I am trying to change the light fixture on a ceiling light controlled
by a dimmer that has an on/off push control, and a rotary dimmer
control. I've changed this light twice before with no problems
(dimming worked as expected), but this 3rd time when I threw the
breaker back on I got no light. I'ver verified that the light works
by connecting it to another power source.

For the record, on this 3rd try to change the light, I was working
with the breaker on and and one point caused a brief short that caused
some sparks to fly and the breaker to trip. Since then I've always
turned off the power when working. Also, the original wiring coming
through the ceiling is very old and brittle with lots of exposed
parts. That's how I caused the short; by moving the wires around some
exposed wires must have brushed up against one another.

Tonight I tried to diagnose what was wrong, pulling out my trusty
fluke voltmeter. This surprised me: when the power is on, the minimum
voltage across the two wires I connect the light to is 60V. I get
this when the dimmer switch is off, or on at its minimum position.
When the dimmer switch is on and at its maximum position, I get 120V.
I was expecting to measure something close to 0V in the off position.

Now more wierdness. When I connected the light to the ceiling wires
(white to white, black to black, same ones that have worked for me
twice before without incident), I got nothing (as described above).
At first I assumed it was just a poor connection (I'm using wiring
nuts) but undid and redid everything and got the same result. When I
measure the voltage across the wires the light is connected to with
the light connected I get 0V, whether or not the switch is on or off,
minimal or maximal.

I have repeated all of these measurements multiple times (ie, turning
off the power, unwiring the light, powering back up, measuring,
turning off the power rewiring the light, powering back up, measuring
again). Always the same result: 60 to 120 V min to max when the light
is not connected, 0 volts when it is.

Can anyone explain to me what is going on, or what I should do to
diagnose this problem?