We know that the potential difference ΔV from the generator starts moving electric charges in a circuit giving raise to an electric current. At constant applied potential difference, the current i that passes in a circuit depends on the characteristics of the conductive material that is used. Each conductor actually manifests a certain resistance to the passage of the current, due to collisions between the electrons in motion inside the conductor and the atoms of the impurities present in the circuit.

In mathematical terms, is defined as the resistance R of a conductor the ratio of the potential difference ΔV applied and the intensity of the current, ie R = ΔV / i. Since the current i appears in the denominator we have that in a conductor with high resistance R circulates a small current i, vice versa, a conductor with a small resistance R is characterized by the high currents. We can say that the resistance R measures the degree of opposition they meet the electrons to move up the potential difference ΔV. The unit of resistance is the ohm (Ω symbol, capital omega): 1 Ω = 1 V / 1 A.

In general, as the potential difference ΔV increases also the current i but there are various possible relationships between i and ΔV depending on the conductor that we take into consideration.

So if we measure with a voltmeter and the potential difference ΔV with an ammeter, the current i discover that in a metal the relationship R = ΔV / i is constant. In metallic conductors the resistance does not depend on the potential difference ΔV that we apply to the conductor. What determines then the resistance in a metallic conductor? The answer is given by the second law of Ohm. The resistance R in a conductor depends upon the material it is made of the wire, its length from its the section A. More precisely, we will have that R = ρ L / A, ie, the resistance is directly proportional to the length l of the wire and inversely proportional to the area of the section. The proportionality constant ρ (symbol that corresponds to the Greek letter ro) depends on the type of metal that we take into consideration and goes under the name of resistivity.

Resistivity small mean small resistance, ie good ability to conduct electricity. Since ρ = R • A / l, the unit of measurement of the resistivity in the International System is the ohm per meter (Ω • m).