Originally Posted by Sewjourner

These machines do not have polarized plugs. With a polarized plug, one blade is wider than the other, so you can only plug the machine into the outlet one way. With these old machines you can insert the plug into your outlet in any of two combinations. If you happen to plug it in one way, and another appliance near you is plugged in the other way, you will get a voltage potential between the appliances. Running the machine will increase the potential, making the tickle stronger. If anyone is interested I can set up some test gear and post a video in a day or so, along with a fix.