[Amps] Resonant chokes

[peter chadwick]

Carl asked "How did Henry and others long before them do it before all that fancy stuff 
became available?"

General Radio (from your part of the world, Carl) did an inductance bridge where you could superimpose quite large amounts of DC. I have a small Marconi Instruments bridge that has provision for putting DC on electrolytics and inductors - you need an external DC source for that, though.
73
Peter G3RZP