I built a stir plate recently using a cooling fan, cigar box, 25oh 3 watt potentiometer from radio shack, and 9 V DC power supply. Currently, no matter how far or close I place the magnets from the surface, it throws my stir bar (42 mm) at a fairly low to low-medium speed before I can get a good vortex. Right now I have 10x2 mm rare earth magnets. Im waiting on some to come in the mail that are 12x5 mm, so I'm hoping that will solve the problem.
Also, I'm very concerned about the potentiometer. It's getting very, very hot. Is this normal?
Thanks for your help.