The balancing length for a cell is 460 cm in a potential experiment. When an external resistance of  10Ω is connected in parallel to the cell. The balancing length changes by 60 cm. If the internal resistance of the cell is N10 , where N is an integer,  the value of N is

Let the emf of cell is ‘E’ internal resistance ‘r’ and potential gradient is  k. When Only cell is connected : E=460k   → (1)  After connecting the resistor :  E×1010+r=400k →2  From (1) and (2) , 460k × 1010+r=400k ⇒46=410+r ⇒6=4r ⇒r=64=32=1.5 Ω Given : r=N10 ⇒N=10 × r =10 × 1.5=15