Yes. This would be a test for resistance between the power supply and the regulator. Back probing at the alternator field connection is the easiest location since the regulator's connector doesn't allow that.
It's not valid. With the wire off, there's less current flowing in the circuit.
Imagine it was a dedicated wire going direct to the power supply.
Remove the wire no current flows through. The voltage above ground should be the same as the supply.
Lets use an example with a lamp and a 12 volt battery.
View attachment 1715402303
If the second probe is to ground, the meter will see what voltage?
And if the second probe is on the batteries positive cable, then the meter sees what voltage?
Same example, with current flowing through the light bulb.
View attachment 1715402305
If the connections and wire are big enough and in perfect condition, the voltage at the bulb should still be battery voltage. Viewed the other way - there's no voltage drop in the wire.
But if there is a poor connection, or undersized wire, that resistance to flow will cause a voltage drop.
View attachment 1715402308
IMO. That would be really good.
You mean tested voltage - which implies one probe is on a ground (chassis).
Testing across the ammeer terminals should be zero. There should be no loss in the ammeter. Internally its a metal plate with the two studs pressed in. Unless the studs are loose, there's no measurable resistance.