I'm looking at getting a motor to drive a micrometer. It's on an interferometer mirror for the curious. I'll need around 1RPM or so, but I want to be able to vary the speed, because I'm pretty unsure about how fast I'll need it to go for my experiment to work. I'm thinking a range of like 1/4 to 2 RPM would be pretty safe. I won't need much torque, as the motor will just be turning a micrometer knob. So I'm looking at geared DC motors, but I don't know how much variation in speed a given motor will have, and what specified speed I should get. I don't mind building a PWM if I have to, but it'd be really swell if all I had to do is hook it up through a potentiometer. Should I go for the highest RPM I might need at the rated voltage? Will that let me go down in speed by a factor of 8 or so?