Small Voltage control

I am wiring something for a show I am doing. It involves a battery circuit with two D batteries and I would like to be able to increment the output down slowly. I have tried a couple potentiometers that I had here at my house (from when I was in electronics classes 15 years ago) but I believe they aren't meant to work with such small voltage. They don't really increment as much as they basically turn it on and off. Is there a potentiometer that will do what I want in this circuit? If so, how do I select the right one?

The pot resistance you are using is too high. Likely, you effective load resistance is a few Ohms, and you are using 10K or higher pots.

Determine effective load resistance you are trying to drive. Make the rheostat (pot) resistance about 2 to 5 times higher than that.

Suppose you have a 1.5V motor that draws 0.25A when it is running. That means its effective resistance is E/I = 1.5/0.25 = 6Ω. To vary the speed of this motor, it would take rheostat of ~20Ωmax. When the rheostat is set to ~6Ω, the motor would run at half speed.