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华东版九年义务制初中物理课本中，第６２页有这样一道设计性实验题：图１（原书图１８—１６）是简单的水位自动报警装置所需的基本器材．要求当水位尚未达到金属块Ａ的高度时，工作电路中绿灯亮，表示水位正常；当水位升高到Ａ时（通常的水含有杂质，是导体），红灯亮，即发?
In the nine-year compulsory junior high school physics textbook for East China Edition, there is such a design experiment on page 62: Figure 1 (Original Text Figure 18-16) is the basic equipment required for a simple water level automatic alarm device. It is required that when the water level has not reached the height of the metal block A, the green light in the working circuit is on, indicating that the water level is normal; when the water level rises to A (usually the water contains impurities, is a conductor), the red light is on, that is, it is emitted.