Are you saying probe the neutral and the ground of the outlet and the ac will couple to the neutral?
No, I'm saying that the Neutral & ground are already connected together at the building's "entry point".
The return path between the outlet's Neutral pin & the Oscilloscope chassis ground is via all the associated wiring, accounting for quite a lot of feet.(think of it as having a very long lead on your probe ground clip)
If you now probe the Active (hot), without your ground clip connected to anything, (for safety, it is best to remove it entirely when you are using this method, in case it inadvertently touches a "hot"point), you will see the supply Mains waveform.
If you have the DUT open, & have actually measured between the power lead plug's ground pin, & found a point which is definitely connected to that pin, you can clip the ground clip to that, but you still have the long return path to Neutral.
Both of these methods can cause error, due to noise pickup, or just the inductance & resistance of all those many feet of cable.
I would not mess with trying to clip to the socket earth, as there is no real advantage.
A good rule is:-
Treat both the Active & Neutral connections as if they are both "hot" w.r.t groundThis is good practice for those of us in nominally 230v single phase countries, but may save your life if you ever come across a North American "Split phase" 240v outlet.
In these, there is no "Neutral", both (non-ground) pins are at 120v w.r.t ground.
Toroids don't work at low frequencies? that's because of their flux?
More to do with them being designed to operate efficiently at different frequencies, depending upon their intended use.
EDIT: Small m is milli M is Mega? I thought "mohm" was mega ohms? Do they make mili ohm? that would be a 1cm trace or shorter...
Yep, when you come across the "small m" prefix, it means the unit you are using is the base unit divided by 1000, as in millimetre, millilitre, milliamperes, millivolts, & so on.
The "Large M "prefix means you are using a unit which is 1000000 times the base unit, as in Megavolt, Megahertz, Megohm,etc.