An AC ammeter is used to measure current in a circuit. When a direct current (DC) passes through the circuit, the AC ammeter reads `3 A`. When alternating current AC passes through the circuit, the AC ammeter reads 4 A. The reading of this ammeter (in ampere) if DC and AC flow through the circuit simultaneously, is .

Answer

To solve the problem, we need to understand how an AC ammeter measures current and how to combine the readings of DC and AC currents.

### Step-by-Step Solution:

1. **Understand the Readings**:
- The AC ammeter reads 3 A when only DC is present.
- The AC ammeter reads 4 A when only AC is present.

2. **Identify the Nature of the Readings**:
- The reading of 3 A for DC means that the ammeter is calibrated to read the equivalent current in terms of AC.
- The reading of 4 A for AC indicates the effective (RMS) value of the AC current.

3. **Use the Pythagorean Theorem**:
- When both DC and AC currents are present, the total current can be calculated using the Pythagorean theorem. This is because DC and AC currents are orthogonal to each other in terms of their phase.
- The formula to find the resultant current (I_total) when both currents are present is:
\[ I_{\text{total}} = \sqrt{I_{\text{DC}}^2 + I_{\text{AC}}^2} \]
- Here, \(I_{\text{DC}} = 3 \, \text{A}\) and \(I_{\text{AC}} = 4 \, \text{A}\).

4. **Calculate the Resultant Current**:
- Substitute the values into the formula:
\[ I_{\text{total}} = \sqrt{(3)^2 + (4)^2} \]
- Calculate the squares:
\[ I_{\text{total}} = \sqrt{9 + 16} = \sqrt{25} \]
- Finally, take the square root:
\[ I_{\text{total}} = 5 \, \text{A} \]

5. **Conclusion**:
- The reading of the AC ammeter when both DC and AC flow through the circuit simultaneously is **5 A**.