Homopolar Motor Limiting factor angular velocity

[expatiated]

I am doing an experiment involving determing the angular velocity of a homopolar motor (simple set up of the screw, battery, magnet and copper wire) and was wondering what the limiting factor on the speed would be. One website suggests that this might be back emf.

 

[CWatters]

Normally that type of motor would accelerate until the back emf equals the battery voltage (roughly). However I suspect that air drag and other sources of friction might represent a significant load for such a low power motor?