A uniformly wound solenoid coil of self inductance 1.8 ´ 10^–4 henry and resistance 6 ohm is broken up into two identical coils. These identical coils are then connected in parallel across a 15-voll battery of negligible resistance. The time constant for the current in the circuit is 

0.3 × 10^-4Sec

The coil is broken into two identical coils.

$L_{eq} = \frac{L/2 \times L/2}{L/2 + L/2} = \frac{L}{4} = 0.45 \times 10{-4} H$

$ R_{eq} = \frac{R/2 \times R/2}{R/2 + R/2} = \frac{R}{4} = 1.5 \Omega$

$ \text{Time constan } = \frac{L_{eq}}{R_{eq}} = \frac{0.45\times 10^{-4}}{1.5} = 0.3\times 10^{-4} Sec$