直流电动机是利用______的原理制成的，其中，有一个周期性改变线圈中电流方向的装置，叫______．
直流电动机是根据通电线圈在磁场中受力转动原理制成的，在线圈转过平衡位置时，如果不改变电流的方向，线圈中两根导体的受力方向不变，在力的作用下线圈将在平衡位置附近来回摆动，为了使线圈能够持续转动，在输入电流时采用换向器，使线圈在刚过平衡位置时及时改变线圈中的电流方向，改变线圈的受力方向，从而使线圈能够实现连续转动．
故答案是：通电线圈在磁场中受力转动、换向器．
| 完形填空。 | |||
| My father brought home a sailboat when I was ten, and almost each Sunday in summers we would gosailing. Dad was quite skilled in sailing, but not good at 1 . As for me, I 2 both before twelve because of living close to Lake Ontario. The last time Dad and l set sail together is really 3 . It was a perfect weekend after I graduated from university. I came home and 4 Dad to go sailing. Out we set soon on the 5 lake. Dad hadn't sailed for years, but everything 6 wen with the tiller (舵柄) in his hands. When we were in the middle of the lake, a 7 wind came all of a sudden. The boat was hit 8 . Dad was always at his best in any 9 , but at this moment he 10 . "John! 11 !" he shouted in a trembling voice, with the tiller still in his hands. In my memory he could fix any 12 . He was the one I always 13 for strength and security. Before I could respond, a 14 of water got into the boat. I rushed to the tiller 15 it was too late. Another huge wave of water 16 the boat in a minute. We were thrown into the water, and Dad was struggling aimlessly. At that moment, I felt fiercely 17 of him. I swam to Dad 18 and assisted him in climbing onto the hull(船壳)of the boat. Upon sitting on the hull, Dad was a little awkward about his flash of 19 . "It's all right, Dad. We are safe now," I comforted him. That was the first time Dad had counted on me in a moment of emergency. More importantly, I found it was myturn to start 20 for my father. | |||
| ( )1. A. boating ( )2. A. enjoyed ( )3. A. unforgivable ( )4. A. sent ( )5. A. calm ( )6. A. finished ( )7. A. strong ( )8. A. repeatedly ( )9. A. danger ( )10. A. suffered ( )11. A. look ( )12. A. problem ( )13. A. turned to ( )14. A. fountain ( )15. A. if ( )16. A. got through ( )17. A. ashamed ( )18. A. hopelessly ( )19. A. pain ( )20. A. making up | B. running B. desired B. unforgettable B. ordered B. icy B. went B. gentle B. lightly B. place B. fell B. Help B. relationship B. lived with B. stream B. for B. poured into B. protective B. quickly B. anger B. getting ready | C. swimming C. hated C. cheerful C. invited C. stormy C. seemed C. cold C. hardly C. sport C. froze C. Run C. machine C. argued with C. shower C. after C. turned over C. tired C. slowly C. fear C. paying off | D. teaching D. learned D. regretful D. allowed D. thundery D. sounded D. hot D. violently D. job D. withdrew D. Jump D. boat D. objected to D. wave D. but D. lifted up D. afraid D. helplessly D. shame D. looking out |