Faraday was not happy with the result of the electromotor, because the motor turned only once. He changed the polarity of the battery and the wire wrapped nail again turned just once. If he changed the polarity every time, when the north pole of the wire wrapped nail is opposite to the south pole of the horseshoe-shaped magnet, then he would have the result he was looking for. The wire wrapped nail would turn and turn around the spindle (as long as the battery is not empty).