I think it is wrong to say a static magnetic field has a RESONANT frequency, because it has many resonant frequencies, or can excite many resonant frequencies.
Take for example a proton precession magnetometer. After the spins are polarized by the coil, the current is stoped and the proton spins begin to precess around the external magnetic field vector, and the resonant frequency of precession depends on the strength of this field, but the static field is not in resonance.
Ferromagnetic materials owe their properties to the electron spins not protons, and the resonant frequency is much higher. I think it is 29 GHz or so at a field strength of 1 T. So in the earth natural magnetic field of about 50 e-6 T, this resonant frequency is about: 1.45 MHz (in comparison, proton in earths field is resonant at about 2130 Hz)
But let's get back to Hendershot, I don't read that he generated a lot of power from his motors, because spinning at constant RPM is really very little power dissipation, just a bit due to friction.
I find it very currious that the secret was premagnetizing the core, or causing a magnetic bias. This is similar to the TPU, a magnetic bias to the iron wire, or ferrite or torroids, etc..
EM
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