The

resitance of a

wire or other

objects is a measure of the

potential difference **V** that must be impressed across the object to cause a

current **I** of one

ampere to flow through it:

**R** = V/I

The unit of resistance is the ohm, for which the symbol Ω is used. 1 Ω = 1 Volt / Ampere

The resistance **R** of a wire of length **L** and cross-sectional area **A** is:

**R** = ρ**L** / **A**

where **ρ** is a constant called the resistivity. The resistivity is a characteristic of the material from which the wire is made. For **L** in mass **m**, **A** in **m**^{2}, and **R** in **Ω**, the units of **ρ** are **ρ · m**

Resistance varies with temperature as well.

If a wire has a resistance **R**_{o} at a temperature **T**_{0}, then its resistance **R** at temperature **T** is:

**R** = **R**_{0} + α**R**_{0}( **T - T**_{0} )

where α is the temperature coefficient of resistance of the material of the wire. Usually α varies with temperature and so this relation is applicable only over a small temperature range. The units of α are **K**^{-1} or **°C**^{-1}.

A similar relation applies to the variation of resistivity with temperature. If ρ_{0} and ρ are the resistivites at **T**_{0} and **T**, respectively, then:

**ρ** = **ρ**_{0} + α**ρ**_{0}( **T - T**_{0} )