Key Idea : This problem is based on the application of potentiometer in which we find the internal resistance of a cell. In potentiometer experiment in which we find internal resistance of a cell, let E be the emf of the cell and V the terminal potential difference, then VE=l2l1 where l1 and l2 are lengths of potentiometer wire with and without short circuited through a resistance. Since, VE=RR+r[∵E=I(R+r) and V=IR] ∴RR+r=l2l1 1+RrRrr=100110=10010=101×10=1Ω r=101×10=1Ω