A galvanometer of resistance $50 \Omega$ is connected to a battery of $3 \cdot \mathrm{~V}$ along with a${ }^{\text { }}$ resistance of $2950 \Omega$ in series shows full-scale deflection of 30 divisions. The additional series resistance required to reduce the deflection to 20 divisions is

A cylindrical conductor of diameter 0.1 mm carries a current of 90 mA . The current density ( in $\mathrm{Am}^{-2}$ ) is ( $\pi \approx 3$ )

A piece of copper is to be shaped into a conducting wire of maximum resistance. The suitable length and diameter are ......................and............................respectively.