A \( 12 \, \text{V} \) battery connected to a coil of resistance \( 6 \, \Omega \) through a switch, drives a constant current in the circuit. The switch is opened in \( 1 \, \text{ms} \). The emf induced across the coil is \( 20 \, \text{V} \). The inductance of the coil is:
