1．测量一根钢管的外径，四种放置直角三角板和刻度尺的方法分别如图l、图人图3、图4所示，其中正确的是图所示的方法．（199年广西初中物理初赛题）分析图1和图3所示的测量，不能保证刻度尺的边缘正好通过钢管的圆心，即不能保证所测的是钢管的直径．图4所示的测量，刻度尺的 1. Measuring the outer diameter of a steel pipe, the four methods for placing the right triangle and scale are shown in Figure 1, Figure 3 and Figure 4, respectively, where the correct method is shown in the figure. (199 junior high school physics preliminary title in Guangxi) Analysis of the measurements shown in Figs. 1 and 3 does not guarantee that the edge of the scale will pass through the center of the steel pipe, that is, the diameter of the steel pipe cannot be guaranteed. Figure 4 shows the measurement, scale