I feed a sine wave into a transformer which is followed by a full-wave bridge rectifier. I expect to see a rectified wave at the output with respect to local ground, but that's not the case. It's flatlining. Why?
My signal generator is not floating and I suppose that, by connecting the (also mains earth referenced) scope ground pin to the circuit, I create a short. But I was under the impression that the transformer would provide isolation from mains earth -- thus floating the part of the circuit on the right of the transformer and allowing me to probe around the circuit at my hearts content. Am I wrong?
Yes, because the scope probe ground reference clip is connected back through the instruments and their line cords to the grounded side of the FG's output. So you are creating a groundloop short that bypasses the transformer. See the marked-up sketch below.
The way to do this measurement is to make a differential measurement using two scope probes with no ground clip connection, and use the scope's Subtract function (or, on many analog scopes, Invert and Add functions) to give the differential measurement on the output side. See the scopeshot below.
(And I just saw the transformer ratio. Yes... you'll need a bit more voltage on the output side of the transformer before you see anything sensible!)