Slow-discharge circuit for gradually diminishing electromagnet field?
I want to create a circuit in which a hand crank charges up one or more batteries or capacitors, which in turn discharge slowly through an electromagnetic coil (such that the magnetic field of the electromagnet gradually diminishes) over the course of an hour or so.
The overall idea is that you crank the crank for a short while, creating a magnetic field so that BBs cling to the electromagnet. But little by little, over the course of maybe 20 or 30 minutes (longer would be OK), the BBs drop off one by one (or a few at a time).
Can anyone suggest what such a circuit might look like? What variables will determine how long the crank has to be turned, how strong the magnetic field will be and how slowly it will discharge? And will the BBs only cling to the ends of the electromagnet? Or will they cling along the length of it as well?