Come on friend, provide the details. What are you using for the voltage source and at what value? How adjustable, over what range and to what accuracy? How stable in either % or an absolute value over what time frame and what range of current? In other words, what is the resistance range of the samples you intend to work with?
If you want a real answer, you have to provide a real question.
At a guess, I would say use step down transformer to get to the 2 to 12 volt range. Then a resistive voltage divider made of a resister and a potentiometer. The details control the values of the components.