Simple Electric Motor

It looks like the system works with a 'dead' short around the battery, with only the wire resistance and the tip of the screw's resistance limiting the current - the web site says the wire gets hot. So quite a lot of energy must be wasted - probably not a very efficient motor!

The interesting thing is that the closer the screw is to dropping out of the magnetic field the faster it will spin. I'm sure it's able to get pretty close to frictionless if you had the right screw and steady enough hands.
As for the wire getting hot, that's probably from friction, not the inherent resistance of the wire.

Interesting. So it generates a back-emf that limits the current? Or is it just the internal resistance of the battery that limits it?

I'd assume it's the internal resistance.

I can't wait to show that to some young kids if that doesn't spark scientific interest nothing will .

I did this demo with my son. Great fun! It worked fine, once we found a charged battery. Then, it mysteriously stopped working. We concluded we'd somehow irreversibly altered the laws of physics! (We'll investigate: maybe the conductivity of the screw tip changed from oxidization -- it actually arc welded into place once... battery deader than a no-load VM reading would suggest, etc...)

BTW if you use stranded wire, and touch the magnet with just one strand, the the heating effect is fairly well concentrated, so the rest of the wire does not get uncomfortably warm.

The screw accelerates surprisingly quickly. We found out that these homopole motors are really pretty efficient. The Navy is planning to build one of 50,000 hp. Although I can mumble stuff about the right hand rule, I really don't completely understand this. The wire, when energized, would seem to be a DC electromagnet, with a stationary field. I am accustomed to thinking of energized electromagnets as pretty interchangeable with permanent magnets. So, if I replaced the wire with a permanent magnet then I'd have a perpetual motion machine. What am I missing?

I haven't built one yet but saw the video does flipping the magnet have any affect ?

Haven't had ours running again yet (amazingly -- what is there to break??!!) The magnet has noticeable spark damage around its periphery, which we will polish off. That and a fresh battery, and maybe even a freshly cut wire end, should put us back in business. I assume that flipping the magnet reverses the rotation direction, but we were just about to try that before our motor quit.