Extending the Range of a Moving Iron Ammeter

A moving iron ammeter is an instrument used to measure alternating current (AC) or direct current (DC). The range of a standard ammeter is limited by the maximum current the internal coil can safely handle without overheating or damaging the instrument.

To measure currents larger than the basic range of the moving iron ammeter, its range must be extended. This is typically done by diverting a portion of the total current away from the ammeter's internal coil. The device used for this purpose is called a shunt.

Understanding Shunts for Ammeters

A shunt is a low-resistance resistor connected in parallel with the ammeter. Here's why this configuration works:

• When a shunt is connected in parallel, the total current flowing into the circuit divides between the ammeter's coil and the shunt resistor.
• Because the shunt has a much lower resistance than the ammeter coil, most of the current passes through the shunt.
• Only a small, proportional fraction of the total current flows through the ammeter coil, which is within its safe measurement limit.
• The scale of the ammeter is calibrated to indicate the total current flowing into the parallel combination (ammeter + shunt).

Connecting a shunt in parallel effectively extends the range of the ammeter by providing an alternative, low-resistance path for the majority of the current.

Why Parallel Connection is Necessary for Ammeter Range Extension

Ammeter coils have very low resistance. To measure current, an ammeter is connected in series with the load. For range extension using a shunt, the shunt must be in parallel with the am ammeter coil so that the current gets divided. If a resistance were added in series, it would increase the total resistance in the circuit significantly, altering the current being measured and potentially reducing it, rather than allowing the ammeter to measure a larger current safely.

Comparison with Multipliers

Multipliers are high-resistance resistors connected in series with the coil of a voltmeter. Voltmeters are connected in parallel across the component or circuit where voltage is to be measured. Multipliers are used to extend the range of voltmeters, not ammeters.

Analyzing the Options

• Option 1: Shunt connected in series with an ammeter. This is incorrect. A shunt must be in parallel to divert current.
• Option 2: Shunt connected in parallel with an ammeter. This is correct. A shunt in parallel diverts excess current, extending the range.
• Option 3: Multiplier connected in parallel with an ammeter. This is incorrect. Multipliers are for voltmeters, and a parallel connection wouldn't work for an ammeter's range extension with a high resistance.
• Option 4: Multiplier connected in series with an ammeter. This is incorrect. Multipliers are for voltmeters, and a series connection would increase the ammeter's resistance, affecting the measurement.

Therefore, the range of a moving iron ammeter can be extended by using a shunt connected in parallel with the ammeter.