A moving-coil milliammeter having a full-scale deflection of 1 mA and an internal resistance of 0.5 ohms is to be converted to a voltmeter of 20 volts full-scale deflection. It would be necessary to insert a:

To convert an ammeter (which measures current) into a voltmeter (which measures voltage), a resistor must be added in series. This is known as a multiplier resistor. Shunt resistors are used in parallel to increase current-measuring capacity, so options suggesting a shunt can be immediately eliminated. Using Ohm's Law (R = E / I), the total resistance required for the circuit to read 20 volts at a full-scale current of 1 mA (0.001 amps) is 20 / 0.001 = 20,000 ohms. Since the meter movement itself already has an internal resistance of 0.5 ohms, you must subtract this from the total necessary resistance to find the value of the added resistor. The required series resistor is 20,000 - 0.5 = 19,999.5 ohms.