A capacitor of capacitance C is connected to a battery of EMF E for a long time and then disconnected. The charged capacitor is then connected across a long solenoid having n turns per meter in its closely packed winding on its core. After connections it is found that the voltage across the capacitor drops to `E//eta` in a time `Delta t`. In this period estimate the average magnetic induction at the centre of solenoid.

Answer

`[(mu_(0)n CE)/(Delta t) (1 - (1)/(eta))]`