Polarized Plug
DH agrees. It shouldn't make any difference on AC. Also, if you can find one, you can use an adaptor and not file the blade. There are probably still some around, if you ask enough people. The older machines weren't well grounded. That is why sometimes you can feel stray voltage when you very slightly touch them. Even my new machine with 3 wires has a little stray voltage. We have a 3 wire, grounded wiring in our house too. Go figure