You want to invite Mr. Williams to give a lecture on "American Literature" in your college. Suppose you are the assistant of the English Department, write a letter and say:

1) the purpose of the invitation;

2) the time of the lecture.

Your letter should be no less than 100 words. You don’t need to write the address. Don’t sign your own name at the end of the letter. Use Ruth instead.

如图，一束电荷量为e的电子以垂直于磁感应强度B并垂直于磁场边界的速度v射入宽度为d的匀强磁场中，穿出磁场时速度方向θ=60°，求：

（1）穿越磁场的轨道半径；

（2）电子的质量；

（3）穿越磁场的时间；

（4）欲使电子从右边界射出，其速度v必须满足什么条件？