After years of wanting one (and pushing off projects because I didn’t have one) I finally bought an oscilloscope ... with the “trigger” which is a voltage threshold at which the signal ...

The oscilloscope photo below shows this changing ... Also, have you done any AC voltage measurements? That’s something I haven’t done. Measuring the voltage of a Tesla coil comes to mind.

To determine peak voltage from an oscilloscope trace, the maximum vertical deviation of the trace from the centre line is measured from the screen. This number of divisions is multiplied by the Y ...