I recently bought an Arduino starter kit which comes with an Arduino Uno and some components. I know very little about electronics or physics. The Arduino has some analog-to-digital pins on it which allow you to measure, in code, the voltage "sent to" that pin. What I don't quite understand is which voltage it measures and how.
I set up a test circuit and some code to print out the voltage measured by the input pin. I wired three 220 ohm resistors in series, then connected a wire to the A0 analog-to-digital input pin on the Arduino, and with the other end of the wire probed different points along the circuit. You can see this in the diagram. Note that the four branches coming out of the A0 pin were never all wired up at once, that's just to represent the four points A, B, C and D on the circuit that I probed. Next to each point I've written the voltage that I measured there.
I understand that what I'm measuring at point X is the voltage "between" X and the ground, dropping by 1.66V after each resistor. What I still don't really understand is how the pin measures this. I'm not really looking for an actual explanation of how the voltmeter inside the pin works, just some kind of high level conceptual understanding. For example, one thing that confuses me is that once the A0 pin is connected to the circuit, what I have isn't really a "circuit" anymore, in the way I usually think about them. It's not a straight line from source to ground with components along it. I clearly can't think of the A0 and the wire connecting it to the circuit as just one big component, like the resistors, because that would imply current is traveling both up and back down the wire connecting A0 to the circuit. So how should I think about the A0 pin? Is it like a secondary connection to the ground? A secondary connection to the source? If it's a connection to the ground, wouldn't that imply, due the principle of the path of least resistance, that if I connect it to say point A, no current will travel through any of the resistors?