Smokin' POT

[kayvee]

Hello everybody. About a month ago I made a post titled "How do I
hook up my potentiometer?" [http://groups.google.com/group/
sci.electronics.basics/browse_thread/thread/c9b1f3cd98cb2e2b/
7faa05598f647c54?hl=en&lnk=gst&q=Potentiometer#7faa05598f647c54]

Now, I ended up hooking it up John Fields' way with the diode in
parallel with the motor [12V]:

..V+>---+---------------+
.. | |
.. [220] |
.. | |
.. P /c
.. O<------------| NPN Power xsistor
.. T \e
.. | |
.. +---[Motor]-----+
.. | |
..GND>--+---[Diode>]----+

Now, I have been using a 12V power supply and a NPN with a high power
rating and I got a hold of a 500ohm POT (unknown power rating). I
hooked it up and all and it runs fine when the POT resistance it
0ohms, but once I start to turn the POT around half way, thus slowing
down the motor, the POT starts to smoke, which in my experience is
bad. Do I just need a POT with a higher power rating, or is there
another way of doing this?

Thanks guys!