I've just started to learn about electrical work, and today I mapped the circuits in my home. However, I have one light switch (a ganged device with one switch and one empty spot) that doesn't control anything. I removed the cover and got out my circuit tester (without killing the power at the service panel) because I wanted to see if the wires are hot. I'd like to figure out why I have this inactive switch in my house. I also did some research online about how to test for hot wires.
I believe that the switch is a simple dipole, as it has only two screw terminals on it (aside from the green grounding screw). I was able to determine that the wire at top is hot by touching one probe on my circuit tester to it, and the other to the box (which must be grounded, because the light came on on my circuit tester). However, I noticed something strange--there are two wires attached to the top screw terminal. When I tried to touch the probe to the second wire (still with the other to ground), I received a big flash, and the end of the metal probe of my circuit tester has taken some serious damage.
Now I'm really confused. For one thing, I'm feeling pretty nervous about working with electricity again! Also, I don't understand why I was able to touch one wire safely, but not the second wire. Shouldn't they be at the same potential when they're attached to the same metal terminal? And, why on earth would two wires be attached to the same terminal on a dipole switch? This doesn't match any diagram in my electrical book.
Any help is greatly appreciated. Thanks so much!