The way to change a DC ammeter into a voltmeter is to remove the low impedance resistor in parallel and connect the high impedance resistor in series. The specific resistance depends on the internal resistance of your meter. (range from several hundred ohms/volt to several thousand ohms/volt)

method of attachment

First, connect the voltmeter to the voltage divider, and connect the resistor box in series and parallel with the voltmeter with a switch. First, disconnect the switch, adjust the sliding hole rheostat to make the voltmeter fully biased, close the switch, and adjust the resistor box to make the voltmeter half biased. The resistance shown in the resistance box is the resistance of the voltmeter, which has errors and is too large.