We have started using double neodyium magnets instead of just one in each coil. This seems to increase the power on very low speed windmills. It also prevents the spheres from going into oscillation at higher speeds. The two magnets are just far enough apart so that they don't interfere TOO much with each other. This does prevent the magnets from free spinning inside the tubes but this has never been a problem. We found that putting a bit of graphite inside the tubes with the magnets keeps the friction down. Also by bevelling the acrylic separator between the spheres keeps the friction down.
What we don't know yet is wether or not a piece of iron outside the coil at right angle to the tube will also increase the stability and power output.
