Is There a Device to Make a Circuit Resonate?

If you are trying to do what it looks like, then have fun, but it will all be to nought. All of the theory that you will involve yourself with is rock solid and has been for 100 years - the best you will be able to do is get it to be self sustaining if you operate at near absolute zero and use super conductors etc. etc.

In simple terms an inductor is something that tries to keep current constant by adding or subtracting a voltage developed from the collapsing or expanding magnetic field. i.e. the current tries to drop, the magnetic field tries (and does a little) drop, this induces a voltage in the direction of the current so that the current is maintained for a little longer - and for so long as the magnetic field is dropping. Its really an exchange of magnetic energy into the circuit driving forces that keeps things going. The key thing is that the next time you want to use that magnetic source of energy it has to be pumped in from somewhere.

The pumping in of magnetic energy always involves some sort of force or voltage and either movement or current, and there are always losses of some sort associated with this, so while the magnetic/voltage exchange might be perfect (and it is not because there are also things called iron losses that arise out of eddy currents in any iron core) a little bit more energy always has to be put back into getting the magnetic field back up than is actually stored by the magnetic field.

The bottom line is that there is not and never will be any self sustaining vibrating magnetic system that produces net energy - unless as one respondent says, "you invent a new physics". Now I won't say it's impossible, though it probably is, but you sure are not going to find the answer with coils of wire or permanent magnets and clever switches.

Unless you have some truly amazing, new and different idea - and there are hundreds who have trod the path bedore you - all you will gain is a good insight into how electricity works. This would be useful.