As well - don't forget the ground return - it's through the body of the jack to the A-frame. there should be a lock washer on one of the bolts, sandwiched between the frame and the jack's flange, which pierce through the paint in order to make a connection. I experienced some intermittent operation, as well as some sparking, until I tightened down the bolt.
To check the ground, remove the jack head, and measure voltage between battery positive terminal and bare metal at the top of the jack tube. if zero volts, it's probably a ground problem.