R = ρL/A

R = ρL/A — proportional to length, inversely proportional to cross-section.

Resistivity ρ is material-dependent: copper ~1.7 × 10⁻⁸ Ω·m, nichrome ~1.1 × 10⁻⁶ Ω·m.

Stretching a wire to length nL keeps mass constant → A drops to A/n → R becomes n²R.