The property of a conductor which tends to restrict the flow of current through the conductor is called resistance.SI unit of resistance is ohm ( $\Omega$ ). When a potential difference of $1\,\mathrm{V}$ applied across a wire gives rise to $1\,\mathrm{A}$ of current through the wire, then the resistance is said to be $1\,\mathrm{ohm}$ $(1\,\Omega)$.The resistance of conductor depends on length, thickness, nature of material and temperature of conductor.(i) If the length is doubled, then $R$ is doubled because the resistance of a conductor is directly proportional to length i.e., $R \propto l$.(ii) Resistance of a conductor is inversely proportional to the square of its diameter or area of cross-section. So, if the radius of conductor is doubled, its resistance becomes one-fourth. i.e., $\;\; R \propto \;\frac{1}{A}$and$A = \pi r^2$If$r' = 2 r$, then$A' = \pi r^2 = \pi (2r)^2 = 4\pi r^2 = 4 A$If $A' = 4 A$, then$R' = \rho \; \frac{l'}{A'} \; = \rho \; \frac{l}{4 A} = \; \frac{1}{4} \left(\rho \; \frac{l}{A}\right)$$\; = \; \frac{1}{4} R$Therefore, if radius is doubled, the resistance becomes one fourth.