The magneticlinesofforce don't exist , they are defined by scientists.
磁力线并不存在，它是由科学家所定义。
2
It has been suggested that birds can sense the magneticlinesofforce stretching from the north to south magnetic poles of the earth, and so direct themselves.
有人认为，鸟类能觉察得出地球自北向南磁极的磁力线，因而能为自己导向。
3
The existence of electric current in the conductor(not closed)which is cut-ting the magneticlinesofforce(B-lines) variably in the magnetic field is pointed out, and the acceleration is discussed.