Understanding Ammeter Connection and Internal Resistance

An ammeter is an instrument used to measure the electric current flowing through a circuit. To measure the current passing through a specific component or part of a circuit, the ammeter must be connected in series with that component or part.

When any device is inserted into a circuit in series, it adds its own resistance to the total resistance of the circuit. According to Ohm's Law, which states that voltage ($V$) is equal to current ($I$) multiplied by resistance ($R$), represented as:

\[V = IR\]

If the total resistance of the circuit changes significantly due to the inserted device, the current flowing in the circuit will also change, potentially leading to an inaccurate measurement.

Why Low Internal Resistance is Crucial for Ammeters

An ideal ammeter would have zero internal resistance so that its presence in the series circuit does not alter the original current. However, real ammeters have some internal resistance.

When an ammeter with internal resistance ($R_{ammeter}$) is connected in series with a circuit component having resistance ($R_{circuit}$), the total resistance of that part of the circuit becomes $R_{total} = R_{circuit} + R_{ammeter}$.

If the ammeter's internal resistance ($R_{ammeter}$) is high, the total resistance increases significantly. This increase in total resistance would cause the current flowing in the circuit to decrease (assuming the voltage source is constant), resulting in the ammeter measuring a current that is lower than the original current that flowed before the ammeter was inserted.

Therefore, to minimize the impact on the circuit and measure the current accurately, the internal resistance of an ammeter must be kept as low as possible.

Low Voltage Drop on the Ammeter

The voltage drop across the ammeter itself can be calculated using Ohm's Law:

\[V_{ammeter} = I_{circuit} \times R_{ammeter}\]

If the internal resistance ($R_{ammeter}$) is low, then the voltage drop across the ammeter ($V_{ammeter}$) will also be low, even when there is a significant current ($I_{circuit}$) flowing through it. A low voltage drop across the ammeter means that most of the circuit's voltage is applied across the components being measured, and the ammeter itself consumes very little voltage. This is a direct consequence of having low internal resistance and is the primary reason why ammeters are designed this way when connected in series: to avoid altering the circuit's operating conditions and to provide an accurate current reading.

Analyzing the Options

• Low selectivity: This is not related to the internal resistance of an ammeter used for basic current measurement in a circuit.
• High sensitivity: Sensitivity relates to the meter's ability to detect small changes in current. While a desirable feature, low internal resistance is about minimizing circuit disturbance, not directly sensitivity.
• Low voltage drop on ammeter: This is a direct consequence of low internal resistance ($V_{ammeter} = I \times R_{ammeter}$). A low voltage drop ensures the ammeter does not significantly affect the circuit's operation or the current being measured.
• High voltage drop across ammeter: This would happen with high internal resistance, which would significantly alter the circuit's current and lead to inaccurate measurements.

Based on the function of an ammeter and the principles of circuit analysis, having a low internal resistance when connected in series is essential to cause a low voltage drop across the ammeter, thus minimizing its effect on the circuit and ensuring accurate current measurement.