Thomson's jumping ring: Why net repulsion?

[greypilgrim]

Hi.

According to Lenz's law, a conducting ring responds to an increasing external magnetic field with a repelling field and to a decreasing external magnetic field with an attracting one.

Thomson's experiment uses an AC coil, so its field strength changes between increasing and decreasing a 100 times per second (at 50 Hz). Yet there is a net repelling force making the ring jump. Why is there more repulsion than attraction?

 

[Doc Al]

I haven't looked at it in detail, but it has to do with the inductance of the ring, which makes the current in the ring out of phase with that of the coil enough so that the average force is repulsive. I think several papers have been written on that demo.