Another way is to build separate, binary valued coils connected in series. For example, the first coil is x uH, the next coil is 2x uH, the next coil is 4x uH, then 8x uH... and so on. Use relays to short the coils out you don't want to produce any value inductance that is a mutiple of x uH. The relays are switched on and off with electrical signals. The first coil determines the smallest adjustment that can be accomplished. The more coils used, the larger the total inductance that can be accomplished.