如何用等长法观测弧垂？
参考答案：
所谓等长法观测弧垂，就是从观测档两个档端杆AB的电线悬挂点向下各量一个弧垂fm，并绑一个水平标志板（即驰度板），然后调整电线，使它和两弛度板的连线相切，也就是使电线的视线点和两侧的弛度板三点成一直线，这时导线的弧垂即为要求的fm值。等长法使用方便，容易掌握，精度高，但只有在弧垂fm小于悬点高且视线畅通的情况才能用。
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| Once there was an old farmer, with a horse which was almost as old as himself. He 1 one morning with his young son, 2 the horse before it died. Father and son walked, 3 the farmer did not want the horse to be 4 ? They met two men on the road 5 said: "Why are you walking, farmer? You have a horse. It's 6 to market." The farmer knew that this was 7 , so he rode on the horse, 8 his son walked. ? Then they met two old ladies. " 9 are you doing up there, farmer? Can't you 10 how tired your boy is?" 11 the farmer got down, and his son rode 12 ? 13 , three old men stopped them. One said: "Why are you walking, farmer? 14 . It's too hot for an old man like you to walk today." So the farmer got up 15 his son, and they 16 ? 17 , a young woman passed them. "Why aren't you walking?" she asked. "It isn't 18 to market. Give your poor old horse a rest." So the farmer, and his son, got down 19 ? It is a fact that you cannot please 20 the people all the time. ? | ||||
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