Please find the attached picture.. My understanding : Use right hand thumb rule to find the direction of magnetic flux lines when the direction of currents are known. When I have a 'cross', the lines will be in clockwise direction while I have a 'dot', the lines will be in anticlcokwise direction. And as magnetic flux lines start from N pole and end at S pole, i find the N and S poles.. This works only with hollow cylindrical electromagnet, not with solid one.. Where I make mistake?? Please clarify. Thanks.