Important definitions:

Electrical resistance \((R)\):

A resistive element of a circuit is said to be a ohmic resistance if it does not depend on the voltage applied, neither it depends on the direction or intensity of the current (Ohm's Law). For these elements, the electrical resistance is a constant ratio between the potential difference \(\mathbb{V}\) between the conductor terminals and the current intensity \(i\) passing through it, so $$ R = \frac{\mathbb{V}}{i}.$$ The electrical resistance of a solid depends on two factors:

• The number of free electrons in its structure;
• The mobility of free electrons through the solid molecules network.

Ohmmeter:

It is the instrument used to measure the electrical resistance.

Ohm's Law

A conductor obeys Ohm's law if the value of its resistance is independent of the potential difference and current \(i\) applied. That is, the potential drop \(\mathbb{V}\), on an ohmic resistance \(R\), which is traversed by a current \(i\), is given by: $$\mathbb{V} = R i.$$ Ohm's law is an empirical law and is valid for some materials. In general, metal conductors are ohmic, but others may not be, as gases or liquids, and other electronic devices such as transistors and diodes. For the latter, the variation of the \(\mathbb{V}\) with the current intensity is not linear. These are called non-ohmic or non-linear conductors. However, we can state that for small variations of \(\mathbb{V}\), almost all conductors of nature obey Ohm 's Law.