An ammeter can be adapted to measure potential difference by using a

Analyze the question: An ammeter is built from a sensitive galvanometer. We want to make it read potential difference (voltage), so it must work like a voltmeter.

Explain the concept:

• A shunt is a low resistance connected in parallel with a galvanometer. It lets most of the current bypass the coil, which extends the current range. This makes an ammeter.
• A multiplier is a high resistance connected in series with a galvanometer. It limits the current through the coil and makes the instrument read voltage, which makes a voltmeter.
• A rheostat is a variable resistor used to control current in a circuit.
• A resistance box is a set of known resistors used for experiments. It is not a standard way to convert a meter.

Step-by-step solution:

1. Voltage is measured by a voltmeter, which has a high resistance.
2. To get a high total resistance from a galvanometer or ammeter, add a large resistance in series.
3. The voltage across the meter is V = I(R_g + R_m). So the scale can be calibrated in volts, with R_m = V/I_g − R_g.
4. The series resistor that does this is called a multiplier.

Answer: The correct option is B. multiplier.

Exam tip: Remember "Shunt = Parallel = Ammeter" and "Multiplier = Series = Voltmeter". The letters S-P-A and M-S-V can help you recall them.