hi,
Assume you use a PMOS enhancement FET.
When the pot slider is at the top [on the drawing] the FET is not conducting so there is no voltage drop across the Rsense resistor.
If the pot is slowly turned so that the FET starts to conduct then the current thru the sense resistor will create a voltage drop across the Rsense.
This voltage drop will cause the current thru the Rsense and the FET to stablise. By choosing the value of Rsense and FET gate on voltage, values of 4mA thru 20mA can be achieved by adjusting the pot.
The Vsupply must be high enough to allow for the voltage drop across the Rsense, FET and Rload at 20mA.
A PNP transistor can be used in place of the FET.
For a Constant Current source, the gate voltage is derived from a fixed reference voltage. So as the load resistance varies the current in the load remains constant.
What are you using the current source for.?