Alternating current can not be measured by D.C. Ammeter because

Options

A. ac cannot pass through dc ammeter
B. Average value of complete cycle is zero
C. ac is virtual
D. ac changes its direction

Answer

### Step-by-Step Solution:

1. **Understanding the Nature of AC and DC**:
- Alternating Current (AC) changes its direction periodically, while Direct Current (DC) flows in a single direction.
- AC can be represented as a sinusoidal wave, oscillating between positive and negative values, whereas DC is a constant value above zero.

2. **Graphical Representation**:
- Draw the graphs for both AC and DC:
- The DC graph is a straight horizontal line above the time axis (positive constant).
- The AC graph is a sine wave oscillating above and below the time axis.

3. **Calculating Average Values**:
- For DC, the average value over time is simply the constant value of the current.
- For AC, when you calculate the average value over one complete cycle, the positive and negative halves cancel each other out, resulting in an average value of zero.

4. **Implication for Measurement**:
- Since the DC ammeter measures the average current, if it is connected to an AC source, it will read zero because the average value of the AC current over a complete cycle is zero.

5. **Conclusion**:
- Therefore, the reason why an AC cannot be measured by a DC ammeter is that the average value of the complete cycle of AC is zero.

### Final Answer:
Alternating current cannot be measured by a DC ammeter because the average value of the complete cycle of AC is zero.