Sounds a little too aggressive. I like the idea of the balls gently "knocking" around, subtley building to a big bang, as it were.
Think of the balls as truly extended foreplay.
I like it when you talk dirty.......
> ...too aggressive.
You're thinking of a magnet attracting a free metal object on open space. Yes, under those conditions, the action is rapid as the attraction snaps them together. Within the confines of the intended operating environment, however, the sliding ball will not be free to move nearly so rapidly. It will be restricted to a slow, sssslllllliiiiiiddiiinnnggg motion, from one end to the other. In fact, position sensing could be built in so that the eletromegnets would not switch until the sliding ball reached the active end of the strand, thus ensuring a full traverse of the length of the strand each time.
What's not to like? Slow. Deliberate. Highly tactile. Repetitive. KA-BOOM!