My house is ~100 years old, and I just bought it. I have an outlet that has 2 wires, what I assume are hot and neutral but no ground wire. I'm using a Klein receptacle tester. When I plug the tester into the outlet, it says that it is correct. When I pull the outlet out of the wall away from the metal box it sits in (while still wired), the tester says there is no ground. I assume that is because the screw that I use to screw the outlet receptacle to the box it sits in is metal to metal thus "grounding" the outlet. Is this incorrect? I just want to make sure I'm not misled by this tester. I thought the purpose of ground was to provide an "error" path that leads a charge back to the panel. I've been told many different things about ground, but trying to understand in regards to an outlet how ground works and what is safe for a 2 wire outlet.
Metal junction boxes are typically serviced by metallic conduit that is grounded in the panel. This is particularly common in older homes that have been updated to whatever the current code was when it was rewired. Homes 100 years ago didn't require grounded outlets so when they were brought up to date it was the best way to provide proper grounding. Although a receptacle will usually ground through the attachment ears to the metal box you should still attach a ground wire to the green screw on each recptacle and run it to a grounding screw on the box.
One suggestion for the future, instead of a receptacle tester, which can be squirrely at times, you should invest in a multimeter that will give you more precise information.
This all fits with "metal box". With a metal box and the right configuration/type of receptacle (typically called "self-grounding") you don't need a ground wire to the receptacle, and if you don't use a ground wire then as soon as you pull out the receptacle from the box the tester won't see a ground.
Everything sounds good to me.