An ammeter is an electrical instrument used to measure electric current in a circuit. It is always connected in series with the component through which the current is to be measured. An ideal ammeter has very low (ideally zero) internal resistance to ensure it does not significantly alter the current it is measuring.

A voltmeter is an electrical instrument used to measure the potential difference (voltage) between two points in a circuit. It is always connected in parallel across the component whose voltage is to be measured. An ideal voltmeter has very high (ideally infinite) internal resistance to ensure it draws negligible current from the circuit branch it is measuring, thus not affecting the voltage.

Ammeter to Voltmeter Conversion

Since an ammeter has a very low internal resistance and a voltmeter requires a very high internal resistance, to use an ammeter in place of a voltmeter, we must increase its effective resistance significantly.

• Increasing Resistance: To increase the overall resistance of the ammeter for voltage measurement, a high value resistance (often called a multiplier resistance) must be connected.
• Connection Type: For resistances to add up and form a high total resistance, they must be connected in series. When the high resistance is connected in series with the ammeter, the total resistance of this combination becomes the sum of the ammeter's internal resistance and the added high resistance.
• Function of Series Resistance:
  • It limits the current flowing through the ammeter to a safe, measurable level, even when high voltages are applied across the combination.
  • Most of the potential difference drops across this high series resistance, allowing the ammeter (which has low resistance) to handle only a small portion of the total voltage. This entire combination then acts as a high-resistance device capable of measuring large potential differences.

Therefore, to convert an ammeter into a voltmeter, a high value resistance is connected in series with the ammeter. The resulting combination can then be connected in parallel across the points where voltage is to be measured.

Analyzing Connection Options

• A high value resistance in series: This is the correct method. By adding a high resistance in series, the total resistance of the ammeter-resistance combination becomes very high, which is essential for a voltmeter to measure voltage accurately without drawing much current from the main circuit.
• A low value resistance in parallel: Connecting a low resistance in parallel with an ammeter (or galvanometer) is typically done to convert it into an ammeter with a higher range. This lowers the effective resistance of the device, making it unsuitable for use as a voltmeter. This parallel resistance is called a shunt.
• A high value resistance in parallel: Connecting a high resistance in parallel with the ammeter would reduce the overall effective resistance of the combination, as resistances in parallel combine to yield a smaller equivalent resistance than the smallest individual resistance. This configuration would not be suitable for a voltmeter.

The principle is that a voltmeter needs to have a very high internal resistance. An ammeter, by itself, has a very low internal resistance. Adding a high resistance in series increases the total resistance of the measuring device, thus converting the low-resistance ammeter into a high-resistance voltmeter.