A potentiometer wire of resistance per unit length  $r=r_0(1+\frac{x}{4\mathcal{l}})$  (where x is distance from left end of the potentiometer wire) is connected to four ideal batteries as shown in the figure. Resistance of connecting wires is negligible. A cell of e.m.f. 1 volt is balanced against potential drop on potentiometer wire, then null point can be obtained :