Short Answer:

An ammeter should have very low resistance so that it does not affect the current flowing in the circuit where it is connected. Since an ammeter is always connected in series, a high resistance would reduce the current and cause incorrect readings, which can lead to errors in measurement.

Low resistance ensures that the ammeter allows almost the same amount of current to pass through as if it wasn’t there at all. This helps in getting accurate and reliable current measurements without disturbing the normal operation of the circuit.

Detailed Explanation:

Why ammeter has low resistance

An ammeter is an instrument used to measure the electric current in a circuit. It is always connected in series with the component or section of the circuit where current needs to be measured. In a series connection, the same current flows through all components, so the ammeter becomes part of the actual current path.

If the ammeter had high internal resistance, it would oppose the flow of current. This would lead to a drop in current in the entire circuit, making the reading lower than the actual current. To avoid this, the internal resistance of the ammeter is kept as low as possible, often close to zero ohms.

Main Reasons for Low Resistance in Ammeter:

1. Avoids voltage drop:
   A high resistance would cause a noticeable voltage drop across the ammeter, which could affect the rest of the circuit. Low resistance minimizes this voltage drop.
2. Prevents alteration of current:
   Since the purpose of an ammeter is to measure current without influencing it, keeping its resistance low ensures it doesn’t change the current it is meant to measure.
3. Improves measurement accuracy:
   Low resistance ensures that the ammeter measures the true current flowing through the circuit, leading to accurate and dependable readings.
4. Protects the circuit:
   High resistance could introduce unexpected behavior in the circuit, possibly causing damage in sensitive components. Low resistance avoids such risks.

Working Mechanism:

Internally, an ammeter uses a sensitive current-measuring device like a moving coil galvanometer. Since this coil can handle only small currents, a shunt resistor (a low-value resistor) is connected in parallel with it. The shunt allows the majority of the current to bypass the delicate coil, protecting it while still allowing accurate current measurement. This entire arrangement ensures that the overall internal resistance of the ammeter remains very low.

Consequences of High Resistance:

• If an ammeter had high resistance, it would block part of the current, just like any resistor in series.
• It would cause a voltage drop across itself, reducing the voltage across the actual load, which is not desirable.
• It may heat up or get damaged if excessive voltage is dropped across it due to high resistance.
• The circuit being measured may malfunction because of altered current values.

Design Consideration:

When designing or selecting an ammeter, manufacturers ensure:

• It has minimal resistance.
• It can carry rated current without overheating.
• It gives linear and accurate readings over its full scale.

Applications:

• In electronics for testing and fault detection.
• In power systems to monitor current flow in lines or machines.
• In laboratories for measuring currents during experiments.

Conclusion

An ammeter must have low resistance because it is connected in series and must not change the current flowing in the circuit. A high-resistance ammeter would affect the measurement and the operation of the circuit itself. By keeping the internal resistance low, the ammeter ensures accurate and safe current measurement without disturbing the original current path.