·实验预习·１．测量电路＿，要用电压表，其符号是＿． ２．图３－１是电压表的表盘，当选接“－”与“３”两接线柱时，其量程是＿，每一大格表示＿，每一小格表示＿．当选接“－”与“１５”两接线柱时，其量程是＿，每一大格表示＿，每一小格表示＿．图３－１中表盘指针指 ·Experimental preparations 1. Measuring circuit _, to use a voltmeter, the symbol is _. 2. Figure 3-1 is the dial of the voltmeter. When the two terminals “-” and “3” are connected, the range is _, each big box represents _, and each small box represents _. When selecting the “-” and “15” terminals, the range is _, each big box represents _, and each small box represents _. The dial pointer in Figure 3-1