In the potentiometer, when the cell in the secondary circuit is shunted with \( 4\Omega \) resistance, the balance is obtained at the length \( 120 \, \text{cm} \) of wire. Now when the same cell is shunted with \( 12\Omega \) resistance, the balance is shifted to a length of \( 180 \, \text{cm} \). The internal resistance of cell is ________ \( \Omega \).
