✔ 最佳答案
You can use the "Right Hand Screw Rule". Curl the fingers of your right hand in the direction of current flowing in the solenoid. The thumb then points in the direction of the North Pole.
See the following diagram:
圖片參考:
http://imgcld.yimg.com/8/n/HA00458726/o/20130415224924.jpg