You want soft iron balls. The terms hard (for permanent) and soft for
transient magnetization can be taken literally. Dislocations and
impurities that form hard steel makes it difficult for steel to form
into permanent magnets and for permanent magnets, once formed, to
demagnetize.
To summarize, you want pure annealed iron. Maybe wrought iron could do
the job.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit.
> >... Problem - the steel
> > balls may eventually
> > become magnetized and then begin stick together, ruining the kinetic
> > display.
> You want soft iron balls.
And, they're easily available, by the pound (as shotgun or BB gun
loads).
It's possible they'll clump anyhow, because the applied magnet
field will both move the balls AND cause them to attract each other.