I have a building originally wired for electricity in 1952, with other sections done more piecemeal within the last thirty years or so. This old wiring is a two-wire, cloth-sheathed wire that feeds into metal electrical boxes. The receptacles are two-prong outlets.
I know that one cannot simply replace these plugs with grounded three-plug outlets without ensuring a proper ground. After doing some research, I've seen that sometimes the box itself can be grounded with either AC or a grounding wire outside the box. Using a test plug and my multimeter, I measured right around 120V between hot and the metal box. I then switched out the two-plug with a three-plug, attached a pigtail ground wire to the back of the box, and tested it again with the ground plug. Again, I get right around 120V. I tried most of the other plugs around the building with similar results.
I know that in a properly grounded, modern (for lack of a better word) system, this is normal. My question here is this: does this reading indicate that the line is, in fact grounded back to the box? Is there any way that the reading on my multimeter is "lying" to me? I am assuming, if it is grounded, that there is AC line behind the wall or a grounding wire run somewhere. If this is the case, is attaching the plugs to the box through a pigtail ground wire a reliable ground? I know code allows for conduit or AC cable to act as a ground.
Thanks for your help and advice!