A potentiometer wire of length \( 300 \, \text{cm} \) is connected in series with a resistance \( 780 \, \Omega \) and a standard cell of emf \( 4 \, \text{V} \). A constant current flows through potentiometer wire. The length of the null point for cell of emf \( 20 \, \text{mV} \) is found to be \( 60 \, \text{cm} \). The resistance of the potentiometer wire is __________ \( \Omega \).
