题目

What is “Walk to School”? Now we are working on a programmed of  “Walk to School”．We would like as many parents and children as possible to take part, even if in a small way．Children who walk to and from school along with their parents can learn essential （基本的） road safety and life skills． You can take part by… ■ Walking to and from school with your child every day. ■ Walking for one or two days during the week. ■ Encouraging others to walk, if you already do. Walking is great! ■ Walking is great exercise！A walk is good for your body and can keep you fit. 为探究“影响电磁铁磁性强弱的因素”，某同学用电池(电压一定)、滑动变阻器、数量足够多的大头针、大铁钉及较长的导线为主要材料，连接了如图所示的电路进行实验。      (1)实验中，他巧妙地通过______________来显示电磁铁磁性的强弱。 (2)闭合电路后，变阻器接入电路的电阻较大时的情景如图甲；变阻器接入电路的电阻较小时的情景如图乙。通过比较这两次实验的现象，可得出的初步结论是：通过电磁铁的________越大时，它的磁性越强。 (3)在图中标出电磁铁的N极、S极 (4)你认为该实验的局限性是：______________________________________________________________________________________________________