Grounding & Shielding to Reduce Guitar Noise Part 1

Jeff, the 3rd prong is not actually a ground. It is an unutilized low impedance conductive path to return fault current to the service point to allow the fault current to trip the branch circuit overcurrent protection device (breaker). The only ground occurs at the service. It is a bond between the grounded conductor ((neutral) and the actual ground or earth. Yes I know it has been called ground forever but is being changed in the 2014 national electrical code. What you are accomplishing is creating a bond to provide a path to drain the unwanted interference to earth at the service ground. Now at issue is those 2 prong plugs do not all have a metal box that has a wired connection to complete that path. Just because you attach the screw on an adapter you do not necessarily complete this path. Only if the box is metal and has a separate ground wire or is supplied through a conduit system with metal conduit. Only 2 wires were in original non metallic cable, same with knob and tube. What would happen is the electrician in some area ran a separate wire between the metal boxes and a water pipe (old were all metal)somewhere nearby, but usually only in the bath and kitchen sink areas. In the early days the insurance institute actually fought the electrical industry in the need to have a ground connection. They were of the feeling it was the ground that would cause the fires and electrocutions. Funny don’t you think. Deadly actually. So in closing, the only way to actually know is as you showed with your tester, or by actually measuring with a multi meter.

PS. the reason the wire type adapter went away was the ability of the adapter to be plugged in reversed when attached. The modern version will not allow this, keeping the ungrounded and grounded (hot and neutral) conductor orientation intact.
You can create a temporary bond for your gig with some insulated stranded (green or green yellow) #12 wire from your power strip to any exterior metal point on the electrical panel without ever opening it up. Do not install a new screw but use any external accessible screw and slide a forked lug in and retighten. Remove when done. Best if all system power is taken from same side of service and ground bond point anyway. If you ever had the sound system power accessed from sound mans location while instrument amps near stage and touched your lips to the mic while playing you probably have experienced the blue flame I call it. The difference in potential between the two point of supply creates a ground loop and that is actual power shocking the piss out of you. historically there have been many deaths from this situation.12