Oh, and following up from Pufferfish's post:
If they could actually change the voltage / pain level during use, that makes me suspect a picana: adapted from cattleprods but redesigned specifically and solely for the purpose of torturing humans. In use since the 1930s and the application mechanism is a perfect match for what you describe.http://en.m.wikipedia.org/wiki/Picana
I advise against googling it since one of the first results are pictures of real use on real people in real torture - somewhere in South America. "Faces of Death" level omega trigger shit.