In the given electrical circuit, which one of the following equations is a correct equation?

With a resistance of ' $X$ ' in the left gap and resistance of $9 \Omega$ in the right gap of a meter bridge, the balance point is obtained at 40 cm from the left end. In what way and to which resistance $3 \Omega$ resistance be connected to obtain the balance at 50 cm from the left end?

In moving coil galvanometer, strong horse shoe magnet of concave shaped pole pieces is used to

For a metallic wire, the ratio of voltage to corresponding current is