Which statement about the shunt-type ammeter is correct?

Which statement about the shunt-type ammeter is correct?

• A. It uses a high-value resistor to measure current.
• B. It measures current by the voltage drop across a low-value resistor (shunt) to provide a proportional reading.
• C. It measures current based on transformer action.
• D. It only indicates voltage.

Answer: (B)

The key idea is to measure current with a very small, known voltage drop. A low-value resistor (the shunt) is connected in parallel with the meter’s movement so the same voltage appears across both. Most of the circuit current flows through the shunt, while a tiny portion goes through the meter coil. The meter is calibrated so that this small coil current, driven by that voltage, corresponds to the total current in the circuit. Because the voltage drop across a small shunt is proportional to the current (V = I·R) and the shunt is sized to be very small, the circuit isn’t noticeably affected while giving a proportional reading of the current. This approach is in contrast to using a high-value resistor, which would waste a large portion of the circuit voltage and disturb operation; a current transformer measures current by magnetic induction (not a direct voltage drop); and a device that only indicates voltage would be a voltmeter, not an ammeter.

The key idea is to measure current with a very small, known voltage drop. A low-value resistor (the shunt) is connected in parallel with the meter’s movement so the same voltage appears across both. Most of the circuit current flows through the shunt, while a tiny portion goes through the meter coil. The meter is calibrated so that this small coil current, driven by that voltage, corresponds to the total current in the circuit. Because the voltage drop across a small shunt is proportional to the current (V = I·R) and the shunt is sized to be very small, the circuit isn’t noticeably affected while giving a proportional reading of the current.

This approach is in contrast to using a high-value resistor, which would waste a large portion of the circuit voltage and disturb operation; a current transformer measures current by magnetic induction (not a direct voltage drop); and a device that only indicates voltage would be a voltmeter, not an ammeter.