In a potentiometer wire experiment the emf of a battery in the primary circuit is 20volt and its internal resistance is 5$\Omega$  . There is a resistance box (in series with the battery and the potentiometer wire) whose resistance can be varied from 120$\Omega$   to 170$\Omega$  . Resistance of the potentiometer wire is 75$\Omega$  . The following potential difference can't be measured using this potentiometer