Resistivity is the key to measuring electrical resistance

When a voltage is applied to a material or device, current will flow through it. How much current will flow is based on the resistance that the material applies to a circuit. The resistance of a material is based on a number of factors, the most important being its resistivity. Resistance and resistivity are often used interchangeably, but they have slightly different meanings. Knowing the difference helps you understand how electrons will flow.

Basic physics and Ohm's law teaches us that resistance (R) of a material or device is the quotient of the voltage, also called electromagnetic force or electromotive force, (E) applied to a circuit divided by the resulting current (I) through the circuit. By simple substitution of Ohm's law (I=E/R), resistance is

R=E/I

In general terms, resistance is the capacity of a circuit or material to oppose the flow of an electrical current and is referred to as Ohms (Ω). Resistivity is the measurement of a device's resistance. Like all units in the metric system, resistance of a pure element is given in a standard unit (Ω-m) at room temperature. The resistivity of pure copper, for example, is 1.68E-8 Ω-m.

Elements that are highly resistant to the flow of electrons are considered insulators. Insulators are typically tested for their resistance as well as their dielectric strength. Elements that are low on the periodic table are conductors.

Solid elements are classified as insulators, semi-insulators, or conductors by their "static resistivity" in the periodic table of elements. Resistance in an insulator, semiconductor, or conductive material is a primary property attribute that needs to be considered in any application. 

The measured resistivity of a material sample depends on its size and thickness. Temperature, humidity, and electrification time, among other factors, also affect resistivity. In general, when two otherwise identical material specimens are compared from the same sample and all other factors are the same, the resistance of the wider specimen is less than the smaller specimen and the resistance of a longer specimen is greater than a shorter specimen. Resistance is dependent on a specimen's size, but resistivity isn't.

This article continues on Test&Measurement World (direct link) where it includes common resistivity measurements, how resistance is measured, and an example measurement.