where ℓ ℓ is the length of the conductor, measured in metres (m), A is the cross-sectional area of the conductor measured in square metres (m), σ (sigma) is the electrical conductivity measured in siemens per meter (S·m), and ρ (rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals : ρ ρ = 1 / σ σ . Resistivity is a measure of the material's ability to oppose electric current.