Electronic – Newbie question: How do pots limit current

note: question was originally about a current limiter

The LM317 with the single series resistor between output and adjust input is actually a fixed current source, not a current limiter. You don't need the LM317 to create a current limiter, a few discrete components will do:

For a limiting at 2mA you select a 330\$\Omega\$ resistor for \$R_{SENSE}\$. If there flows 2mA through it Q2 will start to conduct and reduce the base voltage of Q1, so that its current is cut off.

edit (re changed question)
Maybe you're focusing too much on the LM317. If you need a constant current you could use the LM234 which is a programmable current source for up to 10mA. You set the current with a resistor.

There will still be a positive gate-to-source voltage if the source is negative with respect to ground. Note that the current source isn't grounded; it's connected to a negative voltage. So there's room for some GS voltage.

EDIT: Yes, the magnitude of the current through the source can affect other parts of the circuit. It's common to think of \$V{gs}\$ as the input to a MOSFET, and the drain current as the output, but really they're just two variables with a defined relationship to each other. You can put it in a circuit that forces a \$Vgs\$ and allows drain current to adjust to where it wants, or you can put it in a circuit that forces the drain current and allows the gate voltage to adjust. Provided you're operating within the allowable range of parameters of the device, the two variables should always obey the equations. But which is "input" and which is "output" is just a point of view.