Dear Mr. JimSmith and everyone:

For the first question:
Thanks a lot for reminding me of Mr. LarryBible's reply to my post in the tech help forum. Yes, measuring AC current amperage is best done via the indirect measurement of the voltage induced in a coil around the conductor carrying the AC current, and that's what my original question was trying to address.

However, Mordecai's trick of using a low resistance and high precision (but must be NOT variable due to temperature increase)resistor to trigger a voltage drop can only be best applied to DC because that trick will become more complicated and involved for AC (formula manipulation and equation solving). Why?, because an AC current I is equal to the ratio of the RMS voltage to the impedance (not the simple resistance). An AC circuit impedance is dependent upon [the AC frequency, the inductance, the capacitance, the resistance and the circuit schematic]


For the other question:

Yes, the resistance concept for DC is mostly replaced the impedance concept for AC, because the inductance (an inductor such as a coil), or the capacitance (a condenser/capacitor) imposes a resistance-like and frequency-dependent effect on AC. DC treats an inductor as a regular resistor and a capacitor as an insulator (infinite resistance), i.e. a DC circuit with a resistor, an inductor (a coil) and a capacitor in series will have a DC current of zero ampere because of the capacitor's infinite resistance.


I will post a more detailed explanation later this weekend (using some formula manipulations). I hope someone will try to refresh his/her memory about AC behavior and formulaes and post a clearer and more informative reply before I do. It has been more than 10 years since I learnt about AC, so I think it's good for me and everyone to refresh our memory (closed book of course :-)

Best regards,

Eric