I have waved my hands to explain this one several times so I figured it was time to make a posting of exactly how it all works.
The attached JPG shows how the ammeter wiring in a factory 1966 harness works. I have only drawn the wires involved in the ammeter circuit and given their colors and size/length where appropriate. The red and yellow wires go through a connector in the firewall which I have not shown to keep the drawing somewhat simple. The lengths given for those wires includes the wire on both sides of the firewall.
The ammeter is actually reading the voltage across what I drew as the big purple wire in the drawing. (Its actually a black/yellow wire but that's hard to draw.
) The little rectangle shown in the middle of the yellow wire by the solenoid is a connector built into the wiring harness. These wires are all wrapped together inside the underhood harness so you have to scratch your head a little while to figure out where these all are and how they connect. The 4-way wire connection down by the alternator is a thick lump you can feel inside the taped harness, its not anything you can actually see without removing some tape.
The ammeter coil itself is about 1/6 Ohm but when you include the red and yellow wires to and from the purple wire the total impedance for current flow through the ammeter wiring is right at 1/4 Ohm. I give example numbers for an alternator charging 100 Amps into the battery. About 1 1/4 A, or 1/80 of that charge current will go through the wires to the ammeter if all the connections are good. Unfortunately, full scale deflection of the ammeter takes about 2.8 A so that means the charge rate at the upper end of the ammeter scale is almost 230 A. Neither 230 or even 100 A will ever happen with the stock alternators which explains why this meter needle never moves a whole lot even if it does move.
On many cars, the connection at the solenoid end of the red and yellow wires are dirty or broken and there will be NOTHING going through the ammeter. That doesn't hurt anything else in the car so few ever take the time to figure out there is even a problem with the wiring.