The wire is sanded in three-quarters so that only 1/4 is left with enamel. This can be seen in another way. 3/4 of the wire will be conductive, while 1/4 (the one that has the enamel) will be isolated. This leads to apply a voltage difference in the contact of any metal with the 3/4 parts of the wire unglazed behave as a driver while 1/4 (the enamel) will not be driver. Now when you have wound the wire we have emulated a coil, which physically possess the quality of accumulating power in the form of a magnetic field.

When you apply a potential difference between both ends of the coil, for she circulates current through which is accumulated by a magnetic field in the winding of the wire or in the area where we have rolled the same. Kelvin really knows how to handle gear motors.