Many materials exhibit a simple relationship between the values of current, voltage, and resistance, known as Ohm’s law. The current that flows through most substances is directly proportional to the voltage applied to them. The German physicist Georg Simon Ohm (1787–1854) was the first to demonstrate experimentally that the current in a metal wire is directly proportional to the voltage applied. Any material, component, or device that obeys Ohm’s law, where the current through the device is proportional to the voltage applied, is known as an ohmic material or ohmic component. Any material or component that does not obey Ohm’s law is known as a nonohmic material or nonohmic component. In an article published in 1827, Georg Ohm described an experiment in which he measured the voltage across and current through various simple electrical circuits containing various lengths of wire. The experiment was used to observe the current through a resistor that results from an applied voltage.

Another similar experiment was used to observe the current through a resistor that results from an applied voltage. In this simple circuit, a resistor is connected in series with a battery. The voltage is measured with a voltmeter, which is placed in parallel with the resistor. The current is measured using an ammeter and is placed in series with the resistor. Several measurements of the current were carried out for several different voltages. The current through the resistor and the voltage across the resistor were measured, and a graph was plotted between the voltage and the current, which showed an approximately linear relationship. The slope of the line is the resistance, or the voltage divided by the current. The result became known as Ohm’s law.