With a small gap they run from 575 Hy to 600 Hy from .5 vac @ 60 cps to 40vac. The gap does its job just like it should, and there is plenty of inductance. About 30kOhm per half at 20 cps( 2Pi*20*(L/2)).

I plan to do full spectrum measurements on it soon. My Heathkit signal generator needs two 9V batteries, and I can't rob the smoke alarms anymore...:)

I plan to set voltage level and measure Z across the spectrum. I wonder how it will do. And of course a few commercial grid chokes for comparison will make for some entertaining comparisons.
cheers,
Douglas