You need a load for best results. Open circuit voltage should be the same as the source. When current flows through the wire to a load and back you get a voltage drop along those two legs according to V=IxR. That's current in a dc circuit or real current in an ac circuit going to a resistive load like a heater. Also, under load, you may find excessive voltage drop across the extension connection, or at either end of the plug, especially if corroded. It may get warm due to the high resistance from corrosion when current is flowing, that's due to I^2xR heating ...you are dissipating power where ever there is resistance. That's why cords get warm and bad connections get hot and burn. That's a big reason Mainsail recommends the Smart Plug BTW because it has greater contact area and therefore less resistance. At any rate, proceed with caution when measuring. AC is unforgiving and can stop you heart.
A simple way to look at it is that's why your skill saw can stall under load on a 50 foot 16awg extension cord... the voltage drop is great to the point where the saws not happy with 90v. Thicker wire has less resistance per foot so less voltage drop. Plus the higher voltage at the load, the less current is drawn.
BTW, an out of phase reactive current also flows to inductive loads through the AC cord like motors and transformers that aren't power factor corrected. PF = 1 is purely resistive current (think toaster oven load) and the lowest total current flow for the real power produced at the load. PFs of 0.6 - 0.9 are typical. Good appliances are 0.9-0.95. For example, the good battery chargers like sterling and pronautic p are PF corrected and draw less current (less reactive current) for the DC amps produced.
