A commutator is a mechanical device that switches the direction of the current in the coil.

In regular motors the commutation is more complex and the current reverses so the coil is always being turned to get to alignment with the field magnet. .. In THIS simple motor the current doesn't reverse! As he explains it: Removing the insulation carefully on only one 'side' of the wire (the correct side) causes current so the coil turns, trying to align, but the insulation then blocks the current and the rotor continues, coasting past the alignment point to the next angle where the current starts again and attracts it again the same as before. This way, the coil only gets a shot of current one way to spin it TOWARD alignment, but the current stops before it gets there and it coasts past alignment and past the opposite alignment where a reverse current would be needed. . The magnetic attraction only pulls it half way around and it coasts the other half. Hope that helps.

I would think the coil would just stop when the magnetic field of the coil is aligned with the magnet's field. I will have to try it and see.

@@electricandmagneticfields2314 Of course, but at that time I didn't see where they explained to remove it partly. I an sure I saw others that explained it correctly after that. This is so old I don't even recall writing that above comment... . My motor winding days go back to the 50s probably from Popular Electronics. The armature was a nail shaft with crossed nails for the coil. The commutator was zig-zag wires for the "segments" on a masking tape insulator. It was held up by double nail supports. The field was more nails in a "U" with the field coil on this. It was powered by one of those "Dry Cells" that was about 2" in diameter and 6" tall. I found it a while ago on the web, but can't seem to find the plans now. Much more elaborate. I've recently done THIS one with students with good success.

@@electricandmagneticfields2314 It was very similar to this one: kzfaq.info/get/bejne/jr6JmKioqt_bkn0.html

Or this one: kzfaq.info/get/bejne/rM5zftKAx9m3eZ8.html

@@electricandmagneticfields2314 Thinking, now... Shouldn't the bare wire on the commutator be 90 degrees from where you show it? You want the coil _axis_ horizontal (coil vertical) when current flows, so the two fields are at 90 degrees and generates a torque. If you sand a side as you show it, it can stop when reverse aligned and try to push the coil upward. It appears your method requires a starting push. Done this way, you should "tilt' the support wires ever so slightly (off balance) so the armature will come to rest (battery unconnected) with the coil vertical (the axis horizontal) ... and the commutator making contact. Then it is self starting due to torque when at rest.