When a voltage measuring device is connected to a.c. mains the meter shows the steady input voltage of `220 V`. This means

Options

A. input voltage cannot be AC voltage, but a DC voltage
B. maximum input voltage is 220V.
C. the meter reads not v but `lt v^(2) gt` and is calibrated to read `sqrt(lt v^(2) gt)`
D. the pointer of the meter is stuck by some mechanical defect.

Explanation

To solve the question regarding the voltage measuring device connected to the AC mains showing a steady input voltage of 220 V, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Understanding AC Voltage Measurement**:
- The voltage measuring device connected to the AC mains is showing a reading of 220 V. This value represents the RMS (Root Mean Square) voltage of the AC supply.

2. **Analyzing the Options**:
- **Option A**: "Input voltage cannot be AC voltage but DC voltage."
- This option is incorrect because the mains supply is AC, and the device is designed to measure AC voltage.

- **Option B**: "Maximum input voltage is 220 V."
- This option is also incorrect. The maximum voltage (peak voltage) of the AC supply can be calculated using the formula \( V_{max} = V_{rms} \times \sqrt{2} \). For 220 V RMS, the peak voltage is approximately \( 220 \times \sqrt{2} \approx 311 V \).

- **Option C**: "The meter reads \( V_{rms} \) but is calibrated to read \( V^2 \)."
- This option is correct. The meter indeed measures the RMS voltage, which is the standard way to express AC voltage. It does not read \( V^2 \) but is calibrated to read the RMS value directly.

- **Option D**: "The pointer of the meter is stopped by some mechanical effect."
- This option is false because the meter is functioning correctly and is not stuck due to mechanical issues. The reading of 220 V is valid.

3. **Conclusion**:
- The correct interpretation of the meter reading of 220 V is that it represents the RMS voltage of the AC mains supply, and the correct option is **Option C**.

### Final Answer:
The meter reads the RMS voltage of the AC supply, which is 220 V.

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