In a potentiometer experiment a cell of e.m.f $1.5\mathrm{ }\mathrm{V}$ connected in the secondary circuit gives a balancing length of $165\mathrm{ }\mathrm{c}\mathrm{m}$ of the wire. If a resistance of $5\mathrm{ }\mathrm{\Omega }$ is connected in parallel to the cell, the balancing length of the wire is $150\mathrm{ }\mathrm{c}\mathrm{m}$ . The internal resistance of the cell is