Since, $\;R=P\;\frac{L}{A}$ $\;I=\;\frac{V}{R}=\;\frac{V}{\frac{\rho L}{A}}=\;\frac{V A}{\rho L}$ $\;I=n e A V_d=\;\frac{V A}{\rho L}$ when another conductor is joint in series $\;\;\text{Effective Resistance}\;=R+R=\;\frac{2\rho L}{A}$ $\;\;\text{Effective current}\;=\;\frac{V}{2R}=\;\frac{V}{\frac{2\rho L}{A}}=\;\frac{V A}{2\rho L}$ Now $\;\frac{V A}{2\rho L}=n e A V_{d'}$ or, $\;\;\;\frac{1}{2}\left(\;\frac{V A}{\rho L a}\right)=n e A V_d$ or, $\;\;\;\frac{1}{2}\times n e A V_d=n e A V_{d'}$ or $V_d=2 V_d'$ Drift velocity in second case will be half of the initial drift velocity Current density $=J=n e V_d$ Initially current density $=J=n e V_d$ When another conductor injured in series current density $J'=n e V_{d'}$ $\;\frac{J}{J'}\;=\;\frac{V_d}{V_d'}\;=\;\frac{2 V d'}{V_d'}=2$ ∴ $J\;=2 J'$ Current density in second case will be half of the initial current density.