Which of the following statements is correct ?

A. Magnetic field due to earth changes with time and position
B. For paramagnetic material magnetic susceptibility is negative
C. Tangent galvanometer is used to measure power loss
D. `oint vecB* vecd s` may be non zero quantity

Options

A. Magnetic field due to earth changes with time and position
B. For paramagnetic material magnetic susceptibility is negative
C. Tangent galvanometer is used to measure power loss
D. `oint vecB* vecd s` may be non zero quantity

Answer

To solve the question, we need to analyze each of the given statements one by one and determine which one is correct.

### Step 1: Analyze Statement A
**Statement A:** "Magnetic field due to earth changes with time and position."

- The Earth's magnetic field is influenced by various factors, including its rotation and the movement of molten iron in its outer core. This leads to changes in the magnetic field over time and at different locations on the Earth's surface.
- **Conclusion:** This statement is correct.

### Step 2: Analyze Statement B
**Statement B:** "For paramagnetic material, magnetic susceptibility is negative."

- Paramagnetic materials have a positive magnetic susceptibility. This means that when placed in a magnetic field, they become magnetized in the direction of the field.
- **Conclusion:** This statement is incorrect.

### Step 3: Analyze Statement C
**Statement C:** "Tangent galvanometer is used to measure power loss."

- A tangent galvanometer is actually used to measure the horizontal component of the Earth's magnetic field or the current flowing through a coil. It is not used for measuring power loss.
- **Conclusion:** This statement is incorrect.

### Step 4: Analyze Statement D
**Statement D:** "`oint vecB* vecd s` may be a non-zero quantity."

- According to Maxwell's equations, the line integral of the magnetic field (B) around a closed loop is equal to the current passing through the loop (Ampere's Law). However, the divergence of B is always zero, meaning that the net magnetic field lines do not begin or end at any point (no magnetic monopoles). Therefore, the integral can be zero or non-zero depending on the current enclosed.
- **Conclusion:** This statement can be misleading. The integral can be non-zero if there is current, but the context of the question suggests it is incorrect in the sense of divergence.

### Final Conclusion
After analyzing all the statements:
- **Correct Answer:** Statement A is correct.