georg Simon Ohm in 1826. It is expressed by the following equation:

V=I X R

In this equation V represents the potential difference between one end of the conductor and the other (that is, the voltage applied to the conductor); I is the current flowing through the conductor; and R is called the resistance of the conductor. If V is given in volts and I is given in amperes, R will be in ohms.

The law offers a simple method of calculating the voltage, current, or resistance in a conductor when two of these three quantities are known. For example, if the direct-current voltage applied to an electric light bulb is 120 volts and the filament in the bulb has a resistance of 240 ohms, the current flowing through the filament is

I = V/R = 120 volts/240 ohms = 0.5 ampere.

Ohm's Law is valid for metallic conductors (for example, copper and tungsten) in direct-current circuits as long as the current is relatively low. High currents will heat a metallic conductor and cause its resistance to change, so that the ratio of voltage to current in the conductor will also change. Ohm's Law holds for a complete direct-current circuit as well as for any part of the circuit, provided that l is the total current flowing between the points across which the voltage, V, is measured.

In alternating-current circuits, Ohm's Law seldom is valid because the current flow is affected not only by resistance but also by factors known as inductance and capacitance.