my jig saw sparks?

Carbon brushes deliver power to the rotating part of the motor - they rest against a segmented cylinder on the rotor of the motor called a commutator. As each segment of the commutator passes under the brush it's quite normal for a small spark to jump across the gap.

You can see a home made electric motor sparking away in this video...

http://www.youtube.com/watch?v=MxA3pQlpu84

Peccavi
September 2009