From Appropedia

Electrical Resistance of a circuit element is a property of the type of material and its size (length and diameter). The units of electrical resistance (R) is Ohms. The larger the surface area of a wire, the smaller the resistance, and the longer the the length of the wire, the greater the resistance.

The resistance of a metal also increases with an increase in temperature. The quantities of resistance, potential difference and current are related to each other by a formula known as Ohm's Law:
I = V/R or V = IR

where I is the current and V is the potential difference or voltage.

Different materials have different reactivities. Superconductors have a resistivity of 0 at low temperatures.

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