A building has several apartments. Each one has a doorbell, and 4 wires
coming to the doorbell. One set is (I assume) for the downstairs main door
and the second one is for the door of the apartment.

When I went to replace the old doorbell, the button at the main entry door
worked and two wires were connected. The apt. door button didn't work and
two wires were disconnected. I couldn't get the apt door bell to work by
connecting the two wires to the doorbell at the "rear door" section. In
fact one of the wires sparked when I touched it to the terminal.

When I installed the new doorbell, the same thing happened. The entlry
door still works when on the "front door" terminal and "transformer", but
no joy with the apt door. I put a cheap meter on all four wires to
discover which was the transformer wire for each set of two. I got odd
readings, something like 50 and 75 volts AC. I figured this was incorrect
since the actual voltage should be around 16VAC, and indeed I could touch
the wires with no problem.

So what's happening?

Why do I get a higher reading than it should be? Is it because the wires
were not connected to anything? Oh, I put the neg test lead into an
extension cord on the neutral side (assuming that the plug it was pluged
into was wired correctly) and use the pos test lead on the wires coming
out of the wall.

What likely caused the arcing when I touched one of the 2nd pair of wires
to the doorbell?

Is it likely that the second pair of wires has its own "transformer" lead
rather than just taking power from the transformer wire from the first
pair?

I assume there's been this problem for a while since the 2nd pair of wires
(apt front door) was disconnected. Any way to track down what the problem
might be?

Any additional info you need?

--
charles