Normally the threads on the socket is connected to the grounded conductor and the button is hot but you can't rely on that. The "jumping around" behavior is indicative that the probe is connected to a conductor that is connected to neither a ground nor a hot. You're seeing a lot stray inducted values. They can be quite large but have next to no current to them.

The first thing to look for is if the wires leading to the photocell have any power (then look for a breaker, a switch, or some poor connection if not).

My sensor burns out about every three years because it faces into the sun. You might be suffering from the same ill. In my case, it's easier to just replace the sensor than try to re-orient a pole that's buried in concrete.